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