225 строки
7.4 KiB
C
225 строки
7.4 KiB
C
/*
|
|
* 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 ((INT32) 98) /* FIX(0.382683433) */
|
|
#define FIX_0_541196100 ((INT32) 139) /* FIX(0.541196100) */
|
|
#define FIX_0_707106781 ((INT32) 181) /* FIX(0.707106781) */
|
|
#define FIX_1_306562965 ((INT32) 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 INT32 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 */
|