jidctflt.c (8754B)
1 /* 2 * jidctflt.c 3 * 4 * This file was part of the Independent JPEG Group's software: 5 * Copyright (C) 1994-1998, Thomas G. Lane. 6 * Modified 2010 by Guido Vollbeding. 7 * libjpeg-turbo Modifications: 8 * Copyright (C) 2014, 2022, D. R. Commander. 9 * For conditions of distribution and use, see the accompanying README.ijg 10 * file. 11 * 12 * This file contains a floating-point implementation of the 13 * inverse DCT (Discrete Cosine Transform). In the IJG code, this routine 14 * must also perform dequantization of the input coefficients. 15 * 16 * This implementation should be more accurate than either of the integer 17 * IDCT implementations. However, it may not give the same results on all 18 * machines because of differences in roundoff behavior. Speed will depend 19 * on the hardware's floating point capacity. 20 * 21 * A 2-D IDCT can be done by 1-D IDCT on each column followed by 1-D IDCT 22 * on each row (or vice versa, but it's more convenient to emit a row at 23 * a time). Direct algorithms are also available, but they are much more 24 * complex and seem not to be any faster when reduced to code. 25 * 26 * This implementation is based on Arai, Agui, and Nakajima's algorithm for 27 * scaled DCT. Their original paper (Trans. IEICE E-71(11):1095) is in 28 * Japanese, but the algorithm is described in the Pennebaker & Mitchell 29 * JPEG textbook (see REFERENCES section in file README.ijg). The following 30 * code is based directly on figure 4-8 in P&M. 31 * While an 8-point DCT cannot be done in less than 11 multiplies, it is 32 * possible to arrange the computation so that many of the multiplies are 33 * simple scalings of the final outputs. These multiplies can then be 34 * folded into the multiplications or divisions by the JPEG quantization 35 * table entries. The AA&N method leaves only 5 multiplies and 29 adds 36 * to be done in the DCT itself. 37 * The primary disadvantage of this method is that with a fixed-point 38 * implementation, accuracy is lost due to imprecise representation of the 39 * scaled quantization values. However, that problem does not arise if 40 * we use floating point arithmetic. 41 */ 42 43 #define JPEG_INTERNALS 44 #include "jinclude.h" 45 #include "jpeglib.h" 46 #include "jdct.h" /* Private declarations for DCT subsystem */ 47 48 #ifdef DCT_FLOAT_SUPPORTED 49 50 51 /* 52 * This module is specialized to the case DCTSIZE = 8. 53 */ 54 55 #if DCTSIZE != 8 56 Sorry, this code only copes with 8x8 DCTs. /* deliberate syntax err */ 57 #endif 58 59 60 /* Dequantize a coefficient by multiplying it by the multiplier-table 61 * entry; produce a float result. 62 */ 63 64 #define DEQUANTIZE(coef, quantval) (((FAST_FLOAT)(coef)) * (quantval)) 65 66 67 /* 68 * Perform dequantization and inverse DCT on one block of coefficients. 69 */ 70 71 GLOBAL(void) 72 _jpeg_idct_float(j_decompress_ptr cinfo, jpeg_component_info *compptr, 73 JCOEFPTR coef_block, _JSAMPARRAY output_buf, 74 JDIMENSION output_col) 75 { 76 FAST_FLOAT tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7; 77 FAST_FLOAT tmp10, tmp11, tmp12, tmp13; 78 FAST_FLOAT z5, z10, z11, z12, z13; 79 JCOEFPTR inptr; 80 FLOAT_MULT_TYPE *quantptr; 81 FAST_FLOAT *wsptr; 82 _JSAMPROW outptr; 83 _JSAMPLE *range_limit = (_JSAMPLE *)cinfo->sample_range_limit; 84 int ctr; 85 FAST_FLOAT workspace[DCTSIZE2]; /* buffers data between passes */ 86 #define _0_125 ((FLOAT_MULT_TYPE)0.125) 87 88 /* Pass 1: process columns from input, store into work array. */ 89 90 inptr = coef_block; 91 quantptr = (FLOAT_MULT_TYPE *)compptr->dct_table; 92 wsptr = workspace; 93 for (ctr = DCTSIZE; ctr > 0; ctr--) { 94 /* Due to quantization, we will usually find that many of the input 95 * coefficients are zero, especially the AC terms. We can exploit this 96 * by short-circuiting the IDCT calculation for any column in which all 97 * the AC terms are zero. In that case each output is equal to the 98 * DC coefficient (with scale factor as needed). 99 * With typical images and quantization tables, half or more of the 100 * column DCT calculations can be simplified this way. 101 */ 102 103 if (inptr[DCTSIZE * 1] == 0 && inptr[DCTSIZE * 2] == 0 && 104 inptr[DCTSIZE * 3] == 0 && inptr[DCTSIZE * 4] == 0 && 105 inptr[DCTSIZE * 5] == 0 && inptr[DCTSIZE * 6] == 0 && 106 inptr[DCTSIZE * 7] == 0) { 107 /* AC terms all zero */ 108 FAST_FLOAT dcval = DEQUANTIZE(inptr[DCTSIZE * 0], 109 quantptr[DCTSIZE * 0] * _0_125); 110 111 wsptr[DCTSIZE * 0] = dcval; 112 wsptr[DCTSIZE * 1] = dcval; 113 wsptr[DCTSIZE * 2] = dcval; 114 wsptr[DCTSIZE * 3] = dcval; 115 wsptr[DCTSIZE * 4] = dcval; 116 wsptr[DCTSIZE * 5] = dcval; 117 wsptr[DCTSIZE * 6] = dcval; 118 wsptr[DCTSIZE * 7] = dcval; 119 120 inptr++; /* advance pointers to next column */ 121 quantptr++; 122 wsptr++; 123 continue; 124 } 125 126 /* Even part */ 127 128 tmp0 = DEQUANTIZE(inptr[DCTSIZE * 0], quantptr[DCTSIZE * 0] * _0_125); 129 tmp1 = DEQUANTIZE(inptr[DCTSIZE * 2], quantptr[DCTSIZE * 2] * _0_125); 130 tmp2 = DEQUANTIZE(inptr[DCTSIZE * 4], quantptr[DCTSIZE * 4] * _0_125); 131 tmp3 = DEQUANTIZE(inptr[DCTSIZE * 6], quantptr[DCTSIZE * 6] * _0_125); 132 133 tmp10 = tmp0 + tmp2; /* phase 3 */ 134 tmp11 = tmp0 - tmp2; 135 136 tmp13 = tmp1 + tmp3; /* phases 5-3 */ 137 tmp12 = (tmp1 - tmp3) * ((FAST_FLOAT)1.414213562) - tmp13; /* 2*c4 */ 138 139 tmp0 = tmp10 + tmp13; /* phase 2 */ 140 tmp3 = tmp10 - tmp13; 141 tmp1 = tmp11 + tmp12; 142 tmp2 = tmp11 - tmp12; 143 144 /* Odd part */ 145 146 tmp4 = DEQUANTIZE(inptr[DCTSIZE * 1], quantptr[DCTSIZE * 1] * _0_125); 147 tmp5 = DEQUANTIZE(inptr[DCTSIZE * 3], quantptr[DCTSIZE * 3] * _0_125); 148 tmp6 = DEQUANTIZE(inptr[DCTSIZE * 5], quantptr[DCTSIZE * 5] * _0_125); 149 tmp7 = DEQUANTIZE(inptr[DCTSIZE * 7], quantptr[DCTSIZE * 7] * _0_125); 150 151 z13 = tmp6 + tmp5; /* phase 6 */ 152 z10 = tmp6 - tmp5; 153 z11 = tmp4 + tmp7; 154 z12 = tmp4 - tmp7; 155 156 tmp7 = z11 + z13; /* phase 5 */ 157 tmp11 = (z11 - z13) * ((FAST_FLOAT)1.414213562); /* 2*c4 */ 158 159 z5 = (z10 + z12) * ((FAST_FLOAT)1.847759065); /* 2*c2 */ 160 tmp10 = z5 - z12 * ((FAST_FLOAT)1.082392200); /* 2*(c2-c6) */ 161 tmp12 = z5 - z10 * ((FAST_FLOAT)2.613125930); /* 2*(c2+c6) */ 162 163 tmp6 = tmp12 - tmp7; /* phase 2 */ 164 tmp5 = tmp11 - tmp6; 165 tmp4 = tmp10 - tmp5; 166 167 wsptr[DCTSIZE * 0] = tmp0 + tmp7; 168 wsptr[DCTSIZE * 7] = tmp0 - tmp7; 169 wsptr[DCTSIZE * 1] = tmp1 + tmp6; 170 wsptr[DCTSIZE * 6] = tmp1 - tmp6; 171 wsptr[DCTSIZE * 2] = tmp2 + tmp5; 172 wsptr[DCTSIZE * 5] = tmp2 - tmp5; 173 wsptr[DCTSIZE * 3] = tmp3 + tmp4; 174 wsptr[DCTSIZE * 4] = tmp3 - tmp4; 175 176 inptr++; /* advance pointers to next column */ 177 quantptr++; 178 wsptr++; 179 } 180 181 /* Pass 2: process rows from work array, store into output array. */ 182 183 wsptr = workspace; 184 for (ctr = 0; ctr < DCTSIZE; ctr++) { 185 outptr = output_buf[ctr] + output_col; 186 /* Rows of zeroes can be exploited in the same way as we did with columns. 187 * However, the column calculation has created many nonzero AC terms, so 188 * the simplification applies less often (typically 5% to 10% of the time). 189 * And testing floats for zero is relatively expensive, so we don't bother. 190 */ 191 192 /* Even part */ 193 194 /* Apply signed->unsigned and prepare float->int conversion */ 195 z5 = wsptr[0] + ((FAST_FLOAT)_CENTERJSAMPLE + (FAST_FLOAT)0.5); 196 tmp10 = z5 + wsptr[4]; 197 tmp11 = z5 - wsptr[4]; 198 199 tmp13 = wsptr[2] + wsptr[6]; 200 tmp12 = (wsptr[2] - wsptr[6]) * ((FAST_FLOAT)1.414213562) - tmp13; 201 202 tmp0 = tmp10 + tmp13; 203 tmp3 = tmp10 - tmp13; 204 tmp1 = tmp11 + tmp12; 205 tmp2 = tmp11 - tmp12; 206 207 /* Odd part */ 208 209 z13 = wsptr[5] + wsptr[3]; 210 z10 = wsptr[5] - wsptr[3]; 211 z11 = wsptr[1] + wsptr[7]; 212 z12 = wsptr[1] - wsptr[7]; 213 214 tmp7 = z11 + z13; 215 tmp11 = (z11 - z13) * ((FAST_FLOAT)1.414213562); 216 217 z5 = (z10 + z12) * ((FAST_FLOAT)1.847759065); /* 2*c2 */ 218 tmp10 = z5 - z12 * ((FAST_FLOAT)1.082392200); /* 2*(c2-c6) */ 219 tmp12 = z5 - z10 * ((FAST_FLOAT)2.613125930); /* 2*(c2+c6) */ 220 221 tmp6 = tmp12 - tmp7; 222 tmp5 = tmp11 - tmp6; 223 tmp4 = tmp10 - tmp5; 224 225 /* Final output stage: float->int conversion and range-limit */ 226 227 outptr[0] = range_limit[((int)(tmp0 + tmp7)) & RANGE_MASK]; 228 outptr[7] = range_limit[((int)(tmp0 - tmp7)) & RANGE_MASK]; 229 outptr[1] = range_limit[((int)(tmp1 + tmp6)) & RANGE_MASK]; 230 outptr[6] = range_limit[((int)(tmp1 - tmp6)) & RANGE_MASK]; 231 outptr[2] = range_limit[((int)(tmp2 + tmp5)) & RANGE_MASK]; 232 outptr[5] = range_limit[((int)(tmp2 - tmp5)) & RANGE_MASK]; 233 outptr[3] = range_limit[((int)(tmp3 + tmp4)) & RANGE_MASK]; 234 outptr[4] = range_limit[((int)(tmp3 - tmp4)) & RANGE_MASK]; 235 236 wsptr += DCTSIZE; /* advance pointer to next row */ 237 } 238 } 239 240 #endif /* DCT_FLOAT_SUPPORTED */