jfdctflt.cpp (5820B)
1 /* 2 * jfdctflt.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 * forward DCT (Discrete Cosine Transform). 10 * 11 * This implementation should be more accurate than either of the integer 12 * DCT implementations. However, it may not give the same results on all 13 * machines because of differences in roundoff behavior. Speed will depend 14 * on the hardware's floating point capacity. 15 * 16 * A 2-D DCT can be done by 1-D DCT on each row followed by 1-D DCT 17 * on each column. Direct algorithms are also available, but they are 18 * much more complex and seem not to be any faster when reduced to code. 19 * 20 * This implementation is based on Arai, Agui, and Nakajima's algorithm for 21 * scaled DCT. Their original paper (Trans. IEICE E-71(11):1095) is in 22 * Japanese, but the algorithm is described in the Pennebaker & Mitchell 23 * JPEG textbook (see REFERENCES section in file README). The following code 24 * is based directly on figure 4-8 in P&M. 25 * While an 8-point DCT cannot be done in less than 11 multiplies, it is 26 * possible to arrange the computation so that many of the multiplies are 27 * simple scalings of the final outputs. These multiplies can then be 28 * folded into the multiplications or divisions by the JPEG quantization 29 * table entries. The AA&N method leaves only 5 multiplies and 29 adds 30 * to be done in the DCT itself. 31 * The primary disadvantage of this method is that with a fixed-point 32 * implementation, accuracy is lost due to imprecise representation of the 33 * scaled quantization values. However, that problem does not arise if 34 * we use floating point arithmetic. 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_FLOAT_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 8 x8 DCTs. /* deliberate syntax err */ 51 #endif 52 53 54 /* 55 * Perform the forward DCT on one block of samples. 56 */ 57 58 GLOBAL void 59 jpeg_fdct_float( FAST_FLOAT * data ) { 60 FAST_FLOAT tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7; 61 FAST_FLOAT tmp10, tmp11, tmp12, tmp13; 62 FAST_FLOAT z1, z2, z3, z4, z5, z11, z13; 63 FAST_FLOAT * dataptr; 64 int ctr; 65 66 /* Pass 1: process rows. */ 67 68 dataptr = data; 69 for ( ctr = DCTSIZE - 1; ctr >= 0; ctr-- ) { 70 tmp0 = dataptr[0] + dataptr[7]; 71 tmp7 = dataptr[0] - dataptr[7]; 72 tmp1 = dataptr[1] + dataptr[6]; 73 tmp6 = dataptr[1] - dataptr[6]; 74 tmp2 = dataptr[2] + dataptr[5]; 75 tmp5 = dataptr[2] - dataptr[5]; 76 tmp3 = dataptr[3] + dataptr[4]; 77 tmp4 = dataptr[3] - dataptr[4]; 78 79 /* Even part */ 80 81 tmp10 = tmp0 + tmp3;/* phase 2 */ 82 tmp13 = tmp0 - tmp3; 83 tmp11 = tmp1 + tmp2; 84 tmp12 = tmp1 - tmp2; 85 86 dataptr[0] = tmp10 + tmp11;/* phase 3 */ 87 dataptr[4] = tmp10 - tmp11; 88 89 z1 = ( tmp12 + tmp13 ) * ( (FAST_FLOAT) 0.707106781 );/* c4 */ 90 dataptr[2] = tmp13 + z1;/* phase 5 */ 91 dataptr[6] = tmp13 - z1; 92 93 /* Odd part */ 94 95 tmp10 = tmp4 + tmp5;/* phase 2 */ 96 tmp11 = tmp5 + tmp6; 97 tmp12 = tmp6 + tmp7; 98 99 /* The rotator is modified from fig 4-8 to avoid extra negations. */ 100 z5 = ( tmp10 - tmp12 ) * ( (FAST_FLOAT) 0.382683433 );/* c6 */ 101 z2 = ( (FAST_FLOAT) 0.541196100 ) * tmp10 + z5;/* c2-c6 */ 102 z4 = ( (FAST_FLOAT) 1.306562965 ) * tmp12 + z5;/* c2+c6 */ 103 z3 = tmp11 * ( (FAST_FLOAT) 0.707106781 );/* c4 */ 104 105 z11 = tmp7 + z3; /* phase 5 */ 106 z13 = tmp7 - z3; 107 108 dataptr[5] = z13 + z2;/* phase 6 */ 109 dataptr[3] = z13 - z2; 110 dataptr[1] = z11 + z4; 111 dataptr[7] = z11 - z4; 112 113 dataptr += DCTSIZE; /* advance pointer to next row */ 114 } 115 116 /* Pass 2: process columns. */ 117 118 dataptr = data; 119 for ( ctr = DCTSIZE - 1; ctr >= 0; ctr-- ) { 120 tmp0 = dataptr[DCTSIZE * 0] + dataptr[DCTSIZE * 7]; 121 tmp7 = dataptr[DCTSIZE * 0] - dataptr[DCTSIZE * 7]; 122 tmp1 = dataptr[DCTSIZE * 1] + dataptr[DCTSIZE * 6]; 123 tmp6 = dataptr[DCTSIZE * 1] - dataptr[DCTSIZE * 6]; 124 tmp2 = dataptr[DCTSIZE * 2] + dataptr[DCTSIZE * 5]; 125 tmp5 = dataptr[DCTSIZE * 2] - dataptr[DCTSIZE * 5]; 126 tmp3 = dataptr[DCTSIZE * 3] + dataptr[DCTSIZE * 4]; 127 tmp4 = dataptr[DCTSIZE * 3] - dataptr[DCTSIZE * 4]; 128 129 /* Even part */ 130 131 tmp10 = tmp0 + tmp3;/* phase 2 */ 132 tmp13 = tmp0 - tmp3; 133 tmp11 = tmp1 + tmp2; 134 tmp12 = tmp1 - tmp2; 135 136 dataptr[DCTSIZE * 0] = tmp10 + tmp11;/* phase 3 */ 137 dataptr[DCTSIZE * 4] = tmp10 - tmp11; 138 139 z1 = ( tmp12 + tmp13 ) * ( (FAST_FLOAT) 0.707106781 );/* c4 */ 140 dataptr[DCTSIZE * 2] = tmp13 + z1;/* phase 5 */ 141 dataptr[DCTSIZE * 6] = tmp13 - z1; 142 143 /* Odd part */ 144 145 tmp10 = tmp4 + tmp5;/* phase 2 */ 146 tmp11 = tmp5 + tmp6; 147 tmp12 = tmp6 + tmp7; 148 149 /* The rotator is modified from fig 4-8 to avoid extra negations. */ 150 z5 = ( tmp10 - tmp12 ) * ( (FAST_FLOAT) 0.382683433 );/* c6 */ 151 z2 = ( (FAST_FLOAT) 0.541196100 ) * tmp10 + z5;/* c2-c6 */ 152 z4 = ( (FAST_FLOAT) 1.306562965 ) * tmp12 + z5;/* c2+c6 */ 153 z3 = tmp11 * ( (FAST_FLOAT) 0.707106781 );/* c4 */ 154 155 z11 = tmp7 + z3; /* phase 5 */ 156 z13 = tmp7 - z3; 157 158 dataptr[DCTSIZE * 5] = z13 + z2;/* phase 6 */ 159 dataptr[DCTSIZE * 3] = z13 - z2; 160 dataptr[DCTSIZE * 1] = z11 + z4; 161 dataptr[DCTSIZE * 7] = z11 - z4; 162 163 dataptr++; /* advance pointer to next column */ 164 } 165 } 166 167 #endif /* DCT_FLOAT_SUPPORTED */