JFDCTFST.c
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上传日期:2022-05-11
资源大小:5032k
文件大小:8k
源码类别:
网络截获/分析
开发平台:
Visual C++
- ////////////////////////////////////////////////////////////////////////
- //
- // Note : this file is included as part of the Smaller Animals Software
- // JpegFile package. Though this file has not been modified from it's
- // original IJG 6a form, it is not the responsibility on the Independent
- // JPEG Group to answer questions regarding this code.
- //
- // Any questions you have about this code should be addressed to :
- //
- // CHRISDL@PAGESZ.NET - the distributor of this package.
- //
- // Remember, by including this code in the JpegFile package, Smaller
- // Animals Software assumes all responsibilities for answering questions
- // about it. If we (SA Software) can't answer your questions ourselves, we
- // will direct you to people who can.
- //
- // Thanks, CDL.
- //
- ////////////////////////////////////////////////////////////////////////
- /*
- * jfdctfst.c
- *
- * Copyright (C) 1994-1996, Thomas G. Lane.
- * This file is part of the Independent JPEG Group's software.
- * For conditions of distribution and use, see the accompanying README file.
- *
- * This file contains a fast, not so accurate integer implementation of the
- * forward DCT (Discrete Cosine Transform).
- *
- * A 2-D DCT can be done by 1-D DCT on each row followed by 1-D DCT
- * on each column. Direct algorithms are also available, but they are
- * much more complex and seem not to be any faster when reduced to code.
- *
- * This implementation is based on Arai, Agui, and Nakajima's algorithm for
- * scaled DCT. Their original paper (Trans. IEICE E-71(11):1095) is in
- * Japanese, but the algorithm is described in the Pennebaker & Mitchell
- * JPEG textbook (see REFERENCES section in file README). The following code
- * is based directly on figure 4-8 in P&M.
- * While an 8-point DCT cannot be done in less than 11 multiplies, it is
- * possible to arrange the computation so that many of the multiplies are
- * simple scalings of the final outputs. These multiplies can then be
- * folded into the multiplications or divisions by the JPEG quantization
- * table entries. The AA&N method leaves only 5 multiplies and 29 adds
- * to be done in the DCT itself.
- * The primary disadvantage of this method is that with fixed-point math,
- * accuracy is lost due to imprecise representation of the scaled
- * quantization values. The smaller the quantization table entry, the less
- * precise the scaled value, so this implementation does worse with high-
- * quality-setting files than with low-quality ones.
- */
- #define JPEG_INTERNALS
- #include "jinclude.h"
- #include "jpeglib.h"
- #include "jdct.h" /* Private declarations for DCT subsystem */
- #ifdef DCT_IFAST_SUPPORTED
- /*
- * This module is specialized to the case DCTSIZE = 8.
- */
- #if DCTSIZE != 8
- Sorry, this code only copes with 8x8 DCTs. /* deliberate syntax err */
- #endif
- /* Scaling decisions are generally the same as in the LL&M algorithm;
- * see jfdctint.c for more details. However, we choose to descale
- * (right shift) multiplication products as soon as they are formed,
- * rather than carrying additional fractional bits into subsequent additions.
- * This compromises accuracy slightly, but it lets us save a few shifts.
- * More importantly, 16-bit arithmetic is then adequate (for 8-bit samples)
- * everywhere except in the multiplications proper; this saves a good deal
- * of work on 16-bit-int machines.
- *
- * Again to save a few shifts, the intermediate results between pass 1 and
- * pass 2 are not upscaled, but are represented only to integral precision.
- *
- * A final compromise is to represent the multiplicative constants to only
- * 8 fractional bits, rather than 13. This saves some shifting work on some
- * machines, and may also reduce the cost of multiplication (since there
- * are fewer one-bits in the constants).
- */
- #define CONST_BITS 8
- /* Some C compilers fail to reduce "FIX(constant)" at compile time, thus
- * causing a lot of useless floating-point operations at run time.
- * To get around this we use the following pre-calculated constants.
- * If you change CONST_BITS you may want to add appropriate values.
- * (With a reasonable C compiler, you can just rely on the FIX() macro...)
- */
- #if CONST_BITS == 8
- #define FIX_0_382683433 ((long) 98) /* FIX(0.382683433) */
- #define FIX_0_541196100 ((long) 139) /* FIX(0.541196100) */
- #define FIX_0_707106781 ((long) 181) /* FIX(0.707106781) */
- #define FIX_1_306562965 ((long) 334) /* FIX(1.306562965) */
- #else
- #define FIX_0_382683433 FIX(0.382683433)
- #define FIX_0_541196100 FIX(0.541196100)
- #define FIX_0_707106781 FIX(0.707106781)
- #define FIX_1_306562965 FIX(1.306562965)
- #endif
- /* We can gain a little more speed, with a further compromise in accuracy,
- * by omitting the addition in a descaling shift. This yields an incorrectly
- * rounded result half the time...
- */
- #ifndef USE_ACCURATE_ROUNDING
- #undef DESCALE
- #define DESCALE(x,n) RIGHT_SHIFT(x, n)
- #endif
- /* Multiply a DCTELEM variable by an long constant, and immediately
- * descale to yield a DCTELEM result.
- */
- #define MULTIPLY(var,const) ((DCTELEM) DESCALE((var) * (const), CONST_BITS))
- /*
- * Perform the forward DCT on one block of samples.
- */
- GLOBAL(void)
- jpeg_fdct_ifast (DCTELEM * data)
- {
- DCTELEM tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7;
- DCTELEM tmp10, tmp11, tmp12, tmp13;
- DCTELEM z1, z2, z3, z4, z5, z11, z13;
- DCTELEM *dataptr;
- int ctr;
- SHIFT_TEMPS
- /* Pass 1: process rows. */
- dataptr = data;
- for (ctr = DCTSIZE-1; ctr >= 0; ctr--) {
- tmp0 = dataptr[0] + dataptr[7];
- tmp7 = dataptr[0] - dataptr[7];
- tmp1 = dataptr[1] + dataptr[6];
- tmp6 = dataptr[1] - dataptr[6];
- tmp2 = dataptr[2] + dataptr[5];
- tmp5 = dataptr[2] - dataptr[5];
- tmp3 = dataptr[3] + dataptr[4];
- tmp4 = dataptr[3] - dataptr[4];
- /* Even part */
- tmp10 = tmp0 + tmp3; /* phase 2 */
- tmp13 = tmp0 - tmp3;
- tmp11 = tmp1 + tmp2;
- tmp12 = tmp1 - tmp2;
- dataptr[0] = tmp10 + tmp11; /* phase 3 */
- dataptr[4] = tmp10 - tmp11;
- z1 = MULTIPLY(tmp12 + tmp13, FIX_0_707106781); /* c4 */
- dataptr[2] = tmp13 + z1; /* phase 5 */
- dataptr[6] = tmp13 - z1;
- /* Odd part */
- tmp10 = tmp4 + tmp5; /* phase 2 */
- tmp11 = tmp5 + tmp6;
- tmp12 = tmp6 + tmp7;
- /* The rotator is modified from fig 4-8 to avoid extra negations. */
- z5 = MULTIPLY(tmp10 - tmp12, FIX_0_382683433); /* c6 */
- z2 = MULTIPLY(tmp10, FIX_0_541196100) + z5; /* c2-c6 */
- z4 = MULTIPLY(tmp12, FIX_1_306562965) + z5; /* c2+c6 */
- z3 = MULTIPLY(tmp11, FIX_0_707106781); /* c4 */
- z11 = tmp7 + z3; /* phase 5 */
- z13 = tmp7 - z3;
- dataptr[5] = z13 + z2; /* phase 6 */
- dataptr[3] = z13 - z2;
- dataptr[1] = z11 + z4;
- dataptr[7] = z11 - z4;
- dataptr += DCTSIZE; /* advance pointer to next row */
- }
- /* Pass 2: process columns. */
- dataptr = data;
- for (ctr = DCTSIZE-1; ctr >= 0; ctr--) {
- tmp0 = dataptr[DCTSIZE*0] + dataptr[DCTSIZE*7];
- tmp7 = dataptr[DCTSIZE*0] - dataptr[DCTSIZE*7];
- tmp1 = dataptr[DCTSIZE*1] + dataptr[DCTSIZE*6];
- tmp6 = dataptr[DCTSIZE*1] - dataptr[DCTSIZE*6];
- tmp2 = dataptr[DCTSIZE*2] + dataptr[DCTSIZE*5];
- tmp5 = dataptr[DCTSIZE*2] - dataptr[DCTSIZE*5];
- tmp3 = dataptr[DCTSIZE*3] + dataptr[DCTSIZE*4];
- tmp4 = dataptr[DCTSIZE*3] - dataptr[DCTSIZE*4];
- /* Even part */
- tmp10 = tmp0 + tmp3; /* phase 2 */
- tmp13 = tmp0 - tmp3;
- tmp11 = tmp1 + tmp2;
- tmp12 = tmp1 - tmp2;
- dataptr[DCTSIZE*0] = tmp10 + tmp11; /* phase 3 */
- dataptr[DCTSIZE*4] = tmp10 - tmp11;
- z1 = MULTIPLY(tmp12 + tmp13, FIX_0_707106781); /* c4 */
- dataptr[DCTSIZE*2] = tmp13 + z1; /* phase 5 */
- dataptr[DCTSIZE*6] = tmp13 - z1;
- /* Odd part */
- tmp10 = tmp4 + tmp5; /* phase 2 */
- tmp11 = tmp5 + tmp6;
- tmp12 = tmp6 + tmp7;
- /* The rotator is modified from fig 4-8 to avoid extra negations. */
- z5 = MULTIPLY(tmp10 - tmp12, FIX_0_382683433); /* c6 */
- z2 = MULTIPLY(tmp10, FIX_0_541196100) + z5; /* c2-c6 */
- z4 = MULTIPLY(tmp12, FIX_1_306562965) + z5; /* c2+c6 */
- z3 = MULTIPLY(tmp11, FIX_0_707106781); /* c4 */
- z11 = tmp7 + z3; /* phase 5 */
- z13 = tmp7 - z3;
- dataptr[DCTSIZE*5] = z13 + z2; /* phase 6 */
- dataptr[DCTSIZE*3] = z13 - z2;
- dataptr[DCTSIZE*1] = z11 + z4;
- dataptr[DCTSIZE*7] = z11 - z4;
- dataptr++; /* advance pointer to next column */
- }
- }
- #endif /* DCT_IFAST_SUPPORTED */