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-/*
- * jidctfst.c
- *
- * Copyright (C) 1994-1998, 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
- * inverse DCT (Discrete Cosine Transform). In the IJG code, this routine
- * must also perform dequantization of the input coefficients.
- *
- * A 2-D IDCT can be done by 1-D IDCT on each column followed by 1-D IDCT
- * on each row (or vice versa, but it's more convenient to emit a row at
- * a time). 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 jidctint.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.
- *
- * The dequantized coefficients are not integers because the AA&N scaling
- * factors have been incorporated. We represent them scaled up by PASS1_BITS,
- * so that the first and second IDCT rounds have the same input scaling.
- * For 8-bit JSAMPLEs, we choose IFAST_SCALE_BITS = PASS1_BITS so as to
- * avoid a descaling shift; this compromises accuracy rather drastically
- * for small quantization table entries, but it saves a lot of shifts.
- * For 12-bit JSAMPLEs, there's no hope of using 16x16 multiplies anyway,
- * so we use a much larger scaling factor to preserve accuracy.
- *
- * 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).
- */
-
-#if BITS_IN_JSAMPLE == 8
-#define CONST_BITS 8
-#define PASS1_BITS 2
-#else
-#define CONST_BITS 8
-#define PASS1_BITS 1 /* lose a little precision to avoid overflow */
-#endif
-
-/* 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_1_082392200 ((INT32) 277) /* FIX(1.082392200) */
-#define FIX_1_414213562 ((INT32) 362) /* FIX(1.414213562) */
-#define FIX_1_847759065 ((INT32) 473) /* FIX(1.847759065) */
-#define FIX_2_613125930 ((INT32) 669) /* FIX(2.613125930) */
-#else
-#define FIX_1_082392200 FIX(1.082392200)
-#define FIX_1_414213562 FIX(1.414213562)
-#define FIX_1_847759065 FIX(1.847759065)
-#define FIX_2_613125930 FIX(2.613125930)
-#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))
-
-
-/* Dequantize a coefficient by multiplying it by the multiplier-table
- * entry; produce a DCTELEM result. For 8-bit data a 16x16->16
- * multiplication will do. For 12-bit data, the multiplier table is
- * declared INT32, so a 32-bit multiply will be used.
- */
-
-#if BITS_IN_JSAMPLE == 8
-#define DEQUANTIZE(coef,quantval) (((IFAST_MULT_TYPE) (coef)) * (quantval))
-#else
-#define DEQUANTIZE(coef,quantval) \
- DESCALE((coef)*(quantval), IFAST_SCALE_BITS-PASS1_BITS)
-#endif
-
-
-/* Like DESCALE, but applies to a DCTELEM and produces an int.
- * We assume that int right shift is unsigned if INT32 right shift is.
- */
-
-#ifdef RIGHT_SHIFT_IS_UNSIGNED
-#define ISHIFT_TEMPS DCTELEM ishift_temp;
-#if BITS_IN_JSAMPLE == 8
-#define DCTELEMBITS 16 /* DCTELEM may be 16 or 32 bits */
-#else
-#define DCTELEMBITS 32 /* DCTELEM must be 32 bits */
-#endif
-#define IRIGHT_SHIFT(x,shft) \
- ((ishift_temp = (x)) < 0 ? \
- (ishift_temp >> (shft)) | ((~((DCTELEM) 0)) << (DCTELEMBITS-(shft))) : \
- (ishift_temp >> (shft)))
-#else
-#define ISHIFT_TEMPS
-#define IRIGHT_SHIFT(x,shft) ((x) >> (shft))
-#endif
-
-#ifdef USE_ACCURATE_ROUNDING
-#define IDESCALE(x,n) ((int) IRIGHT_SHIFT((x) + (1 << ((n)-1)), n))
-#else
-#define IDESCALE(x,n) ((int) IRIGHT_SHIFT(x, n))
-#endif
-
-
-/*
- * Perform dequantization and inverse DCT on one block of coefficients.
- */
-
-GLOBAL(void)
-jpeg_idct_ifast (j_decompress_ptr cinfo, jpeg_component_info * compptr,
- JCOEFPTR coef_block,
- JSAMPARRAY output_buf, JDIMENSION output_col)
-{
- DCTELEM tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7;
- DCTELEM tmp10, tmp11, tmp12, tmp13;
- DCTELEM z5, z10, z11, z12, z13;
- JCOEFPTR inptr;
- IFAST_MULT_TYPE * quantptr;
- int * wsptr;
- JSAMPROW outptr;
- JSAMPLE *range_limit = IDCT_range_limit(cinfo);
- int ctr;
- int workspace[DCTSIZE2]; /* buffers data between passes */
- SHIFT_TEMPS /* for DESCALE */
- ISHIFT_TEMPS /* for IDESCALE */
-
- /* Pass 1: process columns from input, store into work array. */
-
- inptr = coef_block;
- quantptr = (IFAST_MULT_TYPE *) compptr->dct_table;
- wsptr = workspace;
- for (ctr = DCTSIZE; ctr > 0; ctr--) {
- /* Due to quantization, we will usually find that many of the input
- * coefficients are zero, especially the AC terms. We can exploit this
- * by short-circuiting the IDCT calculation for any column in which all
- * the AC terms are zero. In that case each output is equal to the
- * DC coefficient (with scale factor as needed).
- * With typical images and quantization tables, half or more of the
- * column DCT calculations can be simplified this way.
- */
-
- if (inptr[DCTSIZE*1] == 0 && inptr[DCTSIZE*2] == 0 &&
- inptr[DCTSIZE*3] == 0 && inptr[DCTSIZE*4] == 0 &&
- inptr[DCTSIZE*5] == 0 && inptr[DCTSIZE*6] == 0 &&
- inptr[DCTSIZE*7] == 0) {
- /* AC terms all zero */
- int dcval = (int) DEQUANTIZE(inptr[DCTSIZE*0], quantptr[DCTSIZE*0]);
-
- wsptr[DCTSIZE*0] = dcval;
- wsptr[DCTSIZE*1] = dcval;
- wsptr[DCTSIZE*2] = dcval;
- wsptr[DCTSIZE*3] = dcval;
- wsptr[DCTSIZE*4] = dcval;
- wsptr[DCTSIZE*5] = dcval;
- wsptr[DCTSIZE*6] = dcval;
- wsptr[DCTSIZE*7] = dcval;
-
- inptr++; /* advance pointers to next column */
- quantptr++;
- wsptr++;
- continue;
- }
-
- /* Even part */
-
- tmp0 = DEQUANTIZE(inptr[DCTSIZE*0], quantptr[DCTSIZE*0]);
- tmp1 = DEQUANTIZE(inptr[DCTSIZE*2], quantptr[DCTSIZE*2]);
- tmp2 = DEQUANTIZE(inptr[DCTSIZE*4], quantptr[DCTSIZE*4]);
- tmp3 = DEQUANTIZE(inptr[DCTSIZE*6], quantptr[DCTSIZE*6]);
-
- tmp10 = tmp0 + tmp2; /* phase 3 */
- tmp11 = tmp0 - tmp2;
-
- tmp13 = tmp1 + tmp3; /* phases 5-3 */
- tmp12 = MULTIPLY(tmp1 - tmp3, FIX_1_414213562) - tmp13; /* 2*c4 */
-
- tmp0 = tmp10 + tmp13; /* phase 2 */
- tmp3 = tmp10 - tmp13;
- tmp1 = tmp11 + tmp12;
- tmp2 = tmp11 - tmp12;
-
- /* Odd part */
-
- tmp4 = DEQUANTIZE(inptr[DCTSIZE*1], quantptr[DCTSIZE*1]);
- tmp5 = DEQUANTIZE(inptr[DCTSIZE*3], quantptr[DCTSIZE*3]);
- tmp6 = DEQUANTIZE(inptr[DCTSIZE*5], quantptr[DCTSIZE*5]);
- tmp7 = DEQUANTIZE(inptr[DCTSIZE*7], quantptr[DCTSIZE*7]);
-
- z13 = tmp6 + tmp5; /* phase 6 */
- z10 = tmp6 - tmp5;
- z11 = tmp4 + tmp7;
- z12 = tmp4 - tmp7;
-
- tmp7 = z11 + z13; /* phase 5 */
- tmp11 = MULTIPLY(z11 - z13, FIX_1_414213562); /* 2*c4 */
-
- z5 = MULTIPLY(z10 + z12, FIX_1_847759065); /* 2*c2 */
- tmp10 = MULTIPLY(z12, FIX_1_082392200) - z5; /* 2*(c2-c6) */
- tmp12 = MULTIPLY(z10, - FIX_2_613125930) + z5; /* -2*(c2+c6) */
-
- tmp6 = tmp12 - tmp7; /* phase 2 */
- tmp5 = tmp11 - tmp6;
- tmp4 = tmp10 + tmp5;
-
- wsptr[DCTSIZE*0] = (int) (tmp0 + tmp7);
- wsptr[DCTSIZE*7] = (int) (tmp0 - tmp7);
- wsptr[DCTSIZE*1] = (int) (tmp1 + tmp6);
- wsptr[DCTSIZE*6] = (int) (tmp1 - tmp6);
- wsptr[DCTSIZE*2] = (int) (tmp2 + tmp5);
- wsptr[DCTSIZE*5] = (int) (tmp2 - tmp5);
- wsptr[DCTSIZE*4] = (int) (tmp3 + tmp4);
- wsptr[DCTSIZE*3] = (int) (tmp3 - tmp4);
-
- inptr++; /* advance pointers to next column */
- quantptr++;
- wsptr++;
- }
-
- /* Pass 2: process rows from work array, store into output array. */
- /* Note that we must descale the results by a factor of 8 == 2**3, */
- /* and also undo the PASS1_BITS scaling. */
-
- wsptr = workspace;
- for (ctr = 0; ctr < DCTSIZE; ctr++) {
- outptr = output_buf[ctr] + output_col;
- /* Rows of zeroes can be exploited in the same way as we did with columns.
- * However, the column calculation has created many nonzero AC terms, so
- * the simplification applies less often (typically 5% to 10% of the time).
- * On machines with very fast multiplication, it's possible that the
- * test takes more time than it's worth. In that case this section
- * may be commented out.
- */
-
-#ifndef NO_ZERO_ROW_TEST
- if (wsptr[1] == 0 && wsptr[2] == 0 && wsptr[3] == 0 && wsptr[4] == 0 &&
- wsptr[5] == 0 && wsptr[6] == 0 && wsptr[7] == 0) {
- /* AC terms all zero */
- JSAMPLE dcval = range_limit[IDESCALE(wsptr[0], PASS1_BITS+3)
- & RANGE_MASK];
-
- outptr[0] = dcval;
- outptr[1] = dcval;
- outptr[2] = dcval;
- outptr[3] = dcval;
- outptr[4] = dcval;
- outptr[5] = dcval;
- outptr[6] = dcval;
- outptr[7] = dcval;
-
- wsptr += DCTSIZE; /* advance pointer to next row */
- continue;
- }
-#endif
-
- /* Even part */
-
- tmp10 = ((DCTELEM) wsptr[0] + (DCTELEM) wsptr[4]);
- tmp11 = ((DCTELEM) wsptr[0] - (DCTELEM) wsptr[4]);
-
- tmp13 = ((DCTELEM) wsptr[2] + (DCTELEM) wsptr[6]);
- tmp12 = MULTIPLY((DCTELEM) wsptr[2] - (DCTELEM) wsptr[6], FIX_1_414213562)
- - tmp13;
-
- tmp0 = tmp10 + tmp13;
- tmp3 = tmp10 - tmp13;
- tmp1 = tmp11 + tmp12;
- tmp2 = tmp11 - tmp12;
-
- /* Odd part */
-
- z13 = (DCTELEM) wsptr[5] + (DCTELEM) wsptr[3];
- z10 = (DCTELEM) wsptr[5] - (DCTELEM) wsptr[3];
- z11 = (DCTELEM) wsptr[1] + (DCTELEM) wsptr[7];
- z12 = (DCTELEM) wsptr[1] - (DCTELEM) wsptr[7];
-
- tmp7 = z11 + z13; /* phase 5 */
- tmp11 = MULTIPLY(z11 - z13, FIX_1_414213562); /* 2*c4 */
-
- z5 = MULTIPLY(z10 + z12, FIX_1_847759065); /* 2*c2 */
- tmp10 = MULTIPLY(z12, FIX_1_082392200) - z5; /* 2*(c2-c6) */
- tmp12 = MULTIPLY(z10, - FIX_2_613125930) + z5; /* -2*(c2+c6) */
-
- tmp6 = tmp12 - tmp7; /* phase 2 */
- tmp5 = tmp11 - tmp6;
- tmp4 = tmp10 + tmp5;
-
- /* Final output stage: scale down by a factor of 8 and range-limit */
-
- outptr[0] = range_limit[IDESCALE(tmp0 + tmp7, PASS1_BITS+3)
- & RANGE_MASK];
- outptr[7] = range_limit[IDESCALE(tmp0 - tmp7, PASS1_BITS+3)
- & RANGE_MASK];
- outptr[1] = range_limit[IDESCALE(tmp1 + tmp6, PASS1_BITS+3)
- & RANGE_MASK];
- outptr[6] = range_limit[IDESCALE(tmp1 - tmp6, PASS1_BITS+3)
- & RANGE_MASK];
- outptr[2] = range_limit[IDESCALE(tmp2 + tmp5, PASS1_BITS+3)
- & RANGE_MASK];
- outptr[5] = range_limit[IDESCALE(tmp2 - tmp5, PASS1_BITS+3)
- & RANGE_MASK];
- outptr[4] = range_limit[IDESCALE(tmp3 + tmp4, PASS1_BITS+3)
- & RANGE_MASK];
- outptr[3] = range_limit[IDESCALE(tmp3 - tmp4, PASS1_BITS+3)
- & RANGE_MASK];
-
- wsptr += DCTSIZE; /* advance pointer to next row */
- }
-}
-
-#endif /* DCT_IFAST_SUPPORTED */
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