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libpng.3：源码内容

mentioned in the PNG specification is to expand each pixel to cover
those pixels that have not been read yet (the "rectangle" method).
This results in a blocky image for the first pass, which gradually
smooths out as more pixels are read. The other method is the "sparkle"
method, where pixels are drawn only in their final locations, with the
rest of the image remaining whatever colors they were initialized to
before the start of the read. The first method usually looks better,
but tends to be slower, as there are more pixels to put in the rows.
If you don't want libpng to handle the interlacing details, just call
png_read_rows() seven times to read in all seven images. Each of the
images is a valid image by itself, or they can all be combined on an
8x8 grid to form a single image (although if you intend to combine them
you would be far better off using the libpng interlace handling).
The first pass will return an image 1/8 as wide as the entire image
(every 8th column starting in column 0) and 1/8 as high as the original
(every 8th row starting in row 0), the second will be 1/8 as wide
(starting in column 4) and 1/8 as high (also starting in row 0). The
third pass will be 1/4 as wide (every 4th pixel starting in column 0) and
1/8 as high (every 8th row starting in row 4), and the fourth pass will
be 1/4 as wide and 1/4 as high (every 4th column starting in column 2,
and every 4th row starting in row 0). The fifth pass will return an
image 1/2 as wide, and 1/4 as high (starting at column 0 and row 2),
while the sixth pass will be 1/2 as wide and 1/2 as high as the original
(starting in column 1 and row 0). The seventh and final pass will be as
wide as the original, and 1/2 as high, containing all of the odd
numbered scanlines. Phew!
If you want libpng to expand the images, call this before calling
png_start_read_image() or png_read_update_info():
if (interlace_type == PNG_INTERLACE_ADAM7)
number_of_passes
= png_set_interlace_handling(png_ptr);
This will return the number of passes needed. Currently, this
is seven, but may change if another interlace type is added.
This function can be called even if the file is not interlaced,
where it will return one pass.
If you are not going to display the image after each pass, but are
going to wait until the entire image is read in, use the sparkle
effect. This effect is faster and the end result of either method
is exactly the same. If you are planning on displaying the image
after each pass, the "rectangle" effect is generally considered the
better looking one.
If you only want the "sparkle" effect, just call png_read_rows() as
normal, with the third parameter NULL. Make sure you make pass over
the image number_of_passes times, and you don't change the data in the
rows between calls. You can change the locations of the data, just
not the data. Each pass only writes the pixels appropriate for that
pass, and assumes the data from previous passes is still valid.
png_read_rows(png_ptr, row_pointers, NULL,
number_of_rows);
If you only want the first effect (the rectangles), do the same as
before except pass the row buffer in the third parameter, and leave
the second parameter NULL.
png_read_rows(png_ptr, NULL, row_pointers,
number_of_rows);
.SS Finishing a sequential read
After you are finished reading the image through the
low-level interface, you can finish reading the file. If you are
interested in comments or time, which may be stored either before or
after the image data, you should pass the separate png_info struct if
you want to keep the comments from before and after the image
separate. If you are not interested, you can pass NULL.
png_read_end(png_ptr, end_info);
When you are done, you can free all memory allocated by libpng like this:
png_destroy_read_struct(&png_ptr, &info_ptr,
&end_info);
It is also possible to individually free the info_ptr members that
point to libpng-allocated storage with the following function:
png_free_data(png_ptr, info_ptr, mask, seq)
mask - identifies data to be freed, a mask
containing the bitwise OR of one or
more of
PNG_FREE_PLTE, PNG_FREE_TRNS,
PNG_FREE_HIST, PNG_FREE_ICCP,
PNG_FREE_PCAL, PNG_FREE_ROWS,
PNG_FREE_SCAL, PNG_FREE_SPLT,
PNG_FREE_TEXT, PNG_FREE_UNKN,
or simply PNG_FREE_ALL
seq - sequence number of item to be freed
(-1 for all items)
This function may be safely called when the relevant storage has
already been freed, or has not yet been allocated, or was allocated
by the user and not by libpng, and will in those
cases do nothing. The "seq" parameter is ignored if only one item
of the selected data type, such as PLTE, is allowed. If "seq" is not
-1, and multiple items are allowed for the data type identified in
the mask, such as text or sPLT, only the n'th item in the structure
is freed, where n is "seq".
The default behavior is only to free data that was allocated internally
by libpng. This can be changed, so that libpng will not free the data,
or so that it will free data that was allocated by the user with png_malloc()
or png_zalloc() and passed in via a png_set_*() function, with
png_data_freer(png_ptr, info_ptr, freer, mask)
mask - which data elements are affected
same choices as in png_free_data()
freer - one of
PNG_DESTROY_WILL_FREE_DATA
PNG_SET_WILL_FREE_DATA
PNG_USER_WILL_FREE_DATA
This function only affects data that has already been allocated.
You can call this function after reading the PNG data but before calling
any png_set_*() functions, to control whether the user or the png_set_*()
function is responsible for freeing any existing data that might be present,
and again after the png_set_*() functions to control whether the user
or png_destroy_*() is supposed to free the data. When the user assumes
responsibility for libpng-allocated data, the application must use
png_free() to free it, and when the user transfers responsibility to libpng
for data that the user has allocated, the user must have used png_malloc()
or png_zalloc() to allocate it.
If you allocated your row_pointers in a single block, as suggested above in
the description of the high level read interface, you must not transfer
responsibility for freeing it to the png_set_rows or png_read_destroy function,
because they would also try to free the individual row_pointers[i].
If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
separately, do not transfer responsibility for freeing text_ptr to libpng,
because when libpng fills a png_text structure it combines these members with
the key member, and png_free_data() will free only text_ptr.key. Similarly,
if you transfer responsibility for free'ing text_ptr from libpng to your
application, your application must not separately free those members.
The png_free_data() function will turn off the "valid" flag for anything
it frees. If you need to turn the flag off for a chunk that was freed by your
application instead of by libpng, you can use
png_set_invalid(png_ptr, info_ptr, mask);
mask - identifies the chunks to be made invalid,
containing the bitwise OR of one or
more of
PNG_INFO_gAMA, PNG_INFO_sBIT,
PNG_INFO_cHRM, PNG_INFO_PLTE,
PNG_INFO_tRNS, PNG_INFO_bKGD,
PNG_INFO_hIST, PNG_INFO_pHYs,
PNG_INFO_oFFs, PNG_INFO_tIME,
PNG_INFO_pCAL, PNG_INFO_sRGB,
PNG_INFO_iCCP, PNG_INFO_sPLT,
PNG_INFO_sCAL, PNG_INFO_IDAT
For a more compact example of reading a PNG image, see the file example.c.
.SS Reading PNG files progressively
The progressive reader is slightly different then the non-progressive
reader. Instead of calling png_read_info(), png_read_rows(), and
png_read_end(), you make one call to png_process_data(), which calls
callbacks when it has the info, a row, or the end of the image. You
set up these callbacks with png_set_progressive_read_fn(). You don't
have to worry about the input/output functions of libpng, as you are
giving the library the data directly in png_process_data(). I will
assume that you have read the section on reading PNG files above,
so I will only highlight the differences (although I will show
all of the code).
png_structp png_ptr;
png_infop info_ptr;
/* An example code fragment of how you would
initialize the progressive reader in your
application. */
int
initialize_png_reader()
{
png_ptr = png_create_read_struct
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
user_error_fn, user_warning_fn);
if (!png_ptr)
return (ERROR);
info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr)
{
png_destroy_read_struct(&png_ptr, (png_infopp)NULL,
(png_infopp)NULL);
return (ERROR);
}
if (setjmp(png_jmpbuf(png_ptr)))
{
png_destroy_read_struct(&png_ptr, &info_ptr,
(png_infopp)NULL);
return (ERROR);
}
/* This one's new. You can provide functions
to be called when the header info is valid,
when each row is completed, and when the image
is finished. If you aren't using all functions,
you can specify NULL parameters. Even when all
three functions are NULL, you need to call
png_set_progressive_read_fn(). You can use
any struct as the user_ptr (cast to a void pointer
for the function call), and retrieve the pointer
from inside the callbacks using the function
png_get_progressive_ptr(png_ptr);
which will return a void pointer, which you have
to cast appropriately.
*/
png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
info_callback, row_callback, end_callback);
return 0;
}
/* A code fragment that you call as you receive blocks
of data */
int
process_data(png_bytep buffer, png_uint_32 length)
{
if (setjmp(png_jmpbuf(png_ptr)))
{
png_destroy_read_struct(&png_ptr, &info_ptr,
(png_infopp)NULL);
return (ERROR);
}
/* This one's new also. Simply give it a chunk
of data from the file stream (in order, of
course). On machines with segmented memory
models machines, don't give it any more than
64K. The library seems to run fine with sizes
of 4K. Although you can give it much less if
necessary (I assume you can give it chunks of
1 byte, I haven't tried less then 256 bytes
yet). When this function returns, you may
want to display any rows that were generated
in the row callback if you don't already do
so there.
*/
png_process_data(png_ptr, info_ptr, buffer, length);
return 0;
}
/* This function is called (as set by
png_set_progressive_read_fn() above) when enough data
has been supplied so all of the header has been
read.
*/
void
info_callback(png_structp png_ptr, png_infop info)
{
/* Do any setup here, including setting any of
the transformations mentioned in the Reading
PNG files section. For now, you _must_ call
either png_start_read_image() or
png_read_update_info() after all the
transformations are set (even if you don't set
any). You may start getting rows before
png_process_data() returns, so this is your
last chance to prepare for that.
*/
}
/* This function is called when each row of image
data is complete */
void
row_callback(png_structp png_ptr, png_bytep new_row,
png_uint_32 row_num, int pass)
{
/* If the image is interlaced, and you turned
on the interlace handler, this function will
be called for every row in every pass. Some
of these rows will not be changed from the
previous pass. When the row is not changed,
the new_row variable will be NULL. The rows
and passes are called in order, so you don't
really need the row_num and pass, but I'm
supplying them because it may make your life
easier.
For the non-NULL rows of interlaced images,
you must call png_progressive_combine_row()
passing in the row and the old row. You can
call this function for NULL rows (it will just
return) and for non-interlaced images (it just
does the memcpy for you) if it will make the
code easier. Thus, you can just do this for
all cases:
*/
png_progressive_combine_row(png_ptr, old_row,
new_row);
/* where old_row is what was displayed for
previously for the row. Note that the first
pass (pass == 0, really) will completely cover
the old row, so the rows do not have to be
initialized. After the first pass (and only
for interlaced images), you will have to pass
the current row, and the function will combine
the old row and the new row.
*/
}
void
end_callback(png_structp png_ptr, png_infop info)
{
/* This function is called after the whole image
has been read, including any chunks after the
image (up to and including the IEND). You
will usually have the same info chunk as you
had in the header, although some data may have
been added to the comments and time fields.
Most people won't do much here, perhaps setting
a flag that marks the image as finished.
*/
}
.SH IV. Writing
Much of this is very similar to reading. However, everything of
importance is repeated here, so you won't have to constantly look
back up in the reading section to understand writing.
.SS Setup
You will want to do the I/O initialization before you get into libpng,
so if it doesn't work, you don't have anything to undo. If you are not
using the standard I/O functions, you will need to replace them with
custom writing functions. See the discussion under Customizing libpng.
FILE *fp = fopen(file_name, "wb");
if (!fp)
{
return (ERROR);
}
Next, png_struct and png_info need to be allocated and initialized.
As these can be both relatively large, you may not want to store these
on the stack, unless you have stack space to spare. Of course, you
will want to check if they return NULL. If you are also reading,
you won't want to name your read structure and your write structure
both "png_ptr"; you can call them anything you like, such as
"read_ptr" and "write_ptr". Look at pngtest.c, for example.
png_structp png_ptr = png_create_write_struct
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
user_error_fn, user_warning_fn);
if (!png_ptr)
return (ERROR);
png_infop info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr)
{
png_destroy_write_struct(&png_ptr,
(png_infopp)NULL);
return (ERROR);
}
If you want to use your own memory allocation routines,
define PNG_USER_MEM_SUPPORTED and use
png_create_write_struct_2() instead of png_create_write_struct():
png_structp png_ptr = png_create_write_struct_2
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
user_error_fn, user_warning_fn, (png_voidp)
user_mem_ptr, user_malloc_fn, user_free_fn);
After you have these structures, you will need to set up the
error handling. When libpng encounters an error, it expects to
longjmp() back to your routine. Therefore, you will need to call
setjmp() and pass the png_jmpbuf(png_ptr). If you
write the file from different routines, you will need to update
the png_jmpbuf(png_ptr) every time you enter a new routine that will
call a png_*() function. See your documentation of setjmp/longjmp
for your compiler for more information on setjmp/longjmp. See
the discussion on libpng error handling in the Customizing Libpng
section below for more information on the libpng error handling.
if (setjmp(png_jmpbuf(png_ptr)))
{
png_destroy_write_struct(&png_ptr, &info_ptr);
fclose(fp);
return (ERROR);
}
...
return;
If you would rather avoid the complexity of setjmp/longjmp issues,
you can compile libpng with PNG_SETJMP_NOT_SUPPORTED, in which case
errors will result in a call to PNG_ABORT() which defaults to abort().
Now you need to set up the output code. The default for libpng is to
use the C function fwrite(). If you use this, you will need to pass a
valid FILE * in the function png_init_io(). Be sure that the file is
opened in binary mode. Again, if you wish to handle writing data in
another way, see the discussion on libpng I/O handling in the Customizing
Libpng section below.
png_init_io(png_ptr, fp);
If you are embedding your PNG into a datastream such as MNG, and don't
want libpng to write the 8-byte signature, or if you have already
written the signature in your application, use
png_set_sig_bytes(png_ptr, 8);
to inform libpng that it should not write a signature.
.SS Write callbacks
At this point, you can set up a callback function that will be
called after each row has been written, which you can use to control
a progress meter or the like. It's demonstrated in pngtest.c.
You must supply a function
void write_row_callback(png_ptr, png_uint_32 row,
int pass);
{
/* put your code here */
}
(You can give it another name that you like instead of "write_row_callback")
To inform libpng about your function, use
png_set_write_status_fn(png_ptr, write_row_callback);
You now have the option of modifying how the compression library will
run. The following functions are mainly for testing, but may be useful
in some cases, like if you need to write PNG files extremely fast and
are willing to give up some compression, or if you want to get the
maximum possible compression at the expense of slower writing. If you
have no special needs in this area, let the library do what it wants by
not calling this function at all, as it has been tuned to deliver a good
speed/compression ratio. The second parameter to png_set_filter() is
the filter method, for which the only valid values are 0 (as of the
July 1999 PNG specification, version 1.2) or 64 (if you are writing
a PNG datastream that is to be embedded in a MNG datastream). The third
parameter is a flag that indicates which filter type(s) are to be tested
for each scanline. See the PNG specification for details on the specific filter
types.
/* turn on or off filtering, and/or choose
specific filters. You can use either a single
PNG_FILTER_VALUE_NAME or the bitwise OR of one
or more PNG_FILTER_NAME masks. */
png_set_filter(png_ptr, 0,
PNG_FILTER_NONE | PNG_FILTER_VALUE_NONE |
PNG_FILTER_SUB | PNG_FILTER_VALUE_SUB |
PNG_FILTER_UP | PNG_FILTER_VALUE_UP |
PNG_FILTER_AVE | PNG_FILTER_VALUE_AVE |
PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
PNG_ALL_FILTERS);
If an application
wants to start and stop using particular filters during compression,
it should start out with all of the filters (to ensure that the previous
row of pixels will be stored in case it's needed later), and then add
and remove them after the start of compression.
If you are writing a PNG datastream that is to be embedded in a MNG
datastream, the second parameter can be either 0 or 64.
The png_set_compression_*() functions interface to the zlib compression
library, and should mostly be ignored unless you really know what you are
doing. The only generally useful call is png_set_compression_level()
which changes how much time zlib spends on trying to compress the image
data. See the Compression Library (zlib.h and algorithm.txt, distributed
with zlib) for details on the compression levels.
/* set the zlib compression level */
png_set_compression_level(png_ptr,
Z_BEST_COMPRESSION);
/* set other zlib parameters */
png_set_compression_mem_level(png_ptr, 8);
png_set_compression_strategy(png_ptr,
Z_DEFAULT_STRATEGY);
png_set_compression_window_bits(png_ptr, 15);
png_set_compression_method(png_ptr, 8);
png_set_compression_buffer_size(png_ptr, 8192)
extern PNG_EXPORT(void,png_set_zbuf_size)
.SS Setting the contents of info for output
You now need to fill in the png_info structure with all the data you
wish to write before the actual image. Note that the only thing you
are allowed to write after the image is the text chunks and the time
chunk (as of PNG Specification 1.2, anyway). See png_write_end() and
the latest PNG specification for more information on that. If you
wish to write them before the image, fill them in now, and flag that
data as being valid. If you want to wait until after the data, don't
fill them until png_write_end(). For all the fields in png_info and
their data types, see png.h. For explanations of what the fields
contain, see the PNG specification.
Some of the more important parts of the png_info are:
png_set_IHDR(png_ptr, info_ptr, width, height,
bit_depth, color_type, interlace_type,
compression_type, filter_method)
width - holds the width of the image
in pixels (up to 2^31).
height - holds the height of the image
in pixels (up to 2^31).
bit_depth - holds the bit depth of one of the
image channels.
(valid values are 1, 2, 4, 8, 16
and depend also on the
color_type. See also significant
bits (sBIT) below).
color_type - describes which color/alpha
channels are present.
PNG_COLOR_TYPE_GRAY
(bit depths 1, 2, 4, 8, 16)
PNG_COLOR_TYPE_GRAY_ALPHA
(bit depths 8, 16)
PNG_COLOR_TYPE_PALETTE
(bit depths 1, 2, 4, 8)
PNG_COLOR_TYPE_RGB
(bit_depths 8, 16)
PNG_COLOR_TYPE_RGB_ALPHA
(bit_depths 8, 16)
PNG_COLOR_MASK_PALETTE
PNG_COLOR_MASK_COLOR
PNG_COLOR_MASK_ALPHA
interlace_type - PNG_INTERLACE_NONE or
PNG_INTERLACE_ADAM7
compression_type - (must be
PNG_COMPRESSION_TYPE_DEFAULT)
filter_method - (must be PNG_FILTER_TYPE_DEFAULT
or, if you are writing a PNG to
be embedded in a MNG datastream,
can also be
PNG_INTRAPIXEL_DIFFERENCING)
If you call png_set_IHDR(), the call must appear before any of the
other png_set_*() functions, which might require access to some of
the IHDR settings. The remaining png_set_*() functions can be called
in any order.
png_set_PLTE(png_ptr, info_ptr, palette,
num_palette);
palette - the palette for the file
(array of png_color)
num_palette - number of entries in the palette
png_set_gAMA(png_ptr, info_ptr, gamma);
gamma - the gamma the image was created
at (PNG_INFO_gAMA)
png_set_sRGB(png_ptr, info_ptr, srgb_intent);
srgb_intent - the rendering intent
(PNG_INFO_sRGB) The presence of
the sRGB chunk means that the pixel
data is in the sRGB color space.
This chunk also implies specific
values of gAMA and cHRM. Rendering
intent is the CSS-1 property that
has been defined by the International
Color Consortium
(http://www.color.org).
It can be one of
PNG_sRGB_INTENT_SATURATION,
PNG_sRGB_INTENT_PERCEPTUAL,
PNG_sRGB_INTENT_ABSOLUTE, or
PNG_sRGB_INTENT_RELATIVE.
png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
srgb_intent);
srgb_intent - the rendering intent
(PNG_INFO_sRGB) The presence of the
sRGB chunk means that the pixel
data is in the sRGB color space.
This function also causes gAMA and
cHRM chunks with the specific values
that are consistent with sRGB to be
written.
png_set_iCCP(png_ptr, info_ptr, name, compression_type,
profile, proflen);
name - The profile name.
compression - The compression type; always
PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
You may give NULL to this argument to
ignore it.
profile - International Color Consortium color
profile data. May contain NULs.
proflen - length of profile data in bytes.
png_set_sBIT(png_ptr, info_ptr, sig_bit);
sig_bit - the number of significant bits for
(PNG_INFO_sBIT) each of the gray, red,
green, and blue channels, whichever are
appropriate for the given color type
(png_color_16)
png_set_tRNS(png_ptr, info_ptr, trans, num_trans,
trans_values);
trans - array of transparent entries for
palette (PNG_INFO_tRNS)
trans_values - graylevel or color sample values of
the single transparent color for
non-paletted images (PNG_INFO_tRNS)
num_trans - number of transparent entries
(PNG_INFO_tRNS)
png_set_hIST(png_ptr, info_ptr, hist);
(PNG_INFO_hIST)
hist - histogram of palette (array of
png_uint_16)
png_set_tIME(png_ptr, info_ptr, mod_time);
mod_time - time image was last modified
(PNG_VALID_tIME)
png_set_bKGD(png_ptr, info_ptr, background);
background - background color (PNG_VALID_bKGD)
png_set_text(png_ptr, info_ptr, text_ptr, num_text);
text_ptr - array of png_text holding image
comments
text_ptr[i].compression - type of compression used
on "text" PNG_TEXT_COMPRESSION_NONE
PNG_TEXT_COMPRESSION_zTXt
PNG_ITXT_COMPRESSION_NONE
PNG_ITXT_COMPRESSION_zTXt
text_ptr[i].key - keyword for comment. Must contain
1-79 characters.
text_ptr[i].text - text comments for current
keyword. Can be NULL or empty.
text_ptr[i].text_length - length of text string,
after decompression, 0 for iTXt
text_ptr[i].itxt_length - length of itxt string,
after decompression, 0 for tEXt/zTXt
text_ptr[i].lang - language of comment (NULL or
empty for unknown).
text_ptr[i].translated_keyword - keyword in UTF-8 (NULL
or empty for unknown).
num_text - number of comments
png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
num_spalettes);
palette_ptr - array of png_sPLT_struct structures
to be added to the list of palettes
in the info structure.
num_spalettes - number of palette structures to be
added.
png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
unit_type);
offset_x - positive offset from the left
edge of the screen
offset_y - positive offset from the top
edge of the screen
unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
unit_type);
res_x - pixels/unit physical resolution
in x direction
res_y - pixels/unit physical resolution
in y direction
unit_type - PNG_RESOLUTION_UNKNOWN,
PNG_RESOLUTION_METER
png_set_sCAL(png_ptr, info_ptr, unit, width, height)
unit - physical scale units (an integer)
width - width of a pixel in physical scale units
height - height of a pixel in physical scale units
(width and height are doubles)
png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)
unit - physical scale units (an integer)
width - width of a pixel in physical scale units
height - height of a pixel in physical scale units
(width and height are strings like "2.54")
png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
num_unknowns)
unknowns - array of png_unknown_chunk
structures holding unknown chunks
unknowns[i].name - name of unknown chunk
unknowns[i].data - data of unknown chunk
unknowns[i].size - size of unknown chunk's data
unknowns[i].location - position to write chunk in file
0: do not write chunk
PNG_HAVE_IHDR: before PLTE
PNG_HAVE_PLTE: before IDAT
PNG_AFTER_IDAT: after IDAT
The "location" member is set automatically according to
what part of the output file has already been written.
You can change its value after calling png_set_unknown_chunks()
as demonstrated in pngtest.c. Within each of the "locations",
the chunks are sequenced according to their position in the
structure (that is, the value of "i", which is the order in which
the chunk was either read from the input file or defined with
png_set_unknown_chunks).
A quick word about text and num_text. text is an array of png_text
structures. num_text is the number of valid structures in the array.
Each png_text structure holds a language code, a keyword, a text value,
and a compression type.
The compression types have the same valid numbers as the compression
types of the image data. Currently, the only valid number is zero.
However, you can store text either compressed or uncompressed, unlike
images, which always have to be compressed. So if you don't want the
text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.
Because tEXt and zTXt chunks don't have a language field, if you
specify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt
any language code or translated keyword will not be written out.
Until text gets around 1000 bytes, it is not worth compressing it.
After the text has been written out to the file, the compression type
is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR,
so that it isn't written out again at the end (in case you are calling
png_write_end() with the same struct.
The keywords that are given in the PNG Specification are:
Title Short (one line) title or
caption for image
Author Name of image's creator
Description Description of image (possibly long)
Copyright Copyright notice
Creation Time Time of original image creation
(usually RFC 1123 format, see below)
Software Software used to create the image
Disclaimer Legal disclaimer
Warning Warning of nature of content
Source Device used to create the image
Comment Miscellaneous comment; conversion
from other image format
The keyword-text pairs work like this. Keywords should be short
simple descriptions of what the comment is about. Some typical
keywords are found in the PNG specification, as is some recommendations
on keywords. You can repeat keywords in a file. You can even write
some text before the image and some after. For example, you may want
to put a description of the image before the image, but leave the
disclaimer until after, so viewers working over modem connections
don't have to wait for the disclaimer to go over the modem before
they start seeing the image. Finally, keywords should be full
words, not abbreviations. Keywords and text are in the ISO 8859-1
(Latin-1) character set (a superset of regular ASCII) and can not
contain NUL characters, and should not contain control or other
unprintable characters. To make the comments widely readable, stick
with basic ASCII, and avoid machine specific character set extensions
like the IBM-PC character set. The keyword must be present, but
you can leave off the text string on non-compressed pairs.
Compressed pairs must have a text string, as only the text string
is compressed anyway, so the compression would be meaningless.
PNG supports modification time via the png_time structure. Two
conversion routines are provided, png_convert_from_time_t() for
time_t and png_convert_from_struct_tm() for struct tm. The
time_t routine uses gmtime(). You don't have to use either of
these, but if you wish to fill in the png_time structure directly,
you should provide the time in universal time (GMT) if possible
instead of your local time. Note that the year number is the full
year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and
that months start with 1.
If you want to store the time of the original image creation, you should
use a plain tEXt chunk with the "Creation Time" keyword. This is
necessary because the "creation time" of a PNG image is somewhat vague,
depending on whether you mean the PNG file, the time the image was
created in a non-PNG format, a still photo from which the image was
scanned, or possibly the subject matter itself. In order to facilitate
machine-readable dates, it is recommended that the "Creation Time"
tEXt chunk use RFC 1123 format dates (e.g. "22 May 1997 18:07:10 GMT"),
although this isn't a requirement. Unlike the tIME chunk, the
"Creation Time" tEXt chunk is not expected to be automatically changed
by the software. To facilitate the use of RFC 1123 dates, a function
png_convert_to_rfc1123(png_timep) is provided to convert from PNG
time to an RFC 1123 format string.
.SS Writing unknown chunks
You can use the png_set_unknown_chunks function to queue up chunks
for writing. You give it a chunk name, raw data, and a size; that's
all there is to it. The chunks will be written by the next following
png_write_info_before_PLTE, png_write_info, or png_write_end function.
Any chunks previously read into the info structure's unknown-chunk
list will also be written out in a sequence that satisfies the PNG
specification's ordering rules.
.SS The high-level write interface
At this point there are two ways to proceed; through the high-level
write interface, or through a sequence of low-level write operations.
You can use the high-level interface if your image data is present
in the info structure. All defined output
transformations are permitted, enabled by the following masks.
PNG_TRANSFORM_IDENTITY No transformation
PNG_TRANSFORM_PACKING Pack 1, 2 and 4-bit samples
PNG_TRANSFORM_PACKSWAP Change order of packed
pixels to LSB first
PNG_TRANSFORM_INVERT_MONO Invert monochrome images
PNG_TRANSFORM_SHIFT Normalize pixels to the
sBIT depth
PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA
to BGRA
PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA
to AG
PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
to transparency
PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
PNG_TRANSFORM_STRIP_FILLER Strip out filler bytes.
If you have valid image data in the info structure (you can use
png_set_rows() to put image data in the info structure), simply do this:
png_write_png(png_ptr, info_ptr, png_transforms, NULL)
where png_transforms is an integer containing the bitwise OR of some set of
transformation flags. This call is equivalent to png_write_info(),
followed the set of transformations indicated by the transform mask,
then png_write_image(), and finally png_write_end().
(The final parameter of this call is not yet used. Someday it might point
to transformation parameters required by some future output transform.)
You must use png_transforms and not call any png_set_transform() functions
when you use png_write_png().
.SS The low-level write interface
If you are going the low-level route instead, you are now ready to
write all the file information up to the actual image data. You do
this with a call to png_write_info().
png_write_info(png_ptr, info_ptr);
Note that there is one transformation you may need to do before
png_write_info(). In PNG files, the alpha channel in an image is the
level of opacity. If your data is supplied as a level of
transparency, you can invert the alpha channel before you write it, so
that 0 is fully transparent and 255 (in 8-bit or paletted images) or
65535 (in 16-bit images) is fully opaque, with
png_set_invert_alpha(png_ptr);
This must appear before png_write_info() instead of later with the
other transformations because in the case of paletted images the tRNS
chunk data has to be inverted before the tRNS chunk is written. If
your image is not a paletted image, the tRNS data (which in such cases
represents a single color to be rendered as transparent) won't need to
be changed, and you can safely do this transformation after your
png_write_info() call.
If you need to write a private chunk that you want to appear before
the PLTE chunk when PLTE is present, you can write the PNG info in
two steps, and insert code to write your own chunk between them:
png_write_info_before_PLTE(png_ptr, info_ptr);
png_set_unknown_chunks(png_ptr, info_ptr, ...);
png_write_info(png_ptr, info_ptr);
After you've written the file information, you can set up the library
to handle any special transformations of the image data. The various
ways to transform the data will be described in the order that they
should occur. This is important, as some of these change the color
type and/or bit depth of the data, and some others only work on
certain color types and bit depths. Even though each transformation
checks to see if it has data that it can do something with, you should
make sure to only enable a transformation if it will be valid for the
data. For example, don't swap red and blue on grayscale data.
PNG files store RGB pixels packed into 3 or 6 bytes. This code tells
the library to strip input data that has 4 or 8 bytes per pixel down
to 3 or 6 bytes (or strip 2 or 4-byte grayscale+filler data to 1 or 2
bytes per pixel).
png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);
where the 0 is unused, and the location is either PNG_FILLER_BEFORE or
PNG_FILLER_AFTER, depending upon whether the filler byte in the pixel
is stored XRGB or RGBX.
PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
they can, resulting in, for example, 8 pixels per byte for 1 bit files.
If the data is supplied at 1 pixel per byte, use this code, which will
correctly pack the pixels into a single byte:
png_set_packing(png_ptr);
PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your
data is of another bit depth, you can write an sBIT chunk into the
file so that decoders can recover the original data if desired.
/* Set the true bit depth of the image data */
if (color_type & PNG_COLOR_MASK_COLOR)
{
sig_bit.red = true_bit_depth;
sig_bit.green = true_bit_depth;
sig_bit.blue = true_bit_depth;
}
else
{
sig_bit.gray = true_bit_depth;
}
if (color_type & PNG_COLOR_MASK_ALPHA)
{
sig_bit.alpha = true_bit_depth;
}
png_set_sBIT(png_ptr, info_ptr, &sig_bit);
If the data is stored in the row buffer in a bit depth other than
one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG),
this will scale the values to appear to be the correct bit depth as
is required by PNG.
png_set_shift(png_ptr, &sig_bit);
PNG files store 16 bit pixels in network byte order (big-endian,
ie. most significant bits first). This code would be used if they are
supplied the other way (little-endian, i.e. least significant bits
first, the way PCs store them):
if (bit_depth > 8)
png_set_swap(png_ptr);
If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
need to change the order the pixels are packed into bytes, you can use:
if (bit_depth < 8)
png_set_packswap(png_ptr);
PNG files store 3 color pixels in red, green, blue order. This code
would be used if they are supplied as blue, green, red:
png_set_bgr(png_ptr);
PNG files describe monochrome as black being zero and white being
one. This code would be used if the pixels are supplied with this reversed
(black being one and white being zero):
png_set_invert_mono(png_ptr);
Finally, you can write your own transformation function if none of
the existing ones meets your needs. This is done by setting a callback
with
png_set_write_user_transform_fn(png_ptr,
write_transform_fn);
You must supply the function
void write_transform_fn(png_ptr ptr, row_info_ptr
row_info, png_bytep data)
See pngtest.c for a working example. Your function will be called
before any of the other transformations are processed.
You can also set up a pointer to a user structure for use by your
callback function.
png_set_user_transform_info(png_ptr, user_ptr, 0, 0);
The user_channels and user_depth parameters of this function are ignored
when writing; you can set them to zero as shown.
You can retrieve the pointer via the function png_get_user_transform_ptr().
For example:
voidp write_user_transform_ptr =
png_get_user_transform_ptr(png_ptr);
It is possible to have libpng flush any pending output, either manually,
or automatically after a certain number of lines have been written. To
flush the output stream a single time call:
png_write_flush(png_ptr);
and to have libpng flush the output stream periodically after a certain
number of scanlines have been written, call:
png_set_flush(png_ptr, nrows);
Note that the distance between rows is from the last time png_write_flush()
was called, or the first row of the image if it has never been called.
So if you write 50 lines, and then png_set_flush 25, it will flush the
output on the next scanline, and every 25 lines thereafter, unless
png_write_flush() is called before 25 more lines have been written.
If nrows is too small (less than about 10 lines for a 640 pixel wide
RGB image) the image compression may decrease noticeably (although this
may be acceptable for real-time applications). Infrequent flushing will
only degrade the compression performance by a few percent over images
that do not use flushing.
.SS Writing the image data
That's it for the transformations. Now you can write the image data.
The simplest way to do this is in one function call. If you have the
whole image in memory, you can just call png_write_image() and libpng
will write the image. You will need to pass in an array of pointers to
each row. This function automatically handles interlacing, so you don't
need to call png_set_interlace_handling() or call this function multiple
times, or any of that other stuff necessary with png_write_rows().
png_write_image(png_ptr, row_pointers);
where row_pointers is:
png_byte *row_pointers[height];
You can point to void or char or whatever you use for pixels.
If you don't want to write the whole image at once, you can
use png_write_rows() instead. If the file is not interlaced,
this is simple:
png_write_rows(png_ptr, row_pointers,
number_of_rows);
row_pointers is the same as in the png_write_image() call.
If you are just writing one row at a time, you can do this with
a single row_pointer instead of an array of row_pointers:
png_bytep row_pointer = row;
png_write_row(png_ptr, row_pointer);
When the file is interlaced, things can get a good deal more
complicated. The only currently (as of the PNG Specification
version 1.2, dated July 1999) defined interlacing scheme for PNG files
is the "Adam7" interlace scheme, that breaks down an
image into seven smaller images of varying size. libpng will build
these images for you, or you can do them yourself. If you want to
build them yourself, see the PNG specification for details of which
pixels to write when.
If you don't want libpng to handle the interlacing details, just
use png_set_interlace_handling() and call png_write_rows() the
correct number of times to write all seven sub-images.
If you want libpng to build the sub-images, call this before you start
writing any rows:
number_of_passes =
png_set_interlace_handling(png_ptr);
This will return the number of passes needed. Currently, this
is seven, but may change if another interlace type is added.
Then write the complete image number_of_passes times.
png_write_rows(png_ptr, row_pointers,
number_of_rows);
As some of these rows are not used, and thus return immediately,
you may want to read about interlacing in the PNG specification,
and only update the rows that are actually used.
.SS Finishing a sequential write
After you are finished writing the image, you should finish writing
the file. If you are interested in writing comments or time, you should
pass an appropriately filled png_info pointer. If you are not interested,
you can pass NULL.
png_write_end(png_ptr, info_ptr);
When you are done, you can free all memory used by libpng like this:
png_destroy_write_struct(&png_ptr, &info_ptr);
It is also possible to individually free the info_ptr members that
point to libpng-allocated storage with the following function:
png_free_data(png_ptr, info_ptr, mask, seq)
mask - identifies data to be freed, a mask
containing the bitwise OR of one or
more of
PNG_FREE_PLTE, PNG_FREE_TRNS,
PNG_FREE_HIST, PNG_FREE_ICCP,
PNG_FREE_PCAL, PNG_FREE_ROWS,
PNG_FREE_SCAL, PNG_FREE_SPLT,
PNG_FREE_TEXT, PNG_FREE_UNKN,
or simply PNG_FREE_ALL
seq - sequence number of item to be freed
(-1 for all items)
This function may be safely called when the relevant storage has
already been freed, or has not yet been allocated, or was allocated
by the user and not by libpng, and will in those
cases do nothing. The "seq" parameter is ignored if only one item
of the selected data type, such as PLTE, is allowed. If "seq" is not
-1, and multiple items are allowed for the data type identified in
the mask, such as text or sPLT, only the n'th item in the structure
is freed, where n is "seq".
If you allocated data such as a palette that you passed
in to libpng with png_set_*, you must not free it until just before the call to
png_destroy_write_struct().
The default behavior is only to free data that was allocated internally
by libpng. This can be changed, so that libpng will not free the data,
or so that it will free data that was allocated by the user with png_malloc()
or png_zalloc() and passed in via a png_set_*() function, with
png_data_freer(png_ptr, info_ptr, freer, mask)
mask - which data elements are affected
same choices as in png_free_data()
freer - one of
PNG_DESTROY_WILL_FREE_DATA
PNG_SET_WILL_FREE_DATA
PNG_USER_WILL_FREE_DATA
For example, to transfer responsibility for some data from a read structure
to a write structure, you could use
png_data_freer(read_ptr, read_info_ptr,
PNG_USER_WILL_FREE_DATA,
PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
png_data_freer(write_ptr, write_info_ptr,
PNG_DESTROY_WILL_FREE_DATA,
PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
thereby briefly reassigning responsibility for freeing to the user but
immediately afterwards reassigning it once more to the write_destroy
function. Having done this, it would then be safe to destroy the read
structure and continue to use the PLTE, tRNS, and hIST data in the write
structure.
This function only affects data that has already been allocated.
You can call this function before calling after the png_set_*() functions
to control whether the user or png_destroy_*() is supposed to free the data.
When the user assumes responsibility for libpng-allocated data, the
application must use
png_free() to free it, and when the user transfers responsibility to libpng
for data that the user has allocated, the user must have used png_malloc()
or png_zalloc() to allocate it.
If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
separately, do not transfer responsibility for freeing text_ptr to libpng,
because when libpng fills a png_text structure it combines these members with
the key member, and png_free_data() will free only text_ptr.key. Similarly,
if you transfer responsibility for free'ing text_ptr from libpng to your
application, your application must not separately free those members.
For a more compact example of writing a PNG image, see the file example.c.
.SH V. Modifying/Customizing libpng:
There are two issues here. The first is changing how libpng does
standard things like memory allocation, input/output, and error handling.
The second deals with more complicated things like adding new chunks,
adding new transformations, and generally changing how libpng works.
Both of those are compile-time issues; that is, they are generally
determined at the time the code is written, and there is rarely a need
to provide the user with a means of changing them.
Memory allocation, input/output, and error handling
All of the memory allocation, input/output, and error handling in libpng
goes through callbacks that are user-settable. The default routines are
in pngmem.c, pngrio.c, pngwio.c, and pngerror.c, respectively. To change
these functions, call the appropriate png_set_*_fn() function.
Memory allocation is done through the functions png_malloc()
and png_free(). These currently just call the standard C functions. If
your pointers can't access more then 64K at a time, you will want to set
MAXSEG_64K in zlib.h. Since it is unlikely that the method of handling
memory allocation on a platform will change between applications, these
functions must be modified in the library at compile time. If you prefer
to use a different method of allocating and freeing data, you can use
png_create_read_struct_2() or png_create_write_struct_2() to register
your own functions as described above.
These functions also provide a void pointer that can be retrieved via
mem_ptr=png_get_mem_ptr(png_ptr);
Your replacement memory functions must have prototypes as follows:
png_voidp malloc_fn(png_structp png_ptr,
png_size_t size);
void free_fn(png_structp png_ptr, png_voidp ptr);
Your malloc_fn() must return NULL in case of failure. The png_malloc()
function will normally call png_error() if it receives a NULL from the
system memory allocator or from your replacement malloc_fn().
Your free_fn() will never be called with a NULL ptr, since libpng's
png_free() checks for NULL before calling free_fn().
Input/Output in libpng is done through png_read() and png_write(),
which currently just call fread() and fwrite(). The FILE * is stored in
png_struct and is initialized via png_init_io(). If you wish to change
the method of I/O, the library supplies callbacks that you can set
through the function png_set_read_fn() and png_set_write_fn() at run
time, instead of calling the png_init_io() function. These functions
also provide a void pointer that can be retrieved via the function
png_get_io_ptr(). For example:
png_set_read_fn(png_structp read_ptr,
voidp read_io_ptr, png_rw_ptr read_data_fn)
png_set_write_fn(png_structp write_ptr,
voidp write_io_ptr, png_rw_ptr write_data_fn,
png_flush_ptr output_flush_fn);
voidp read_io_ptr = png_get_io_ptr(read_ptr);
voidp write_io_ptr = png_get_io_ptr(write_ptr);
The replacement I/O functions must have prototypes as follows:
void user_read_data(png_structp png_ptr,
png_bytep data, png_size_t length);
void user_write_data(png_structp png_ptr,
png_bytep data, png_size_t length);
void user_flush_data(png_structp png_ptr);
Supplying NULL for the read, write, or flush functions sets them back
to using the default C stream functions. It is an error to read from
a write stream, and vice versa.
Error handling in libpng is done through png_error() and png_warning().
Errors handled through png_error() are fatal, meaning that png_error()
should never return to its caller. Currently, this is handled via
setjmp() and longjmp() (unless you have compiled libpng with
PNG_SETJMP_NOT_SUPPORTED, in which case it is handled via PNG_ABORT()),
but you could change this to do things like exit() if you should wish.
On non-fatal errors, png_warning() is called
to print a warning message, and then control returns to the calling code.
By default png_error() and png_warning() print a message on stderr via
fprintf() unless the library is compiled with PNG_NO_CONSOLE_IO defined
(because you don't want the messages) or PNG_NO_STDIO defined (because
fprintf() isn't available). If you wish to change the behavior of the error
functions, you will need to set up your own message callbacks. These
functions are normally supplied at the time that the png_struct is created.
It is also possible to redirect errors and warnings to your own replacement
functions after png_create_*_struct() has been called by calling:
png_set_error_fn(png_structp png_ptr,
png_voidp error_ptr, png_error_ptr error_fn,
png_error_ptr warning_fn);
png_voidp error_ptr = png_get_error_ptr(png_ptr);
If NULL is supplied for either error_fn or warning_fn, then the libpng
default function will be used, calling fprintf() and/or longjmp() if a
problem is encountered. The replacement error functions should have
parameters as follows:
void user_error_fn(png_structp png_ptr,
png_const_charp error_msg);
void user_warning_fn(png_structp png_ptr,
png_const_charp warning_msg);
The motivation behind using setjmp() and longjmp() is the C++ throw and
catch exception handling methods. This makes the code much easier to write,
as there is no need to check every return code of every function call.
However, there are some uncertainties about the status of local variables
after a longjmp, so the user may want to be careful about doing anything after
setjmp returns non-zero besides returning itself. Consult your compiler
documentation for more details. For an alternative approach, you may wish
to use the "cexcept" facility (see http://cexcept.sourceforge.net).
.SS Custom chunks
If you need to read or write custom chunks, you may need to get deeper
into the libpng code. The library now has mechanisms for storing
and writing chunks of unknown type; you can even declare callbacks
for custom chunks. However, this may not be good enough if the
library code itself needs to know about interactions between your
chunk and existing `intrinsic' chunks.
If you need to write a new intrinsic chunk, first read the PNG
specification. Acquire a first level of
understanding of how it works. Pay particular attention to the
sections that describe chunk names, and look at how other chunks were
designed, so you can do things similarly. Second, check out the
sections of libpng that read and write chunks. Try to find a chunk
that is similar to yours and use it as a template. More details can
be found in the comments inside the code. It is best to handle unknown
chunks in a generic method, via callback functions, instead of by
modifying libpng functions.
If you wish to write your own transformation for the data, look through
the part of the code that does the transformations, and check out some of
the simpler ones to get an idea of how they work. Try to find a similar
transformation to the one you want to add and copy off of it. More details
can be found in the comments inside the code itself.
.SS Configuring for 16 bit platforms
You will want to look into zconf.h to tell zlib (and thus libpng) that
it cannot allocate more then 64K at a time. Even if you can, the memory
won't be accessible. So limit zlib and libpng to 64K by defining MAXSEG_64K.
.SS Configuring for DOS
For DOS users who only have access to the lower 640K, you will
have to limit zlib's memory usage via a png_set_compression_mem_level()
call. See zlib.h or zconf.h in the zlib library for more information.
.SS Configuring for Medium Model
Libpng's support for medium model has been tested on most of the popular
compilers. Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets
defined, and FAR gets defined to far in pngconf.h, and you should be
all set. Everything in the library (except for zlib's structure) is
expecting far data. You must use the typedefs with the p or pp on
the end for pointers (or at least look at them and be careful). Make
note that the rows of data are defined as png_bytepp, which is an
unsigned char far * far *.
.SS Configuring for gui/windowing platforms:
You will need to write new error and warning functions that use the GUI
interface, as described previously, and set them to be the error and
warning functions at the time that png_create_*_struct() is called,
in order to have them available during the structure initialization.
They can be changed later via png_set_error_fn(). On some compilers,
you may also have to change the memory allocators (png_malloc, etc.).
.SS Configuring for compiler xxx:
All includes for libpng are in pngconf.h. If you need to add/change/delete
an include, this is the place to do it. The includes that are not
needed outside libpng are protected by the PNG_INTERNAL definition,
which is only defined for those routines inside libpng itself. The
files in libpng proper only include png.h, which includes pngconf.h.
.SS Configuring zlib:
There are special functions to configure the compression. Perhaps the
most useful one changes the compression level, which currently uses
input compression values in the range 0 - 9. The library normally
uses the default compression level (Z_DEFAULT_COMPRESSION = 6). Tests
have shown that for a large majority of images, compression values in
the range 3-6 compress nearly as well as higher levels, and do so much
faster. For online applications it may be desirable to have maximum speed
(Z_BEST_SPEED = 1). With versions of zlib after v0.99, you can also
specify no compression (Z_NO_COMPRESSION = 0), but this would create
files larger than just storing the raw bitmap. You can specify the
compression level by calling:
png_set_compression_level(png_ptr, level);
Another useful one is to reduce the memory level used by the library.
The memory level defaults to 8, but it can be lowered if you are
short on memory (running DOS, for example, where you only have 640K).
Note that the memory level does have an effect on compression; among
other things, lower levels will result in sections of incompressible
data being emitted in smaller stored blocks, with a correspondingly
larger relative overhead of up to 15% in the worst case.
png_set_compression_mem_level(png_ptr, level);
The other functions are for configuring zlib. They are not recommended
for normal use and may result in writing an invalid PNG file. See
zlib.h for more information on what these mean.
png_set_compression_strategy(png_ptr,
strategy);
png_set_compression_window_bits(png_ptr,
window_bits);
png_set_compression_method(png_ptr, method);
png_set_compression_buffer_size(png_ptr, size);
.SS Controlling row filtering
If you want to control whether libpng uses filtering or not, which
filters are used, and how it goes about picking row filters, you
can call one of these functions. The selection and configuration
of row filters can have a significant impact on the size and
encoding speed and a somewhat lesser impact on the decoding speed
of an image. Filtering is enabled by default for RGB and grayscale
images (with and without alpha), but not for paletted images nor
for any images with bit depths less than 8 bits/pixel.
The 'method' parameter sets the main filtering method, which is
currently only '0' in the PNG 1.2 specification. The 'filters'
parameter sets which filter(s), if any, should be used for each
scanline. Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS
to turn filtering on and off, respectively.
Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB,
PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise
ORed together with '|' to specify one or more filters to use.
These filters are described in more detail in the PNG specification.
If you intend to change the filter type during the course of writing
the image, you should start with flags set for all of the filters
you intend to use so that libpng can initialize its internal
structures appropriately for all of the filter types. (Note that this
means the first row must always be adaptively filtered, because libpng
currently does not allocate the filter buffers until png_write_row()
is called for the first time.)
filters = PNG_FILTER_NONE | PNG_FILTER_SUB
PNG_FILTER_UP | PNG_FILTER_AVE |
PNG_FILTER_PAETH | PNG_ALL_FILTERS;
png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
filters);
The second parameter can also be
PNG_INTRAPIXEL_DIFFERENCING if you are
writing a PNG to be embedded in a MNG
datastream. This parameter must be the
same as the value of filter_method used
in png_set_IHDR().
It is also possible to influence how libpng chooses from among the
available filters. This is done in one or both of two ways - by
telling it how important it is to keep the same filter for successive
rows, and by telling it the relative computational costs of the filters.
double weights[3] = {1.5, 1.3, 1.1},
costs[PNG_FILTER_VALUE_LAST] =
{1.0, 1.3, 1.3, 1.5, 1.7};
png_set_filter_heuristics(png_ptr,
PNG_FILTER_HEURISTIC_WEIGHTED, 3,
weights, costs);
The weights are multiplying factors that indicate to libpng that the
row filter should be the same for successive rows unless another row filter
is that many times better than the previous filter. In the above example,
if the previous 3 filters were SUB, SUB, NONE, the SUB filter could have a
"sum of absolute differences" 1.5 x 1.3 times higher than other filters
and still be chosen, while the NONE filter could have a sum 1.1 times
higher than other filters and still be chosen. Unspecified weights are
taken to be 1.0, and the specified weights should probably be declining
like those above in order to emphasize recent filters over older filters.
The filter costs specify for each filter type a relative decoding cost
to be considered when selecting row filters. This means that filters
with higher costs are less likely to be chosen over filters with lower
costs, unless their "sum of absolute differences" is that much smaller.
The costs do not necessarily reflect the exact computational speeds of
the various filters, since this would unduly influence the final image
size.
Note that the numbers above were invented purely for this example and
are given only to help explain the function usage. Little testing has
been done to find optimum values for either the costs or the weights.
.SS Removing unwanted object code
There are a bunch of #define's in pngconf.h that control what parts of
libpng are compiled. All the defines end in _SUPPORTED. If you are
never going to use a capability, you can change the #define to #undef
before recompiling libpng and save yourself code and data space, or
you can turn off individual capabilities with defines that begin with
PNG_NO_.
You can also turn all of the transforms and ancillary chunk capabilities
off en masse with compiler directives that define
PNG_NO_READ[or WRITE]_TRANSFORMS, or PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS,
or all four,
along with directives to turn on any of the capabilities that you do
want. The PNG_NO_READ[or WRITE]_TRANSFORMS directives disable
the extra transformations but still leave the library fully capable of reading
and writing PNG files with all known public chunks
Use of the PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS directive
produces a library that is incapable of reading or writing ancillary chunks.
If you are not using the progressive reading capability, you can
turn that off with PNG_NO_PROGRESSIVE_READ (don't confuse
this with the INTERLACING capability, which you'll still have).
All the reading and writing specific code are in separate files, so the
linker should only grab the files it needs. However, if you want to
make sure, or if you are building a stand alone library, all the
reading files start with pngr and all the writing files start with
pngw. The files that don't match either (like png.c, pngtrans.c, etc.)
are used for both reading and writing, and always need to be included.
The progressive reader is in pngpread.c
If you are creating or distributing a dynamically linked library (a .so
or DLL file), you should not remove or disable any parts of the library,
as this will cause applications linked with different versions of the
library to fail if they call functions not available in your library.
The size of the library itself should not be an issue, because only
those sections that are actually used will be loaded into memory.
.SS Requesting debug printout
The macro definition PNG_DEBUG can be used to request debugging
printout. Set it to an integer value in the range 0 to 3. Higher
numbers result in increasing amounts of debugging information. The
information is printed to the "stderr" file, unless another file
name is specified in the PNG_DEBUG_FILE macro definition.
When PNG_DEBUG > 0, the following functions (macros) become available:
png_debug(level, message)
png_debug1(level, message, p1)
png_debug2(level, message, p1, p2)
in which "level" is compared to PNG_DEBUG to decide whether to print
the message, "message" is the formatted string to be printed,
and p1 and p2 are parameters that are to be embedded in the string
according to printf-style formatting directives. For example,
png_debug1(2, "foo=%dn", foo);
is expanded to
if(PNG_DEBUG > 2)
fprintf(PNG_DEBUG_FILE, "foo=%dn", foo);
When PNG_DEBUG is defined but is zero, the macros aren't defined, but you
can still use PNG_DEBUG to control your own debugging:
#ifdef PNG_DEBUG
fprintf(stderr, ...
#endif
When PNG_DEBUG = 1, the macros are defined, but only png_debug statements
having level = 0 will be printed. There aren't any such statements in
this version of libpng, but if you insert some they will be printed.
.SH VII. MNG support
The MNG specification (available at http://www.libpng.org/pub/mng) allows
certain extensions to PNG for PNG images that are embedded in MNG datastreams.
Libpng can support some of these extensions. To enable them, use the
png_permit_mng_features() function:
feature_set = png_permit_mng_features(png_ptr, mask)
mask is a png_uint_32 containing the bitwise OR of the
features you want to enable. These include
PNG_FLAG_MNG_EMPTY_PLTE
PNG_FLAG_MNG_FILTER_64
PNG_ALL_MNG_FEATURES
feature_set is a png_uint_32 that is the bitwise AND of
your mask with the set of MNG features that is
supported by the version of libpng that you are using.
It is an error to use this function when reading or writing a standalone
PNG file with the PNG 8-byte signature. The PNG datastream must be wrapped
in a MNG datastream. As a minimum, it must have the MNG 8-byte signature
and the MHDR and MEND chunks. Libpng does not provide support for these
or any other MNG chunks; your application must provide its own support for
them. You may wish to consider using libmng (available at
http://www.libmng.com) instead.
.SH VIII. Changes to Libpng from version 0.88
It should be noted that versions of libpng later than 0.96 are not
distributed by the original libpng author, Guy Schalnat, nor by
Andreas Dilger, who had taken over from Guy during 1996 and 1997, and
distributed versions 0.89 through 0.96, but rather by another member
of the original PNG Group, Glenn Randers-Pehrson. Guy and Andreas are
still alive and well, but they have moved on to other things.
The old libpng functions png_read_init(), png_write_init(),
png_info_init(), png_read_destroy(), and png_write_destroy() have been
moved to PNG_INTERNAL in version 0.95 to discourage their use. These
functions will be removed from libpng version 2.0.0.
The preferred method of creating and initializing the libpng structures is
via the png_create_read_struct(), png_create_write_struct(), and
png_create_info_struct() because they isolate the size of the structures
from the application, allow version error checking, and also allow the
use of custom error handling routines during the initialization, which
the old functions do not. The functions png_read_destroy() and
png_write_destroy() do not actually free the memory that libpng
allocated for these structs, but just reset the data structures, so they
can be used instead of png_destroy_read_struct() and
png_destroy_write_struct() if you feel there is too much system overhead
allocating and freeing the png_struct for each image read.
Setting the error callbacks via png_set_message_fn() before
png_read_init() as was suggested in libpng-0.88 is no longer supported
because this caused applications that do not use custom error functions
to fail if the png_ptr was not initialized to zero. It is still possible
to set the error callbacks AFTER png_read_init(), or to change them with
png_set_error_fn(), which is essentially the same function, but with a new
name to force compilation errors with applications that try to use the old
method.
Starting with version 1.0.7, you can find out which version of the library
you are using at run-time:
png_uint_32 libpng_vn = png_access_version_number();
The number libpng_vn is constructed from the major version, minor
version with leading zero, and release number with leading zero,
(e.g., libpng_vn for version 1.0.7 is 10007).
You can also check which version of png.h you used when compiling your
application:
png_uint_32 application_vn = PNG_LIBPNG_VER;
.SH IX. Y2K Compliance in libpng
December 18, 2008
Since the PNG Development group is an ad-hoc body, we can't make
an official declaration.
This is your unofficial assurance that libpng from version 0.71 and
upward through 1.2.34 are Y2K compliant. It is my belief that earlier
versions were also Y2K compliant.
Libpng only has three year fields. One is a 2-byte unsigned integer that
will hold years up to 65535. The other two hold the date in text
format, and will hold years up to 9999.
The integer is
"png_uint_16 year" in png_time_struct.
The strings are
"png_charp time_buffer" in png_struct and
"near_time_buffer", which is a local character string in png.c.
There are seven time-related functions:
png_convert_to_rfc_1123() in png.c
(formerly png_convert_to_rfc_1152() in error)
png_convert_from_struct_tm() in pngwrite.c, called
in pngwrite.c
png_convert_from_time_t() in pngwrite.c
png_get_tIME() in pngget.c
png_handle_tIME() in pngrutil.c, called in pngread.c
png_set_tIME() in pngset.c
png_write_tIME() in pngwutil.c, called in pngwrite.c
All appear to handle dates properly in a Y2K environment. The
png_convert_from_time_t() function calls gmtime() to convert from system
clock time, which returns (year - 1900), which we properly convert to
the full 4-digit year. There is a possibility that applications using
libpng are not passing 4-digit years into the png_convert_to_rfc_1123()
function, or that they are incorrectly passing only a 2-digit year
instead of "year - 1900" into the png_convert_from_struct_tm() function,
but this is not under our control. The libpng documentation has always
stated that it works with 4-digit years, and the APIs have been
documented as such.
The tIME chunk itself is also Y2K compliant. It uses a 2-byte unsigned
integer to hold the year, and can hold years as large as 65535.
zlib, upon which libpng depends, is also Y2K compliant. It contains
no date-related code.
Glenn Randers-Pehrson
libpng maintainer
PNG Development Group
.SH NOTE
Note about libpng version numbers:
Due to various miscommunications, unforeseen code incompatibilities
and occasional factors outside the authors' control, version numbering
on the library has not always been consistent and straightforward.
The following table summarizes matters since version 0.89c, which was
the first widely used release:
source png.h png.h shared-lib
version string int version
------- ------ ----- ----------
0.89c ("beta 3") 0.89 89 1.0.89
0.90 ("beta 4") 0.90 90 0.90
0.95 ("beta 5") 0.95 95 0.95
0.96 ("beta 6") 0.96 96 0.96
0.97b ("beta 7") 1.00.97 97 1.0.1
0.97c 0.97 97 2.0.97
0.98 0.98 98 2.0.98
0.99 0.99 98 2.0.99
0.99a-m 0.99 99 2.0.99
1.00 1.00 100 2.1.0
1.0.0 1.0.0 100 2.1.0
1.0.0 (from here on, the 100 2.1.0
1.0.1 png.h string is 10001 2.1.0
1.0.1a-e identical to the 10002 from here on, the
1.0.2 source version) 10002 shared library is 2.V
1.0.2a-b 10003 where V is the source
1.0.1 10001 code version except as
1.0.1a-e 10002 2.1.0.1a-e noted.
1.0.2 10002 2.1.0.2
1.0.2a-b 10003 2.1.0.2a-b
1.0.3 10003 2.1.0.3
1.0.3a-d 10004 2.1.0.3a-d
1.0.4 10004 2.1.0.4
1.0.4a-f 10005 2.1.0.4a-f
1.0.5 (+ 2 patches) 10005 2.1.0.5
1.0.5a-d 10006 2.1.0.5a-d
1.0.5e-r 10100 2.1.0.5e-r
1.0.5s-v 10006 2.1.0.5s-v
1.0.6 (+ 3 patches) 10006 2.1.0.6
1.0.6d-g 10007 2.1.0.6d-g
1.0.6h 10007 10.6h
1.0.6i 10007 10.6i
1.0.6j 10007 2.1.0.6j
1.0.7beta11-14 DLLNUM 10007 2.1.0.7beta11-14
1.0.7beta15-18 1 10007 2.1.0.7beta15-18
1.0.7rc1-2 1 10007 2.1.0.7rc1-2
1.0.7 1 10007 2.1.0.7
1.0.8beta1-4 1 10008 2.1.0.8beta1-4
1.0.8rc1 1 10008 2.1.0.8rc1
1.0.8 1 10008 2.1.0.8
1.0.9beta1-6 1 10009 2.1.0.9beta1-6
1.0.9rc1 1 10009 2.1.0.9rc1
1.0.9beta7-10 1 10009 2.1.0.9beta7-10
1.0.9rc2 1 10009 2.1.0.9rc2
1.0.9 1 10009 2.1.0.9
1.0.10beta1 1 10010 2.1.0.10beta1
1.0.10rc1 1 10010 2.1.0.10rc1
1.0.10 1 10010 2.1.0.10
1.0.11beta1-3 1 10011 2.1.0.11beta1-3
1.0.11rc1 1 10011 2.1.0.11rc1
1.0.11 1 10011 2.1.0.11
1.0.12beta1-2 2 10012 2.1.0.12beta1-2
1.0.12rc1 2 10012 2.1.0.12rc1
1.0.12 2 10012 2.1.0.12
1.1.0a-f - 10100 2.1.1.0a-f abandoned
1.2.0beta1-2 2 10200 2.1.2.0beta1-2
1.2.0beta3-5 3 10200 3.1.2.0beta3-5
1.2.0rc1 3 10200 3.1.2.0rc1
1.2.0 3 10200 3.1.2.0
1.2.1beta-4 3 10201 3.1.2.1beta1-4
1.2.1rc1-2 3 10201 3.1.2.1rc1-2
1.2.1 3 10201 3.1.2.1
1.2.2beta1-6 12 10202 12.so.0.1.2.2beta1-6
1.0.13beta1 10 10013 10.so.0.1.0.13beta1
1.0.13rc1 10 10013 10.so.0.1.0.13rc1
1.2.2rc1 12 10202 12.so.0.1.2.2rc1
1.0.13 10 10013 10.so.0.1.0.13
1.2.2 12 10202 12.so.0.1.2.2
1.2.3rc1-6 12 10203 12.so.0.1.2.3rc1-6
1.2.3 12 10203 12.so.0.1.2.3
1.2.4beta1-3 13 10204 12.so.0.1.2.4beta1-3
1.2.4rc1 13 10204 12.so.0.1.2.4rc1
1.0.14 10 10014 10.so.0.1.0.14
1.2.4 13 10204 12.so.0.1.2.4
1.2.5beta1-2 13 10205 12.so.0.1.2.5beta1-2
1.0.15rc1 10 10015 10.so.0.1.0.15rc1
1.0.15 10 10015 10.so.0.1.0.15
1.2.5 13 10205 12.so.0.1.2.5
1.2.6beta1-4 13 10206 12.so.0.1.2.6beta1-4
1.2.6rc1-5 13 10206 12.so.0.1.2.6rc1-5
1.0.16 10 10016 10.so.0.1.0.16
1.2.6 13 10206 12.so.0.1.2.6
1.2.7beta1-2 13 10207 12.so.0.1.2.7beta1-2
1.0.17rc1 10 10017 10.so.0.1.0.17rc1
1.2.7rc1 13 10207 12.so.0.1.2.7rc1
1.0.17 10 10017 10.so.0.1.0.17
1.2.7 13 10207 12.so.0.1.2.7
1.2.8beta1-5 13 10208 12.so.0.1.2.8beta1-5
1.0.18rc1-5 10 10018 10.so.0.1.0.18rc1-5
1.2.8rc1-5 13 10208 12.so.0.1.2.8rc1-5
1.0.18 10 10018 10.so.0.1.0.18
1.2.8 13 10208 12.so.0.1.2.8
1.2.9beta1-3 13 10209 12.so.0.1.2.9beta1-3
1.2.9beta4-11 13 10209 12.so.0.9[.0]
1.2.9rc1 13 10209 12.so.0.9[.0]
1.2.9 13 10209 12.so.0.9[.0]
1.2.10beta1-8 13 10210 12.so.0.10[.0]
1.2.10rc1-3 13 10210 12.so.0.10[.0]
1.2.10 13 10210 12.so.0.10[.0]
1.2.11beta1-4 13 10211 12.so.0.11[.0]
1.0.19rc1-5 10 10019 10.so.0.19[.0]
1.2.11rc1-5 13 10211 12.so.0.11[.0]
1.0.19 10 10019 10.so.0.19[.0]
1.2.11 13 10211 12.so.0.11[.0]
1.0.20 10 10020 10.so.0.20[.0]
1.2.12 13 10212 12.so.0.12[.0]
1.2.13beta1 13 10213 12.so.0.13[.0]
1.0.21 10 10021 10.so.0.21[.0]
1.2.13 13 10213 12.so.0.13[.0]
1.2.14beta1-2 13 10214 12.so.0.14[.0]
1.0.22rc1 10 10022 10.so.0.22[.0]
1.2.14rc1 13 10214 12.so.0.14[.0]
1.2.15beta1-6 13 10215 12.so.0.15[.0]
1.0.23rc1-5 10 10023 10.so.0.23[.0]
1.2.15rc1-5 13 10215 12.so.0.15[.0]
1.0.23 10 10023 10.so.0.23[.0]
1.2.15 13 10215 12.so.0.15[.0]
1.2.16beta1-2 13 10216 12.so.0.16[.0]
1.2.16rc1 13 10216 12.so.0.16[.0]
1.0.24 10 10024 10.so.0.24[.0]
1.2.16 13 10216 12.so.0.16[.0]
1.2.17beta1-2 13 10217 12.so.0.17[.0]
1.0.25rc1 10 10025 10.so.0.25[.0]
1.2.17rc1-3 13 10217 12.so.0.17[.0]
1.0.25 10 10025 10.so.0.25[.0]
1.2.17 13 10217 12.so.0.17[.0]
1.0.26 10 10026 10.so.0.26[.0]
1.2.18 13 10218 12.so.0.18[.0]
1.2.19beta1-31 13 10219 12.so.0.19[.0]
1.0.27rc1-6 10 10027 10.so.0.27[.0]
1.2.19rc1-6 13 10219 12.so.0.19[.0]
1.0.27 10 10027 10.so.0.27[.0]
1.2.19 13 10219 12.so.0.19[.0]
1.2.20beta01-04 13 10220 12.so.0.20[.0]
1.0.28rc1-6 10 10028 10.so.0.28[.0]
1.2.20rc1-6 13 10220 12.so.0.20[.0]
1.0.28 10 10028 10.so.0.28[.0]
1.2.20 13 10220 12.so.0.20[.0]
1.2.21beta1-2 13 10221 12.so.0.21[.0]
1.2.21rc1-3 13 10221 12.so.0.21[.0]
1.0.29 10 10029 10.so.0.29[.0]
1.2.21 13 10221 12.so.0.21[.0]
1.2.22beta1-4 13 10222 12.so.0.22[.0]
1.0.30rc1 13 10030 10.so.0.30[.0]
1.2.22rc1 13 10222 12.so.0.22[.0]
1.0.30 10 10030 10.so.0.30[.0]
1.2.22 13 10222 12.so.0.22[.0]
1.2.23beta01-05 13 10223 12.so.0.23[.0]
1.2.23rc01 13 10223 12.so.0.23[.0]
1.2.23 13 10223 12.so.0.23[.0]
1.2.24beta01-02 13 10224 12.so.0.24[.0]
1.2.24rc01 13 10224 12.so.0.24[.0]
1.2.24 13 10224 12.so.0.24[.0]
1.2.25beta01-06 13 10225 12.so.0.25[.0]
1.2.25rc01-02 13 10225 12.so.0.25[.0]
1.0.31 10 10031 10.so.0.31[.0]
1.2.25 13 10225 12.so.0.25[.0]
1.2.26beta01-06 13 10226 12.so.0.26[.0]
1.2.26rc01 13 10226 12.so.0.26[.0]
1.2.26 13 10226 12.so.0.26[.0]
1.0.32 10 10032 10.so.0.32[.0]
1.2.27beta01-06 13 10227 12.so.0.27[.0]
1.2.27rc01 13 10227 12.so.0.27[.0]
1.0.33 10 10033 10.so.0.33[.0]
1.2.27 13 10227 12.so.0.27[.0]
1.0.34 10 10034 10.so.0.34[.0]
1.2.28 13 10228 12.so.0.28[.0]
1.2.29beta01-03 13 10229 12.so.0.29[.0]
1.2.29rc01 13 10229 12.so.0.29[.0]
1.0.35 10 10035 10.so.0.35[.0]
1.2.29 13 10229 12.so.0.29[.0]
1.0.37 10 10037 10.so.0.37[.0]
1.2.30beta01-04 13 10230 12.so.0.30[.0]
1.0.38rc01-08 10 10038 10.so.0.38[.0]
1.2.30rc01-08 13 10230 12.so.0.30[.0]
1.0.38 10 10038 10.so.0.38[.0]
1.2.30 13 10230 12.so.0.30[.0]
1.0.39rc01-03 10 10039 10.so.0.39[.0]
1.2.31rc01-03 13 10231 12.so.0.31[.0]
1.0.39 10 10039 10.so.0.39[.0]
1.2.31 13 10231 12.so.0.31[.0]
1.2.32beta01-02 13 10232 12.so.0.32[.0]
1.0.40rc01 10 10040 10.so.0.40[.0]
1.2.32rc01 13 10232 12.so.0.32[.0]
1.0.40 10 10040 10.so.0.40[.0]
1.2.32 13 10232 12.so.0.32[.0]
1.2.33beta01-02 13 10233 12.so.0.33[.0]
1.2.33rc01-02 13 10233 12.so.0.33[.0]
1.0.41rc01 10 10041 10.so.0.41[.0]
1.2.33 13 10233 12.so.0.33[.0]
1.0.41 10 10041 10.so.0.41[.0]
1.2.34beta01-07 13 10234 12.so.0.34[.0]
1.0.42rc01 10 10042 10.so.0.42[.0]
1.2.34rc01 13 10234 12.so.0.34[.0]
1.0.42 10 10042 10.so.0.42[.0]
1.2.34 13 10234 12.so.0.34[.0]
Henceforth the source version will match the shared-library minor
and patch numbers; the shared-library major version number will be
used for changes in backward compatibility, as it is intended. The
PNG_PNGLIB_VER macro, which is not used within libpng but is available
for applications, is an unsigned integer of the form xyyzz corresponding
to the source version x.y.z (leading zeros in y and z). Beta versions
were given the previous public release number plus a letter, until
version 1.0.6j; from then on they were given the upcoming public
release number plus "betaNN" or "rcN".
.SH "SEE ALSO"
.IR libpngpf(3) ", " png(5)
.LP
.IR libpng :
.IP
http://libpng.sourceforge.net (follow the [DOWNLOAD] link)
http://www.libpng.org/pub/png
.LP
.IR zlib :
.IP
(generally) at the same location as
.I libpng
or at
.br
ftp://ftp.info-zip.org/pub/infozip/zlib
.LP
.IR PNG specification: RFC 2083
.IP
(generally) at the same location as
.I libpng
or at
.br
ftp://ftp.rfc-editor.org:/in-notes/rfc2083.txt
.br
or (as a W3C Recommendation) at
.br
http://www.w3.org/TR/REC-png.html
.LP
In the case of any inconsistency between the PNG specification
and this library, the specification takes precedence.
.SH AUTHORS
This man page: Glenn Randers-Pehrson
<glennrp at users.sourceforge.net>
The contributing authors would like to thank all those who helped
with testing, bug fixes, and patience. This wouldn't have been
possible without all of you.
Thanks to Frank J. T. Wojcik for helping with the documentation.
Libpng version 1.2.34 - December 18, 2008:
Initially created in 1995 by Guy Eric Schalnat, then of Group 42, Inc.
Currently maintained by Glenn Randers-Pehrson (glennrp at users.sourceforge.net).
Supported by the PNG development group
.br
png-mng-implement at lists.sf.net
(subscription required; visit
png-mng-implement at lists.sourceforge.net (subscription required; visit
https://lists.sourceforge.net/lists/listinfo/png-mng-implement
to subscribe).
.SH COPYRIGHT NOTICE, DISCLAIMER, and LICENSE:
(This copy of the libpng notices is provided for your convenience. In case of
any discrepancy between this copy and the notices in the file png.h that is
included in the libpng distribution, the latter shall prevail.)
If you modify libpng you may insert additional notices immediately following
this sentence.
libpng versions 1.2.6, August 15, 2004, through 1.2.34, December 18, 2008, are
distributed according to the same disclaimer and license as libpng-1.2.5
with the following individual added to the list of Contributing Authors
Cosmin Truta
libpng versions 1.0.7, July 1, 2000, through 1.2.5 - October 3, 2002, are
distributed according to the same disclaimer and license as libpng-1.0.6
with the following individuals added to the list of Contributing Authors
Simon-Pierre Cadieux
Eric S. Raymond
Gilles Vollant
and with the following additions to the disclaimer:
There is no warranty against interference with your
enjoyment of the library or against infringement.
There is no warranty that our efforts or the library
will fulfill any of your particular purposes or needs.
This library is provided with all faults, and the entire
risk of satisfactory quality, performance, accuracy, and
effort is with the user.
libpng versions 0.97, January 1998, through 1.0.6, March 20, 2000, are
Distributed according to the same disclaimer and license as libpng-0.96,
with the following individuals added to the list of Contributing Authors:
Tom Lane
Glenn Randers-Pehrson
Willem van Schaik
libpng versions 0.89, June 1996, through 0.96, May 1997, are
Distributed according to the same disclaimer and license as libpng-0.88,
with the following individuals added to the list of Contributing Authors:
John Bowler
Kevin Bracey
Sam Bushell
Magnus Holmgren
Greg Roelofs
Tom Tanner
libpng versions 0.5, May 1995, through 0.88, January 1996, are
For the purposes of this copyright and license, "Contributing Authors"
is defined as the following set of individuals:
Andreas Dilger
Dave Martindale
Guy Eric Schalnat
Paul Schmidt
Tim Wegner
The PNG Reference Library is supplied "AS IS". The Contributing Authors
and Group 42, Inc. disclaim all warranties, expressed or implied,
including, without limitation, the warranties of merchantability and of
fitness for any purpose. The Contributing Authors and Group 42, Inc.
assume no liability for direct, indirect, incidental, special, exemplary,
or consequential damages, which may result from the use of the PNG
Reference Library, even if advised of the possibility of such damage.
Permission is hereby granted to use, copy, modify, and distribute this
source code, or portions hereof, for any purpose, without fee, subject
to the following restrictions:
1. The origin of this source code must not be misrepresented.
2. Altered versions must be plainly marked as such and
must not be misrepresented as being the original source.
3. This Copyright notice may not be removed or altered from
any source or altered source distribution.
The Contributing Authors and Group 42, Inc. specifically permit, without
fee, and encourage the use of this source code as a component to
supporting the PNG file format in commercial products. If you use this
source code in a product, acknowledgment is not required but would be
appreciated.
A "png_get_copyright" function is available, for convenient use in "about"
boxes and the like:
printf("%s",png_get_copyright(NULL));
Also, the PNG logo (in PNG format, of course) is supplied in the
files "pngbar.png" and "pngbar.jpg (88x31) and "pngnow.png" (98x31).
Libpng is OSI Certified Open Source Software. OSI Certified Open Source is a
certification mark of the Open Source Initiative.
Glenn Randers-Pehrson
glennrp at users.sourceforge.net
December 18, 2008
." end of man page