src/jpeg/jfdctflt.c

   1 /*
   2  * jfdctflt.c
   3  *
   4  * Copyright (C) 1994-1996, 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 8x8 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 {
  61   FAST_FLOAT tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7;
  62   FAST_FLOAT tmp10, tmp11, tmp12, tmp13;
  63   FAST_FLOAT z1, z2, z3, z4, z5, z11, z13;
  64   FAST_FLOAT *dataptr;
  65   int ctr;
  66
  67   /* Pass 1: process rows. */
  68
  69   dataptr = data;
  70   for (ctr = DCTSIZE-1; ctr >= 0; ctr--) {
  71     tmp0 = dataptr[0] + dataptr[7];
  72     tmp7 = dataptr[0] - dataptr[7];
  73     tmp1 = dataptr[1] + dataptr[6];
  74     tmp6 = dataptr[1] - dataptr[6];
  75     tmp2 = dataptr[2] + dataptr[5];
  76     tmp5 = dataptr[2] - dataptr[5];
  77     tmp3 = dataptr[3] + dataptr[4];
  78     tmp4 = dataptr[3] - dataptr[4];
  79
  80     /* Even part */
  81
  82     tmp10 = tmp0 + tmp3;        /* phase 2 */
  83     tmp13 = tmp0 - tmp3;
  84     tmp11 = tmp1 + tmp2;
  85     tmp12 = tmp1 - tmp2;
  86
  87     dataptr[0] = tmp10 + tmp11; /* phase 3 */
  88     dataptr[4] = tmp10 - tmp11;
  89
  90     z1 = (tmp12 + tmp13) * ((FAST_FLOAT) 0.707106781); /* c4 */
  91     dataptr[2] = tmp13 + z1;    /* phase 5 */
  92     dataptr[6] = tmp13 - z1;
  93
  94     /* Odd part */
  95
  96     tmp10 = tmp4 + tmp5;        /* phase 2 */
  97     tmp11 = tmp5 + tmp6;
  98     tmp12 = tmp6 + tmp7;
  99
 100     /* The rotator is modified from fig 4-8 to avoid extra negations. */
 101     z5 = (tmp10 - tmp12) * ((FAST_FLOAT) 0.382683433); /* c6 */
 102     z2 = ((FAST_FLOAT) 0.541196100) * tmp10 + z5; /* c2-c6 */
 103     z4 = ((FAST_FLOAT) 1.306562965) * tmp12 + z5; /* c2+c6 */
 104     z3 = tmp11 * ((FAST_FLOAT) 0.707106781); /* c4 */
 105
 106     z11 = tmp7 + z3;            /* phase 5 */
 107     z13 = tmp7 - z3;
 108
 109     dataptr[5] = z13 + z2;      /* phase 6 */
 110     dataptr[3] = z13 - z2;
 111     dataptr[1] = z11 + z4;
 112     dataptr[7] = z11 - z4;
 113
 114     dataptr += DCTSIZE;         /* advance pointer to next row */
 115   }
 116
 117   /* Pass 2: process columns. */
 118
 119   dataptr = data;
 120   for (ctr = DCTSIZE-1; ctr >= 0; ctr--) {
 121     tmp0 = dataptr[DCTSIZE*0] + dataptr[DCTSIZE*7];
 122     tmp7 = dataptr[DCTSIZE*0] - dataptr[DCTSIZE*7];
 123     tmp1 = dataptr[DCTSIZE*1] + dataptr[DCTSIZE*6];
 124     tmp6 = dataptr[DCTSIZE*1] - dataptr[DCTSIZE*6];
 125     tmp2 = dataptr[DCTSIZE*2] + dataptr[DCTSIZE*5];
 126     tmp5 = dataptr[DCTSIZE*2] - dataptr[DCTSIZE*5];
 127     tmp3 = dataptr[DCTSIZE*3] + dataptr[DCTSIZE*4];
 128     tmp4 = dataptr[DCTSIZE*3] - dataptr[DCTSIZE*4];
 129
 130     /* Even part */
 131
 132     tmp10 = tmp0 + tmp3;        /* phase 2 */
 133     tmp13 = tmp0 - tmp3;
 134     tmp11 = tmp1 + tmp2;
 135     tmp12 = tmp1 - tmp2;
 136
 137     dataptr[DCTSIZE*0] = tmp10 + tmp11; /* phase 3 */
 138     dataptr[DCTSIZE*4] = tmp10 - tmp11;
 139
 140     z1 = (tmp12 + tmp13) * ((FAST_FLOAT) 0.707106781); /* c4 */
 141     dataptr[DCTSIZE*2] = tmp13 + z1; /* phase 5 */
 142     dataptr[DCTSIZE*6] = tmp13 - z1;
 143
 144     /* Odd part */
 145
 146     tmp10 = tmp4 + tmp5;        /* phase 2 */
 147     tmp11 = tmp5 + tmp6;
 148     tmp12 = tmp6 + tmp7;
 149
 150     /* The rotator is modified from fig 4-8 to avoid extra negations. */
 151     z5 = (tmp10 - tmp12) * ((FAST_FLOAT) 0.382683433); /* c6 */
 152     z2 = ((FAST_FLOAT) 0.541196100) * tmp10 + z5; /* c2-c6 */
 153     z4 = ((FAST_FLOAT) 1.306562965) * tmp12 + z5; /* c2+c6 */
 154     z3 = tmp11 * ((FAST_FLOAT) 0.707106781); /* c4 */
 155
 156     z11 = tmp7 + z3;            /* phase 5 */
 157     z13 = tmp7 - z3;
 158
 159     dataptr[DCTSIZE*5] = z13 + z2; /* phase 6 */
 160     dataptr[DCTSIZE*3] = z13 - z2;
 161     dataptr[DCTSIZE*1] = z11 + z4;
 162     dataptr[DCTSIZE*7] = z11 - z4;
 163
 164     dataptr++;                  /* advance pointer to next column */
 165   }
 166 }
 167
 168 #endif /* DCT_FLOAT_SUPPORTED */