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| 1 /* | |
| 2 * jidctflt.c | |
| 3 * | |
| 4 * Copyright (C) 1994-1998, Thomas G. Lane. | |
| 5 * This file is part of the Independent JPEG Group's software. | |
| 6 * For conditions of distribution and use, see the accompanying README file. | |
| 7 * | |
| 8 * This file contains a floating-point implementation of the | |
| 9 * inverse DCT (Discrete Cosine Transform). In the IJG code, this routine | |
| 10 * must also perform dequantization of the input coefficients. | |
| 11 * | |
| 12 * This implementation should be more accurate than either of the integer | |
| 13 * IDCT implementations. However, it may not give the same results on all | |
| 14 * machines because of differences in roundoff behavior. Speed will depend | |
| 15 * on the hardware's floating point capacity. | |
| 16 * | |
| 17 * A 2-D IDCT can be done by 1-D IDCT on each column followed by 1-D IDCT | |
| 18 * on each row (or vice versa, but it's more convenient to emit a row at | |
| 19 * a time). Direct algorithms are also available, but they are much more | |
| 20 * complex and seem not to be any faster when reduced to code. | |
| 21 * | |
| 22 * This implementation is based on Arai, Agui, and Nakajima's algorithm for | |
| 23 * scaled DCT. Their original paper (Trans. IEICE E-71(11):1095) is in | |
| 24 * Japanese, but the algorithm is described in the Pennebaker & Mitchell | |
| 25 * JPEG textbook (see REFERENCES section in file README). The following code | |
| 26 * is based directly on figure 4-8 in P&M. | |
| 27 * While an 8-point DCT cannot be done in less than 11 multiplies, it is | |
| 28 * possible to arrange the computation so that many of the multiplies are | |
| 29 * simple scalings of the final outputs. These multiplies can then be | |
| 30 * folded into the multiplications or divisions by the JPEG quantization | |
| 31 * table entries. The AA&N method leaves only 5 multiplies and 29 adds | |
| 32 * to be done in the DCT itself. | |
| 33 * The primary disadvantage of this method is that with a fixed-point | |
| 34 * implementation, accuracy is lost due to imprecise representation of the | |
| 35 * scaled quantization values. However, that problem does not arise if | |
| 36 * we use floating point arithmetic. | |
| 37 */ | |
| 38 | |
| 39 #define JPEG_INTERNALS | |
| 40 #include "jinclude.h" | |
| 41 #include "jpeglib.h" | |
| 42 #include "jdct.h" /* Private declarations for DCT subsystem */ | |
| 43 | |
| 44 #ifdef DCT_FLOAT_SUPPORTED | |
| 45 | |
| 46 | |
| 47 /* | |
| 48 * This module is specialized to the case DCTSIZE = 8. | |
| 49 */ | |
| 50 | |
| 51 #if DCTSIZE != 8 | |
| 52 Sorry, this code only copes with 8x8 DCTs. /* deliberate syntax err */ | |
| 53 #endif | |
| 54 | |
| 55 | |
| 56 /* Dequantize a coefficient by multiplying it by the multiplier-table | |
| 57 * entry; produce a float result. | |
| 58 */ | |
| 59 | |
| 60 #define DEQUANTIZE(coef,quantval) (((FAST_FLOAT) (coef)) * (quantval)) | |
| 61 | |
| 62 | |
| 63 /* | |
| 64 * Perform dequantization and inverse DCT on one block of coefficients. | |
| 65 */ | |
| 66 | |
| 67 GLOBAL(void) | |
| 68 jpeg_idct_float (j_decompress_ptr cinfo, jpeg_component_info * compptr, | |
| 69 JCOEFPTR coef_block, | |
| 70 JSAMPARRAY output_buf, JDIMENSION output_col) | |
| 71 { | |
| 72 FAST_FLOAT tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7; | |
| 73 FAST_FLOAT tmp10, tmp11, tmp12, tmp13; | |
| 74 FAST_FLOAT z5, z10, z11, z12, z13; | |
| 75 JCOEFPTR inptr; | |
| 76 FLOAT_MULT_TYPE * quantptr; | |
| 77 FAST_FLOAT * wsptr; | |
| 78 JSAMPROW outptr; | |
| 79 JSAMPLE *range_limit = IDCT_range_limit(cinfo); | |
| 80 int ctr; | |
| 81 FAST_FLOAT workspace[DCTSIZE2]; /* buffers data between passes */ | |
| 82 SHIFT_TEMPS | |
| 83 | |
| 84 /* Pass 1: process columns from input, store into work array. */ | |
| 85 | |
| 86 inptr = coef_block; | |
| 87 quantptr = (FLOAT_MULT_TYPE *) compptr->dct_table; | |
| 88 wsptr = workspace; | |
| 89 for (ctr = DCTSIZE; ctr > 0; ctr--) { | |
| 90 /* Due to quantization, we will usually find that many of the input | |
| 91 * coefficients are zero, especially the AC terms. We can exploit this | |
| 92 * by short-circuiting the IDCT calculation for any column in which all | |
| 93 * the AC terms are zero. In that case each output is equal to the | |
| 94 * DC coefficient (with scale factor as needed). | |
| 95 * With typical images and quantization tables, half or more of the | |
| 96 * column DCT calculations can be simplified this way. | |
| 97 */ | |
| 98 | |
| 99 if (inptr[DCTSIZE*1] == 0 && inptr[DCTSIZE*2] == 0 && | |
| 100 inptr[DCTSIZE*3] == 0 && inptr[DCTSIZE*4] == 0 && | |
| 101 inptr[DCTSIZE*5] == 0 && inptr[DCTSIZE*6] == 0 && | |
| 102 inptr[DCTSIZE*7] == 0) { | |
| 103 /* AC terms all zero */ | |
| 104 FAST_FLOAT dcval = DEQUANTIZE(inptr[DCTSIZE*0], quantptr[DCTSIZE*0]); | |
| 105 | |
| 106 wsptr[DCTSIZE*0] = dcval; | |
| 107 wsptr[DCTSIZE*1] = dcval; | |
| 108 wsptr[DCTSIZE*2] = dcval; | |
| 109 wsptr[DCTSIZE*3] = dcval; | |
| 110 wsptr[DCTSIZE*4] = dcval; | |
| 111 wsptr[DCTSIZE*5] = dcval; | |
| 112 wsptr[DCTSIZE*6] = dcval; | |
| 113 wsptr[DCTSIZE*7] = dcval; | |
| 114 | |
| 115 inptr++; /* advance pointers to next column */ | |
| 116 quantptr++; | |
| 117 wsptr++; | |
| 118 continue; | |
| 119 } | |
| 120 | |
| 121 /* Even part */ | |
| 122 | |
| 123 tmp0 = DEQUANTIZE(inptr[DCTSIZE*0], quantptr[DCTSIZE*0]); | |
| 124 tmp1 = DEQUANTIZE(inptr[DCTSIZE*2], quantptr[DCTSIZE*2]); | |
| 125 tmp2 = DEQUANTIZE(inptr[DCTSIZE*4], quantptr[DCTSIZE*4]); | |
| 126 tmp3 = DEQUANTIZE(inptr[DCTSIZE*6], quantptr[DCTSIZE*6]); | |
| 127 | |
| 128 tmp10 = tmp0 + tmp2; /* phase 3 */ | |
| 129 tmp11 = tmp0 - tmp2; | |
| 130 | |
| 131 tmp13 = tmp1 + tmp3; /* phases 5-3 */ | |
| 132 tmp12 = (tmp1 - tmp3) * ((FAST_FLOAT) 1.414213562) - tmp13; /* 2*c4 */ | |
| 133 | |
| 134 tmp0 = tmp10 + tmp13; /* phase 2 */ | |
| 135 tmp3 = tmp10 - tmp13; | |
| 136 tmp1 = tmp11 + tmp12; | |
| 137 tmp2 = tmp11 - tmp12; | |
| 138 | |
| 139 /* Odd part */ | |
| 140 | |
| 141 tmp4 = DEQUANTIZE(inptr[DCTSIZE*1], quantptr[DCTSIZE*1]); | |
| 142 tmp5 = DEQUANTIZE(inptr[DCTSIZE*3], quantptr[DCTSIZE*3]); | |
| 143 tmp6 = DEQUANTIZE(inptr[DCTSIZE*5], quantptr[DCTSIZE*5]); | |
| 144 tmp7 = DEQUANTIZE(inptr[DCTSIZE*7], quantptr[DCTSIZE*7]); | |
| 145 | |
| 146 z13 = tmp6 + tmp5; /* phase 6 */ | |
| 147 z10 = tmp6 - tmp5; | |
| 148 z11 = tmp4 + tmp7; | |
| 149 z12 = tmp4 - tmp7; | |
| 150 | |
| 151 tmp7 = z11 + z13; /* phase 5 */ | |
| 152 tmp11 = (z11 - z13) * ((FAST_FLOAT) 1.414213562); /* 2*c4 */ | |
| 153 | |
| 154 z5 = (z10 + z12) * ((FAST_FLOAT) 1.847759065); /* 2*c2 */ | |
| 155 tmp10 = ((FAST_FLOAT) 1.082392200) * z12 - z5; /* 2*(c2-c6) */ | |
| 156 tmp12 = ((FAST_FLOAT) -2.613125930) * z10 + z5; /* -2*(c2+c6) */ | |
| 157 | |
| 158 tmp6 = tmp12 - tmp7; /* phase 2 */ | |
| 159 tmp5 = tmp11 - tmp6; | |
| 160 tmp4 = tmp10 + tmp5; | |
| 161 | |
| 162 wsptr[DCTSIZE*0] = tmp0 + tmp7; | |
| 163 wsptr[DCTSIZE*7] = tmp0 - tmp7; | |
| 164 wsptr[DCTSIZE*1] = tmp1 + tmp6; | |
| 165 wsptr[DCTSIZE*6] = tmp1 - tmp6; | |
| 166 wsptr[DCTSIZE*2] = tmp2 + tmp5; | |
| 167 wsptr[DCTSIZE*5] = tmp2 - tmp5; | |
| 168 wsptr[DCTSIZE*4] = tmp3 + tmp4; | |
| 169 wsptr[DCTSIZE*3] = tmp3 - tmp4; | |
| 170 | |
| 171 inptr++; /* advance pointers to next column */ | |
| 172 quantptr++; | |
| 173 wsptr++; | |
| 174 } | |
| 175 | |
| 176 /* Pass 2: process rows from work array, store into output array. */ | |
| 177 /* Note that we must descale the results by a factor of 8 == 2**3. */ | |
| 178 | |
| 179 wsptr = workspace; | |
| 180 for (ctr = 0; ctr < DCTSIZE; ctr++) { | |
| 181 outptr = output_buf[ctr] + output_col; | |
| 182 /* Rows of zeroes can be exploited in the same way as we did with columns. | |
| 183 * However, the column calculation has created many nonzero AC terms, so | |
| 184 * the simplification applies less often (typically 5% to 10% of the time). | |
| 185 * And testing floats for zero is relatively expensive, so we don't bother. | |
| 186 */ | |
| 187 | |
| 188 /* Even part */ | |
| 189 | |
| 190 tmp10 = wsptr[0] + wsptr[4]; | |
| 191 tmp11 = wsptr[0] - wsptr[4]; | |
| 192 | |
| 193 tmp13 = wsptr[2] + wsptr[6]; | |
| 194 tmp12 = (wsptr[2] - wsptr[6]) * ((FAST_FLOAT) 1.414213562) - tmp13; | |
| 195 | |
| 196 tmp0 = tmp10 + tmp13; | |
| 197 tmp3 = tmp10 - tmp13; | |
| 198 tmp1 = tmp11 + tmp12; | |
| 199 tmp2 = tmp11 - tmp12; | |
| 200 | |
| 201 /* Odd part */ | |
| 202 | |
| 203 z13 = wsptr[5] + wsptr[3]; | |
| 204 z10 = wsptr[5] - wsptr[3]; | |
| 205 z11 = wsptr[1] + wsptr[7]; | |
| 206 z12 = wsptr[1] - wsptr[7]; | |
| 207 | |
| 208 tmp7 = z11 + z13; | |
| 209 tmp11 = (z11 - z13) * ((FAST_FLOAT) 1.414213562); | |
| 210 | |
| 211 z5 = (z10 + z12) * ((FAST_FLOAT) 1.847759065); /* 2*c2 */ | |
| 212 tmp10 = ((FAST_FLOAT) 1.082392200) * z12 - z5; /* 2*(c2-c6) */ | |
| 213 tmp12 = ((FAST_FLOAT) -2.613125930) * z10 + z5; /* -2*(c2+c6) */ | |
| 214 | |
| 215 tmp6 = tmp12 - tmp7; | |
| 216 tmp5 = tmp11 - tmp6; | |
| 217 tmp4 = tmp10 + tmp5; | |
| 218 | |
| 219 /* Final output stage: scale down by a factor of 8 and range-limit */ | |
| 220 | |
| 221 outptr[0] = range_limit[(int) DESCALE((INT32) (tmp0 + tmp7), 3) | |
| 222 & RANGE_MASK]; | |
| 223 outptr[7] = range_limit[(int) DESCALE((INT32) (tmp0 - tmp7), 3) | |
| 224 & RANGE_MASK]; | |
| 225 outptr[1] = range_limit[(int) DESCALE((INT32) (tmp1 + tmp6), 3) | |
| 226 & RANGE_MASK]; | |
| 227 outptr[6] = range_limit[(int) DESCALE((INT32) (tmp1 - tmp6), 3) | |
| 228 & RANGE_MASK]; | |
| 229 outptr[2] = range_limit[(int) DESCALE((INT32) (tmp2 + tmp5), 3) | |
| 230 & RANGE_MASK]; | |
| 231 outptr[5] = range_limit[(int) DESCALE((INT32) (tmp2 - tmp5), 3) | |
| 232 & RANGE_MASK]; | |
| 233 outptr[4] = range_limit[(int) DESCALE((INT32) (tmp3 + tmp4), 3) | |
| 234 & RANGE_MASK]; | |
| 235 outptr[3] = range_limit[(int) DESCALE((INT32) (tmp3 - tmp4), 3) | |
| 236 & RANGE_MASK]; | |
| 237 | |
| 238 wsptr += DCTSIZE; /* advance pointer to next row */ | |
| 239 } | |
| 240 } | |
| 241 | |
| 242 #endif /* DCT_FLOAT_SUPPORTED */ | |
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