/* * reserved comment block * DO NOT REMOVE OR ALTER!
*/ /* * jidctred.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 inverse-DCT routines that produce reduced-size output: * either 4x4, 2x2, or 1x1 pixels from an 8x8 DCT block. * * The implementation is based on the Loeffler, Ligtenberg and Moschytz (LL&M) * algorithm used in jidctint.c. We simply replace each 8-to-8 1-D IDCT step * with an 8-to-4 step that produces the four averages of two adjacent outputs * (or an 8-to-2 step producing two averages of four outputs, for 2x2 output). * These steps were derived by computing the corresponding values at the end * of the normal LL&M code, then simplifying as much as possible. * * 1x1 is trivial: just take the DC coefficient divided by 8. * * See jidctint.c for additional comments.
*/
/* * 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 is the same as in jidctint.c. */
#if BITS_IN_JSAMPLE == 8 #define CONST_BITS 13 #define PASS1_BITS 2 #else #define CONST_BITS 13 #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...)
*/
/* Multiply an INT32 variable by an INT32 constant to yield an INT32 result. * For 8-bit samples with the recommended scaling, all the variable * and constant values involved are no more than 16 bits wide, so a * 16x16->32 bit multiply can be used instead of a full 32x32 multiply. * For 12-bit samples, a full 32-bit multiplication will be needed.
*/
/* Dequantize a coefficient by multiplying it by the multiplier-table * entry; produce an int result. In this module, both inputs and result * are 16 bits or less, so either int or short multiply will work.
*/
/* Pass 2: process 4 rows from work array, store into output array. */
wsptr = workspace; for (ctr = 0; ctr < 4; ctr++) {
outptr = output_buf[ctr] + output_col; /* It's not clear whether a zero row test is worthwhile here ... */
/* Pass 2: process 2 rows from work array, store into output array. */
wsptr = workspace; for (ctr = 0; ctr < 2; ctr++) {
outptr = output_buf[ctr] + output_col; /* It's not clear whether a zero row test is worthwhile here ... */
#ifndef NO_ZERO_ROW_TEST if (wsptr[1] == 0 && wsptr[3] == 0 && wsptr[5] == 0 && wsptr[7] == 0) { /* AC terms all zero */
JSAMPLE dcval = range_limit[(int) DESCALE((INT32) wsptr[0], PASS1_BITS+3)
& RANGE_MASK];
outptr[0] = dcval;
outptr[1] = dcval;
wsptr += DCTSIZE; /* advance pointer to next row */ continue;
} #endif
/* We hardly need an inverse DCT routine for this: just take the * average pixel value, which is one-eighth of the DC coefficient.
*/
quantptr = (ISLOW_MULT_TYPE *) compptr->dct_table;
dcval = DEQUANTIZE(coef_block[0], quantptr[0]);
dcval = (int) DESCALE((INT32) dcval, 3);
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