1 /* 2 * IDCT implementation using the MIPS DSP ASE (little endian version) 3 * 4 * jidctfst.c 5 * 6 * Copyright (C) 1994-1998, Thomas G. Lane. 7 * This file is part of the Independent JPEG Group's software. 8 * For conditions of distribution and use, see the accompanying README file. 9 * 10 * This file contains a fast, not so accurate integer implementation of the 11 * inverse DCT (Discrete Cosine Transform). In the IJG code, this routine 12 * must also perform dequantization of the input coefficients. 13 * 14 * A 2-D IDCT can be done by 1-D IDCT on each column followed by 1-D IDCT 15 * on each row (or vice versa, but it's more convenient to emit a row at 16 * a time). Direct algorithms are also available, but they are much more 17 * complex and seem not to be any faster when reduced to code. 18 * 19 * This implementation is based on Arai, Agui, and Nakajima's algorithm for 20 * scaled DCT. Their original paper (Trans. IEICE E-71(11):1095) is in 21 * Japanese, but the algorithm is described in the Pennebaker & Mitchell 22 * JPEG textbook (see REFERENCES section in file README). The following code 23 * is based directly on figure 4-8 in P&M. 24 * While an 8-point DCT cannot be done in less than 11 multiplies, it is 25 * possible to arrange the computation so that many of the multiplies are 26 * simple scalings of the final outputs. These multiplies can then be 27 * folded into the multiplications or divisions by the JPEG quantization 28 * table entries. The AA&N method leaves only 5 multiplies and 29 adds 29 * to be done in the DCT itself. 30 * The primary disadvantage of this method is that with fixed-point math, 31 * accuracy is lost due to imprecise representation of the scaled 32 * quantization values. The smaller the quantization table entry, the less 33 * precise the scaled value, so this implementation does worse with high- 34 * quality-setting files than with low-quality ones. 35 */ 36 37 #define JPEG_INTERNALS 38 #include "jinclude.h" 39 #include "jpeglib.h" 40 #include "jdct.h" /* Private declarations for DCT subsystem */ 41 42 #ifdef DCT_IFAST_SUPPORTED 43 44 45 /* 46 * This module is specialized to the case DCTSIZE = 8. 47 */ 48 49 #if DCTSIZE != 8 50 Sorry, this code only copes with 8x8 DCTs. /* deliberate syntax err */ 51 #endif 52 53 54 /* Scaling decisions are generally the same as in the LL&M algorithm; 55 * see jidctint.c for more details. However, we choose to descale 56 * (right shift) multiplication products as soon as they are formed, 57 * rather than carrying additional fractional bits into subsequent additions. 58 * This compromises accuracy slightly, but it lets us save a few shifts. 59 * More importantly, 16-bit arithmetic is then adequate (for 8-bit samples) 60 * everywhere except in the multiplications proper; this saves a good deal 61 * of work on 16-bit-int machines. 62 * 63 * The dequantized coefficients are not integers because the AA&N scaling 64 * factors have been incorporated. We represent them scaled up by PASS1_BITS, 65 * so that the first and second IDCT rounds have the same input scaling. 66 * For 8-bit JSAMPLEs, we choose IFAST_SCALE_BITS = PASS1_BITS so as to 67 * avoid a descaling shift; this compromises accuracy rather drastically 68 * for small quantization table entries, but it saves a lot of shifts. 69 * For 12-bit JSAMPLEs, there's no hope of using 16x16 multiplies anyway, 70 * so we use a much larger scaling factor to preserve accuracy. 71 * 72 * A final compromise is to represent the multiplicative constants to only 73 * 8 fractional bits, rather than 13. This saves some shifting work on some 74 * machines, and may also reduce the cost of multiplication (since there 75 * are fewer one-bits in the constants). 76 */ 77 78 #if BITS_IN_JSAMPLE == 8 79 #define CONST_BITS 8 80 #define PASS1_BITS 2 81 #else 82 #define CONST_BITS 8 83 #define PASS1_BITS 1 /* lose a little precision to avoid overflow */ 84 #endif 85 86 /* Some C compilers fail to reduce "FIX(constant)" at compile time, thus 87 * causing a lot of useless floating-point operations at run time. 88 * To get around this we use the following pre-calculated constants. 89 * If you change CONST_BITS you may want to add appropriate values. 90 * (With a reasonable C compiler, you can just rely on the FIX() macro...) 91 */ 92 93 #if CONST_BITS == 8 94 #define FIX_1_082392200 ((INT32) 277) /* FIX(1.082392200) */ 95 #define FIX_1_414213562 ((INT32) 362) /* FIX(1.414213562) */ 96 #define FIX_1_847759065 ((INT32) 473) /* FIX(1.847759065) */ 97 #define FIX_2_613125930 ((INT32) 669) /* FIX(2.613125930) */ 98 #else 99 #define FIX_1_082392200 FIX(1.082392200) 100 #define FIX_1_414213562 FIX(1.414213562) 101 #define FIX_1_847759065 FIX(1.847759065) 102 #define FIX_2_613125930 FIX(2.613125930) 103 #endif 104 105 106 /* We can gain a little more speed, with a further compromise in accuracy, 107 * by omitting the addition in a descaling shift. This yields an incorrectly 108 * rounded result half the time... 109 */ 110 111 #ifndef USE_ACCURATE_ROUNDING 112 #undef DESCALE 113 #define DESCALE(x,n) RIGHT_SHIFT(x, n) 114 #endif 115 116 117 /* Multiply a DCTELEM variable by an INT32 constant, and immediately 118 * descale to yield a DCTELEM result. 119 */ 120 121 #define MULTIPLY(var,const) ((DCTELEM) DESCALE((var) * (const), CONST_BITS)) 122 123 124 /* Dequantize a coefficient by multiplying it by the multiplier-table 125 * entry; produce a DCTELEM result. For 8-bit data a 16x16->16 126 * multiplication will do. For 12-bit data, the multiplier table is 127 * declared INT32, so a 32-bit multiply will be used. 128 */ 129 130 #if BITS_IN_JSAMPLE == 8 131 #define DEQUANTIZE(coef,quantval) (((IFAST_MULT_TYPE) (coef)) * (quantval)) 132 #else 133 #define DEQUANTIZE(coef,quantval) \ 134 DESCALE((coef)*(quantval), IFAST_SCALE_BITS-PASS1_BITS) 135 #endif 136 137 138 /* Like DESCALE, but applies to a DCTELEM and produces an int. 139 * We assume that int right shift is unsigned if INT32 right shift is. 140 */ 141 142 #ifdef RIGHT_SHIFT_IS_UNSIGNED 143 #define ISHIFT_TEMPS DCTELEM ishift_temp; 144 #if BITS_IN_JSAMPLE == 8 145 #define DCTELEMBITS 16 /* DCTELEM may be 16 or 32 bits */ 146 #else 147 #define DCTELEMBITS 32 /* DCTELEM must be 32 bits */ 148 #endif 149 #define IRIGHT_SHIFT(x,shft) \ 150 ((ishift_temp = (x)) < 0 ? \ 151 (ishift_temp >> (shft)) | ((~((DCTELEM) 0)) << (DCTELEMBITS-(shft))) : \ 152 (ishift_temp >> (shft))) 153 #else 154 #define ISHIFT_TEMPS 155 #define IRIGHT_SHIFT(x,shft) ((x) >> (shft)) 156 #endif 157 158 #ifdef USE_ACCURATE_ROUNDING 159 #define IDESCALE(x,n) ((int) IRIGHT_SHIFT((x) + (1 << ((n)-1)), n)) 160 #else 161 #define IDESCALE(x,n) ((int) IRIGHT_SHIFT(x, n)) 162 #endif 163 164 165 // this table of constants has been moved from mips_idct_le/_be.s to 166 // avoid having to make the assembler code position independent 167 static const int mips_idct_coefs[4] = { 168 0x45464546, // FIX( 1.082392200 / 2) = 17734 = 0x4546 169 0x5A825A82, // FIX( 1.414213562 / 2) = 23170 = 0x5A82 170 0x76427642, // FIX( 1.847759065 / 2) = 30274 = 0x7642 171 0xAC61AC61 // FIX(-2.613125930 / 4) = -21407 = 0xAC61 172 }; 173 174 void mips_idct_columns(JCOEF * inptr, IFAST_MULT_TYPE * quantptr, 175 DCTELEM * wsptr, const int * mips_idct_coefs); 176 void mips_idct_rows(DCTELEM * wsptr, JSAMPARRAY output_buf, 177 JDIMENSION output_col, const int * mips_idct_coefs); 178 179 180 /* 181 * Perform dequantization and inverse DCT on one block of coefficients. 182 */ 183 184 GLOBAL(void) 185 jpeg_idct_mips (j_decompress_ptr cinfo, jpeg_component_info * compptr, 186 JCOEFPTR coef_block, 187 JSAMPARRAY output_buf, JDIMENSION output_col) 188 { 189 JCOEFPTR inptr; 190 IFAST_MULT_TYPE * quantptr; 191 DCTELEM workspace[DCTSIZE2]; /* buffers data between passes */ 192 193 /* Pass 1: process columns from input, store into work array. */ 194 195 inptr = coef_block; 196 quantptr = (IFAST_MULT_TYPE *) compptr->dct_table; 197 198 mips_idct_columns(inptr, quantptr, workspace, mips_idct_coefs); 199 200 /* Pass 2: process rows from work array, store into output array. */ 201 /* Note that we must descale the results by a factor of 8 == 2**3, */ 202 /* and also undo the PASS1_BITS scaling. */ 203 204 mips_idct_rows(workspace, output_buf, output_col, mips_idct_coefs); 205 206 } 207 208 #endif /* DCT_IFAST_SUPPORTED */ 209
