/*+ −
* 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 */+ −