jidctflt.cpp (9308B)
1 /* 2 * jidctflt.c 3 * 4 * Copyright (C) 1994, 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 8 x8 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 FAST_FLOAT tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7; 72 FAST_FLOAT tmp10, tmp11, tmp12, tmp13; 73 FAST_FLOAT z5, z10, z11, z12, z13; 74 JCOEFPTR inptr; 75 FLOAT_MULT_TYPE * quantptr; 76 FAST_FLOAT * wsptr; 77 JSAMPROW outptr; 78 JSAMPLE * range_limit = IDCT_range_limit( cinfo ); 79 int ctr; 80 FAST_FLOAT workspace[DCTSIZE2];/* buffers data between passes */ 81 SHIFT_TEMPS 82 83 /* Pass 1: process columns from input, store into work array. */ 84 85 inptr = coef_block; 86 quantptr = (FLOAT_MULT_TYPE *) compptr->dct_table; 87 wsptr = workspace; 88 for ( ctr = DCTSIZE; ctr > 0; ctr-- ) { 89 /* Due to quantization, we will usually find that many of the input 90 * coefficients are zero, especially the AC terms. We can exploit this 91 * by short-circuiting the IDCT calculation for any column in which all 92 * the AC terms are zero. In that case each output is equal to the 93 * DC coefficient (with scale factor as needed). 94 * With typical images and quantization tables, half or more of the 95 * column DCT calculations can be simplified this way. 96 */ 97 98 if ( ( inptr[DCTSIZE * 1] | inptr[DCTSIZE * 2] | inptr[DCTSIZE * 3] | 99 inptr[DCTSIZE * 4] | inptr[DCTSIZE * 5] | inptr[DCTSIZE * 6] | 100 inptr[DCTSIZE * 7] ) == 0 ) { 101 /* AC terms all zero */ 102 FAST_FLOAT dcval = DEQUANTIZE( inptr[DCTSIZE * 0], quantptr[DCTSIZE * 0] ); 103 104 wsptr[DCTSIZE * 0] = dcval; 105 wsptr[DCTSIZE * 1] = dcval; 106 wsptr[DCTSIZE * 2] = dcval; 107 wsptr[DCTSIZE * 3] = dcval; 108 wsptr[DCTSIZE * 4] = dcval; 109 wsptr[DCTSIZE * 5] = dcval; 110 wsptr[DCTSIZE * 6] = dcval; 111 wsptr[DCTSIZE * 7] = dcval; 112 113 inptr++; /* advance pointers to next column */ 114 quantptr++; 115 wsptr++; 116 continue; 117 } 118 119 /* Even part */ 120 121 tmp0 = DEQUANTIZE( inptr[DCTSIZE * 0], quantptr[DCTSIZE * 0] ); 122 tmp1 = DEQUANTIZE( inptr[DCTSIZE * 2], quantptr[DCTSIZE * 2] ); 123 tmp2 = DEQUANTIZE( inptr[DCTSIZE * 4], quantptr[DCTSIZE * 4] ); 124 tmp3 = DEQUANTIZE( inptr[DCTSIZE * 6], quantptr[DCTSIZE * 6] ); 125 126 tmp10 = tmp0 + tmp2;/* phase 3 */ 127 tmp11 = tmp0 - tmp2; 128 129 tmp13 = tmp1 + tmp3;/* phases 5-3 */ 130 tmp12 = ( tmp1 - tmp3 ) * ( (FAST_FLOAT) 1.414213562 ) - tmp13;/* 2*c4 */ 131 132 tmp0 = tmp10 + tmp13;/* phase 2 */ 133 tmp3 = tmp10 - tmp13; 134 tmp1 = tmp11 + tmp12; 135 tmp2 = tmp11 - tmp12; 136 137 /* Odd part */ 138 139 tmp4 = DEQUANTIZE( inptr[DCTSIZE * 1], quantptr[DCTSIZE * 1] ); 140 tmp5 = DEQUANTIZE( inptr[DCTSIZE * 3], quantptr[DCTSIZE * 3] ); 141 tmp6 = DEQUANTIZE( inptr[DCTSIZE * 5], quantptr[DCTSIZE * 5] ); 142 tmp7 = DEQUANTIZE( inptr[DCTSIZE * 7], quantptr[DCTSIZE * 7] ); 143 144 z13 = tmp6 + tmp5; /* phase 6 */ 145 z10 = tmp6 - tmp5; 146 z11 = tmp4 + tmp7; 147 z12 = tmp4 - tmp7; 148 149 tmp7 = z11 + z13; /* phase 5 */ 150 tmp11 = ( z11 - z13 ) * ( (FAST_FLOAT) 1.414213562 );/* 2*c4 */ 151 152 z5 = ( z10 + z12 ) * ( (FAST_FLOAT) 1.847759065 );/* 2*c2 */ 153 tmp10 = ( (FAST_FLOAT) 1.082392200 ) * z12 - z5;/* 2*(c2-c6) */ 154 tmp12 = ( (FAST_FLOAT) -2.613125930 ) * z10 + z5;/* -2*(c2+c6) */ 155 156 tmp6 = tmp12 - tmp7;/* phase 2 */ 157 tmp5 = tmp11 - tmp6; 158 tmp4 = tmp10 + tmp5; 159 160 wsptr[DCTSIZE * 0] = tmp0 + tmp7; 161 wsptr[DCTSIZE * 7] = tmp0 - tmp7; 162 wsptr[DCTSIZE * 1] = tmp1 + tmp6; 163 wsptr[DCTSIZE * 6] = tmp1 - tmp6; 164 wsptr[DCTSIZE * 2] = tmp2 + tmp5; 165 wsptr[DCTSIZE * 5] = tmp2 - tmp5; 166 wsptr[DCTSIZE * 4] = tmp3 + tmp4; 167 wsptr[DCTSIZE * 3] = tmp3 - tmp4; 168 169 inptr++; /* advance pointers to next column */ 170 quantptr++; 171 wsptr++; 172 } 173 174 /* Pass 2: process rows from work array, store into output array. */ 175 /* Note that we must descale the results by a factor of 8 == 2**3. */ 176 177 wsptr = workspace; 178 for ( ctr = 0; ctr < DCTSIZE; ctr++ ) { 179 outptr = output_buf[ctr] + output_col; 180 /* Rows of zeroes can be exploited in the same way as we did with columns. 181 * However, the column calculation has created many nonzero AC terms, so 182 * the simplification applies less often (typically 5% to 10% of the time). 183 * And testing floats for zero is relatively expensive, so we don't bother. 184 */ 185 186 /* Even part */ 187 188 tmp10 = wsptr[0] + wsptr[4]; 189 tmp11 = wsptr[0] - wsptr[4]; 190 191 tmp13 = wsptr[2] + wsptr[6]; 192 tmp12 = ( wsptr[2] - wsptr[6] ) * ( (FAST_FLOAT) 1.414213562 ) - tmp13; 193 194 tmp0 = tmp10 + tmp13; 195 tmp3 = tmp10 - tmp13; 196 tmp1 = tmp11 + tmp12; 197 tmp2 = tmp11 - tmp12; 198 199 /* Odd part */ 200 201 z13 = wsptr[5] + wsptr[3]; 202 z10 = wsptr[5] - wsptr[3]; 203 z11 = wsptr[1] + wsptr[7]; 204 z12 = wsptr[1] - wsptr[7]; 205 206 tmp7 = z11 + z13; 207 tmp11 = ( z11 - z13 ) * ( (FAST_FLOAT) 1.414213562 ); 208 209 z5 = ( z10 + z12 ) * ( (FAST_FLOAT) 1.847759065 );/* 2*c2 */ 210 tmp10 = ( (FAST_FLOAT) 1.082392200 ) * z12 - z5;/* 2*(c2-c6) */ 211 tmp12 = ( (FAST_FLOAT) -2.613125930 ) * z10 + z5;/* -2*(c2+c6) */ 212 213 tmp6 = tmp12 - tmp7; 214 tmp5 = tmp11 - tmp6; 215 tmp4 = tmp10 + tmp5; 216 217 /* Final output stage: scale down by a factor of 8 and range-limit */ 218 219 outptr[0] = range_limit[(int) DESCALE( (INT32) ( tmp0 + tmp7 ), 3 ) 220 & RANGE_MASK]; 221 outptr[7] = range_limit[(int) DESCALE( (INT32) ( tmp0 - tmp7 ), 3 ) 222 & RANGE_MASK]; 223 outptr[1] = range_limit[(int) DESCALE( (INT32) ( tmp1 + tmp6 ), 3 ) 224 & RANGE_MASK]; 225 outptr[6] = range_limit[(int) DESCALE( (INT32) ( tmp1 - tmp6 ), 3 ) 226 & RANGE_MASK]; 227 outptr[2] = range_limit[(int) DESCALE( (INT32) ( tmp2 + tmp5 ), 3 ) 228 & RANGE_MASK]; 229 outptr[5] = range_limit[(int) DESCALE( (INT32) ( tmp2 - tmp5 ), 3 ) 230 & RANGE_MASK]; 231 outptr[4] = range_limit[(int) DESCALE( (INT32) ( tmp3 + tmp4 ), 3 ) 232 & RANGE_MASK]; 233 outptr[3] = range_limit[(int) DESCALE( (INT32) ( tmp3 - tmp4 ), 3 ) 234 & RANGE_MASK]; 235 236 wsptr += DCTSIZE; /* advance pointer to next row */ 237 } 238 } 239 240 #endif /* DCT_FLOAT_SUPPORTED */