172 lines
4.9 KiB
C
172 lines
4.9 KiB
C
/*
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* jrevdct.c
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*
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* Copyright (C) 1991, Thomas G. Lane.
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* This file is part of the Independent JPEG Group's software.
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* For conditions of distribution and use, see the accompanying README file.
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*
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* This file contains the basic inverse-DCT transformation subroutine.
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*
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* This implementation is based on Appendix A.2 of the book
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* "Discrete Cosine Transform---Algorithms, Advantages, Applications"
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* by K.R. Rao and P. Yip (Academic Press, Inc, London, 1990).
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* It uses scaled fixed-point arithmetic instead of floating point.
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*/
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#include "jinclude.h"
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/* The poop on this scaling stuff is as follows:
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*
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* Most of the numbers (after multiplication by the constants) are
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* (logically) shifted left by LG2_DCT_SCALE. This is undone by UNFIXH
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* before assignment to the output array. Note that we want an additional
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* division by 2 on the output (required by the equations).
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*
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* If right shifts are unsigned, then there is a potential problem.
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* However, shifting right by 16 and then assigning to a short
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* (assuming short = 16 bits) will keep the sign right!!
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*
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* For other shifts,
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*
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* ((x + (1 << 30)) >> shft) - (1 << (30 - shft))
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*
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* gives a nice right shift with sign (assuming no overflow). However, all the
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* scaling is such that this isn't a problem. (Is this true?)
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*/
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#define ONE 1L /* remove L if long > 32 bits */
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#ifdef RIGHT_SHIFT_IS_UNSIGNED
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#define LG2_DCT_SCALE 15
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#define RIGHT_SHIFT(_x,_shft) ((((_x) + (ONE << 30)) >> (_shft)) - (ONE << (30 - (_shft))))
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#else
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#define LG2_DCT_SCALE 16
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#define RIGHT_SHIFT(_x,_shft) ((_x) >> (_shft))
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#endif
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#define DCT_SCALE (ONE << LG2_DCT_SCALE)
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#define LG2_OVERSCALE 2
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#define OVERSCALE (ONE << LG2_OVERSCALE)
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#define FIX(x) ((INT32) ((x) * DCT_SCALE + 0.5))
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#define FIXO(x) ((INT32) ((x) * DCT_SCALE / OVERSCALE + 0.5))
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#define UNFIX(x) RIGHT_SHIFT((x) + (ONE << (LG2_DCT_SCALE-1)), LG2_DCT_SCALE)
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#define UNFIXH(x) RIGHT_SHIFT((x) + (ONE << LG2_DCT_SCALE), LG2_DCT_SCALE+1)
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#define UNFIXO(x) RIGHT_SHIFT((x) + (ONE << (LG2_DCT_SCALE-1-LG2_OVERSCALE)), LG2_DCT_SCALE-LG2_OVERSCALE)
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#define OVERSH(x) ((x) << LG2_OVERSCALE)
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#define SIN_1_4 FIX(0.7071067811856476)
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#define COS_1_4 SIN_1_4
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#define SIN_1_8 FIX(0.3826834323650898)
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#define COS_1_8 FIX(0.9238795325112870)
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#define SIN_3_8 COS_1_8
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#define COS_3_8 SIN_1_8
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#define SIN_1_16 FIX(0.1950903220161282)
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#define COS_1_16 FIX(0.9807852804032300)
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#define SIN_7_16 COS_1_16
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#define COS_7_16 SIN_1_16
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#define SIN_3_16 FIX(0.5555702330196022)
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#define COS_3_16 FIX(0.8314696123025450)
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#define SIN_5_16 COS_3_16
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#define COS_5_16 SIN_3_16
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#define OSIN_1_4 FIXO(0.707106781185647)
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#define OCOS_1_4 OSIN_1_4
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#define OSIN_1_8 FIXO(0.3826834323650898)
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#define OCOS_1_8 FIXO(0.9238795325112870)
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#define OSIN_3_8 OCOS_1_8
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#define OCOS_3_8 OSIN_1_8
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#define OSIN_1_16 FIXO(0.1950903220161282)
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#define OCOS_1_16 FIXO(0.9807852804032300)
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#define OSIN_7_16 OCOS_1_16
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#define OCOS_7_16 OSIN_1_16
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#define OSIN_3_16 FIXO(0.5555702330196022)
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#define OCOS_3_16 FIXO(0.8314696123025450)
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#define OSIN_5_16 OCOS_3_16
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#define OCOS_5_16 OSIN_3_16
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INLINE
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LOCAL void
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fast_idct_8 (DCTELEM *in, int stride)
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{
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/* tmp1x are new values of tmpx -- flashy register colourers
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* should be able to do this lot very well
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*/
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INT32 tmp10, tmp11, tmp12, tmp13;
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INT32 tmp20, tmp21, tmp22, tmp23;
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INT32 tmp30, tmp31;
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INT32 tmp40, tmp41, tmp42, tmp43;
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INT32 tmp50, tmp51, tmp52, tmp53;
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INT32 in0, in1, in2, in3, in4, in5, in6, in7;
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in0 = in[ 0];
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in1 = in[stride ];
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in2 = in[stride*2];
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in3 = in[stride*3];
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in4 = in[stride*4];
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in5 = in[stride*5];
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in6 = in[stride*6];
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in7 = in[stride*7];
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tmp10 = (in0 + in4) * COS_1_4;
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tmp11 = (in0 - in4) * COS_1_4;
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tmp12 = in2 * SIN_1_8 - in6 * COS_1_8;
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tmp13 = in6 * SIN_1_8 + in2 * COS_1_8;
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tmp20 = tmp10 + tmp13;
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tmp21 = tmp11 + tmp12;
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tmp22 = tmp11 - tmp12;
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tmp23 = tmp10 - tmp13;
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tmp30 = UNFIXO((in3 + in5) * COS_1_4);
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tmp31 = UNFIXO((in3 - in5) * COS_1_4);
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tmp40 = OVERSH(in1) + tmp30;
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tmp41 = OVERSH(in7) + tmp31;
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tmp42 = OVERSH(in1) - tmp30;
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tmp43 = OVERSH(in7) - tmp31;
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tmp50 = tmp40 * OCOS_1_16 + tmp41 * OSIN_1_16;
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tmp51 = tmp40 * OSIN_1_16 - tmp41 * OCOS_1_16;
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tmp52 = tmp42 * OCOS_5_16 + tmp43 * OSIN_5_16;
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tmp53 = tmp42 * OSIN_5_16 - tmp43 * OCOS_5_16;
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in[ 0] = UNFIXH(tmp20 + tmp50);
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in[stride ] = UNFIXH(tmp21 + tmp53);
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in[stride*2] = UNFIXH(tmp22 + tmp52);
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in[stride*3] = UNFIXH(tmp23 + tmp51);
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in[stride*4] = UNFIXH(tmp23 - tmp51);
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in[stride*5] = UNFIXH(tmp22 - tmp52);
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in[stride*6] = UNFIXH(tmp21 - tmp53);
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in[stride*7] = UNFIXH(tmp20 - tmp50);
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}
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/*
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* Perform the inverse DCT on one block of coefficients.
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*
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* Note that this code is specialized to the case DCTSIZE = 8.
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*/
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GLOBAL void
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j_rev_dct (DCTBLOCK data)
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{
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int i;
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for (i = 0; i < DCTSIZE; i++)
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fast_idct_8(data+i*DCTSIZE, 1);
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for (i = 0; i < DCTSIZE; i++)
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fast_idct_8(data+i, DCTSIZE);
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}
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