/* * jidctflt.c * * Copyright (C) 1994-1998, Thomas G. Lane. * This file is part of the Independent JPEG Group's software. * * The authors make NO WARRANTY or representation, either express or implied, * with respect to this software, its quality, accuracy, merchantability, or * fitness for a particular purpose. This software is provided "AS IS", and you, * its user, assume the entire risk as to its quality and accuracy. * * This software is copyright (C) 1991-1998, Thomas G. Lane. * All Rights Reserved except as specified below. * * Permission is hereby granted to use, copy, modify, and distribute this * software (or portions thereof) for any purpose, without fee, subject to these * conditions: * (1) If any part of the source code for this software is distributed, then this * README file must be included, with this copyright and no-warranty notice * unaltered; and any additions, deletions, or changes to the original files * must be clearly indicated in accompanying documentation. * (2) If only executable code is distributed, then the accompanying * documentation must state that "this software is based in part on the work of * the Independent JPEG Group". * (3) Permission for use of this software is granted only if the user accepts * full responsibility for any undesirable consequences; the authors accept * NO LIABILITY for damages of any kind. * * These conditions apply to any software derived from or based on the IJG code, * not just to the unmodified library. If you use our work, you ought to * acknowledge us. * * Permission is NOT granted for the use of any IJG author's name or company name * in advertising or publicity relating to this software or products derived from * it. This software may be referred to only as "the Independent JPEG Group's * software". * * We specifically permit and encourage the use of this software as the basis of * commercial products, provided that all warranty or liability claims are * assumed by the product vendor. * * * This file contains a floating-point implementation of the * inverse DCT (Discrete Cosine Transform). In the IJG code, this routine * must also perform dequantization of the input coefficients. * * This implementation should be more accurate than either of the integer * IDCT implementations. However, it may not give the same results on all * machines because of differences in roundoff behavior. Speed will depend * on the hardware's floating point capacity. * * 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 a fixed-point * implementation, accuracy is lost due to imprecise representation of the * scaled quantization values. However, that problem does not arise if * we use floating point arithmetic. */ #include <stdint.h> #include "tinyjpeg-internal.h" #define FAST_FLOAT float #define DCTSIZE 8 #define DCTSIZE2 (DCTSIZE*DCTSIZE) #define DEQUANTIZE(coef,quantval) (((FAST_FLOAT) (coef)) * (quantval)) #if defined(__GNUC__) && (defined(__i686__) || defined(__x86_64__)) static inline unsigned char descale_and_clamp(int x, int shift) { __asm__ ( "add %3,%1\n" "\tsar %2,%1\n" "\tsub $-128,%1\n" "\tcmovl %5,%1\n" /* Use the sub to compare to 0 */ "\tcmpl %4,%1\n" "\tcmovg %4,%1\n" : "=r"(x) : "0"(x), "Ic"((unsigned char)shift), "ir"(1U<<(shift-1)), "r" (0xff), "r" (0) ); return x; } #else static inline unsigned char descale_and_clamp(int x, int shift) { x += (1UL<<(shift-1)); if (x<0) x = (x >> shift) | ((~(0UL)) << (32-(shift))); else x >>= shift; x += 128; if (x>255) return 255; else if (x<0) return 0; else return x; } #endif /* * Perform dequantization and inverse DCT on one block of coefficients. */ void tinyjpeg_idct_float (struct component *compptr, uint8_t *output_buf, int stride) { FAST_FLOAT tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7; FAST_FLOAT tmp10, tmp11, tmp12, tmp13; FAST_FLOAT z5, z10, z11, z12, z13; int16_t *inptr; FAST_FLOAT *quantptr; FAST_FLOAT *wsptr; uint8_t *outptr; int ctr; FAST_FLOAT workspace[DCTSIZE2]; /* buffers data between passes */ /* Pass 1: process columns from input, store into work array. */ inptr = compptr->DCT; quantptr = compptr->Q_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 */ FAST_FLOAT dcval = 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 = (tmp1 - tmp3) * ((FAST_FLOAT) 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 = (z11 - z13) * ((FAST_FLOAT) 1.414213562); /* 2*c4 */ z5 = (z10 + z12) * ((FAST_FLOAT) 1.847759065); /* 2*c2 */ tmp10 = ((FAST_FLOAT) 1.082392200) * z12 - z5; /* 2*(c2-c6) */ tmp12 = ((FAST_FLOAT) -2.613125930) * z10 + z5; /* -2*(c2+c6) */ tmp6 = tmp12 - tmp7; /* phase 2 */ tmp5 = tmp11 - tmp6; tmp4 = tmp10 + tmp5; wsptr[DCTSIZE*0] = tmp0 + tmp7; wsptr[DCTSIZE*7] = tmp0 - tmp7; wsptr[DCTSIZE*1] = tmp1 + tmp6; wsptr[DCTSIZE*6] = tmp1 - tmp6; wsptr[DCTSIZE*2] = tmp2 + tmp5; wsptr[DCTSIZE*5] = tmp2 - tmp5; wsptr[DCTSIZE*4] = tmp3 + tmp4; wsptr[DCTSIZE*3] = 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. */ wsptr = workspace; outptr = output_buf; for (ctr = 0; ctr < DCTSIZE; ctr++) { /* 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). * And testing floats for zero is relatively expensive, so we don't bother. */ /* Even part */ tmp10 = wsptr[0] + wsptr[4]; tmp11 = wsptr[0] - wsptr[4]; tmp13 = wsptr[2] + wsptr[6]; tmp12 = (wsptr[2] - wsptr[6]) * ((FAST_FLOAT) 1.414213562) - tmp13; tmp0 = tmp10 + tmp13; tmp3 = tmp10 - tmp13; tmp1 = tmp11 + tmp12; tmp2 = tmp11 - tmp12; /* Odd part */ z13 = wsptr[5] + wsptr[3]; z10 = wsptr[5] - wsptr[3]; z11 = wsptr[1] + wsptr[7]; z12 = wsptr[1] - wsptr[7]; tmp7 = z11 + z13; tmp11 = (z11 - z13) * ((FAST_FLOAT) 1.414213562); z5 = (z10 + z12) * ((FAST_FLOAT) 1.847759065); /* 2*c2 */ tmp10 = ((FAST_FLOAT) 1.082392200) * z12 - z5; /* 2*(c2-c6) */ tmp12 = ((FAST_FLOAT) -2.613125930) * z10 + z5; /* -2*(c2+c6) */ tmp6 = tmp12 - tmp7; tmp5 = tmp11 - tmp6; tmp4 = tmp10 + tmp5; /* Final output stage: scale down by a factor of 8 and range-limit */ outptr[0] = descale_and_clamp((int)(tmp0 + tmp7), 3); outptr[7] = descale_and_clamp((int)(tmp0 - tmp7), 3); outptr[1] = descale_and_clamp((int)(tmp1 + tmp6), 3); outptr[6] = descale_and_clamp((int)(tmp1 - tmp6), 3); outptr[2] = descale_and_clamp((int)(tmp2 + tmp5), 3); outptr[5] = descale_and_clamp((int)(tmp2 - tmp5), 3); outptr[4] = descale_and_clamp((int)(tmp3 + tmp4), 3); outptr[3] = descale_and_clamp((int)(tmp3 - tmp4), 3); wsptr += DCTSIZE; /* advance pointer to next row */ outptr += stride; } }

Generated by Doxygen 1.6.0 Back to index