compiler/libjpeg/main/jfdctfst.c

   1 /*
   2     $Id$
   3 */
   4
   5 /*
   6  * jfdctfst.c
   7  *
   8  * Copyright (C) 1994-1996, Thomas G. Lane.
   9  * This file is part of the Independent JPEG Group's software.
  10  * For conditions of distribution and use, see the accompanying README file.
  11  *
  12  * This file contains a fast, not so accurate integer implementation of the
  13  * forward DCT (Discrete Cosine Transform).
  14  *
  15  * A 2-D DCT can be done by 1-D DCT on each row followed by 1-D DCT
  16  * on each column.  Direct algorithms are also available, but they are
  17  * much more complex and seem not to be any faster when reduced to code.
  18  *
  19  * This implementation is based on Arai, Agui, and Nakajima's algorithm for
  20  * scaled DCT.  Their original paper (Trans. IEICE E-71(11):1095) is in
  21  * Japanese, but the algorithm is described in the Pennebaker & Mitchell
  22  * JPEG textbook (see REFERENCES section in file README).  The following code
  23  * is based directly on figure 4-8 in P&M.
  24  * While an 8-point DCT cannot be done in less than 11 multiplies, it is
  25  * possible to arrange the computation so that many of the multiplies are
  26  * simple scalings of the final outputs.  These multiplies can then be
  27  * folded into the multiplications or divisions by the JPEG quantization
  28  * table entries.  The AA&N method leaves only 5 multiplies and 29 adds
  29  * to be done in the DCT itself.
  30  * The primary disadvantage of this method is that with fixed-point math,
  31  * accuracy is lost due to imprecise representation of the scaled
  32  * quantization values.  The smaller the quantization table entry, the less
  33  * precise the scaled value, so this implementation does worse with high-
  34  * quality-setting files than with low-quality ones.
  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_IFAST_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 /* Scaling decisions are generally the same as in the LL&M algorithm;
  55  * see jfdctint.c for more details.  However, we choose to descale
  56  * (right shift) multiplication products as soon as they are formed,
  57  * rather than carrying additional fractional bits into subsequent additions.
  58  * This compromises accuracy slightly, but it lets us save a few shifts.
  59  * More importantly, 16-bit arithmetic is then adequate (for 8-bit samples)
  60  * everywhere except in the multiplications proper; this saves a good deal
  61  * of work on 16-bit-int machines.
  62  *
  63  * Again to save a few shifts, the intermediate results between pass 1 and
  64  * pass 2 are not upscaled, but are represented only to integral precision.
  65  *
  66  * A final compromise is to represent the multiplicative constants to only
  67  * 8 fractional bits, rather than 13.  This saves some shifting work on some
  68  * machines, and may also reduce the cost of multiplication (since there
  69  * are fewer one-bits in the constants).
  70  */
  71
  72 #define CONST_BITS  8
  73
  74
  75 /* Some C compilers fail to reduce "FIX(constant)" at compile time, thus
  76  * causing a lot of useless floating-point operations at run time.
  77  * To get around this we use the following pre-calculated constants.
  78  * If you change CONST_BITS you may want to add appropriate values.
  79  * (With a reasonable C compiler, you can just rely on the FIX() macro...)
  80  */
  81
  82 #if CONST_BITS == 8
  83 #define FIX_0_382683433  ((INT32)   98)         /* FIX(0.382683433) */
  84 #define FIX_0_541196100  ((INT32)  139)         /* FIX(0.541196100) */
  85 #define FIX_0_707106781  ((INT32)  181)         /* FIX(0.707106781) */
  86 #define FIX_1_306562965  ((INT32)  334)         /* FIX(1.306562965) */
  87 #else
  88 #define FIX_0_382683433  FIX(0.382683433)
  89 #define FIX_0_541196100  FIX(0.541196100)
  90 #define FIX_0_707106781  FIX(0.707106781)
  91 #define FIX_1_306562965  FIX(1.306562965)
  92 #endif
  93
  94
  95 /* We can gain a little more speed, with a further compromise in accuracy,
  96  * by omitting the addition in a descaling shift.  This yields an incorrectly
  97  * rounded result half the time...
  98  */
  99
 100 #ifndef USE_ACCURATE_ROUNDING
 101 #undef DESCALE
 102 #define DESCALE(x,n)  RIGHT_SHIFT(x, n)
 103 #endif
 104
 105
 106 /* Multiply a DCTELEM variable by an INT32 constant, and immediately
 107  * descale to yield a DCTELEM result.
 108  */
 109
 110 #define MULTIPLY(var,const)  ((DCTELEM) DESCALE((var) * (const), CONST_BITS))
 111
 112
 113 /*
 114  * Perform the forward DCT on one block of samples.
 115  */
 116
 117 JGLOBAL(void)
 118 jpeg_fdct_ifast (DCTELEM * data)
 119 {
 120   DCTELEM tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7;
 121   DCTELEM tmp10, tmp11, tmp12, tmp13;
 122   DCTELEM z1, z2, z3, z4, z5, z11, z13;
 123   DCTELEM *dataptr;
 124   int ctr;
 125   SHIFT_TEMPS
 126
 127   /* Pass 1: process rows. */
 128
 129   dataptr = data;
 130   for (ctr = DCTSIZE-1; ctr >= 0; ctr--) {
 131     tmp0 = dataptr[0] + dataptr[7];
 132     tmp7 = dataptr[0] - dataptr[7];
 133     tmp1 = dataptr[1] + dataptr[6];
 134     tmp6 = dataptr[1] - dataptr[6];
 135     tmp2 = dataptr[2] + dataptr[5];
 136     tmp5 = dataptr[2] - dataptr[5];
 137     tmp3 = dataptr[3] + dataptr[4];
 138     tmp4 = dataptr[3] - dataptr[4];
 139
 140     /* Even part */
 141
 142     tmp10 = tmp0 + tmp3;        /* phase 2 */
 143     tmp13 = tmp0 - tmp3;
 144     tmp11 = tmp1 + tmp2;
 145     tmp12 = tmp1 - tmp2;
 146
 147     dataptr[0] = tmp10 + tmp11; /* phase 3 */
 148     dataptr[4] = tmp10 - tmp11;
 149
 150     z1 = MULTIPLY(tmp12 + tmp13, FIX_0_707106781); /* c4 */
 151     dataptr[2] = tmp13 + z1;    /* phase 5 */
 152     dataptr[6] = tmp13 - z1;
 153
 154     /* Odd part */
 155
 156     tmp10 = tmp4 + tmp5;        /* phase 2 */
 157     tmp11 = tmp5 + tmp6;
 158     tmp12 = tmp6 + tmp7;
 159
 160     /* The rotator is modified from fig 4-8 to avoid extra negations. */
 161     z5 = MULTIPLY(tmp10 - tmp12, FIX_0_382683433); /* c6 */
 162     z2 = MULTIPLY(tmp10, FIX_0_541196100) + z5; /* c2-c6 */
 163     z4 = MULTIPLY(tmp12, FIX_1_306562965) + z5; /* c2+c6 */
 164     z3 = MULTIPLY(tmp11, FIX_0_707106781); /* c4 */
 165
 166     z11 = tmp7 + z3;            /* phase 5 */
 167     z13 = tmp7 - z3;
 168
 169     dataptr[5] = z13 + z2;      /* phase 6 */
 170     dataptr[3] = z13 - z2;
 171     dataptr[1] = z11 + z4;
 172     dataptr[7] = z11 - z4;
 173
 174     dataptr += DCTSIZE;         /* advance pointer to next row */
 175   }
 176
 177   /* Pass 2: process columns. */
 178
 179   dataptr = data;
 180   for (ctr = DCTSIZE-1; ctr >= 0; ctr--) {
 181     tmp0 = dataptr[DCTSIZE*0] + dataptr[DCTSIZE*7];
 182     tmp7 = dataptr[DCTSIZE*0] - dataptr[DCTSIZE*7];
 183     tmp1 = dataptr[DCTSIZE*1] + dataptr[DCTSIZE*6];
 184     tmp6 = dataptr[DCTSIZE*1] - dataptr[DCTSIZE*6];
 185     tmp2 = dataptr[DCTSIZE*2] + dataptr[DCTSIZE*5];
 186     tmp5 = dataptr[DCTSIZE*2] - dataptr[DCTSIZE*5];
 187     tmp3 = dataptr[DCTSIZE*3] + dataptr[DCTSIZE*4];
 188     tmp4 = dataptr[DCTSIZE*3] - dataptr[DCTSIZE*4];
 189
 190     /* Even part */
 191
 192     tmp10 = tmp0 + tmp3;        /* phase 2 */
 193     tmp13 = tmp0 - tmp3;
 194     tmp11 = tmp1 + tmp2;
 195     tmp12 = tmp1 - tmp2;
 196
 197     dataptr[DCTSIZE*0] = tmp10 + tmp11; /* phase 3 */
 198     dataptr[DCTSIZE*4] = tmp10 - tmp11;
 199
 200     z1 = MULTIPLY(tmp12 + tmp13, FIX_0_707106781); /* c4 */
 201     dataptr[DCTSIZE*2] = tmp13 + z1; /* phase 5 */
 202     dataptr[DCTSIZE*6] = tmp13 - z1;
 203
 204     /* Odd part */
 205
 206     tmp10 = tmp4 + tmp5;        /* phase 2 */
 207     tmp11 = tmp5 + tmp6;
 208     tmp12 = tmp6 + tmp7;
 209
 210     /* The rotator is modified from fig 4-8 to avoid extra negations. */
 211     z5 = MULTIPLY(tmp10 - tmp12, FIX_0_382683433); /* c6 */
 212     z2 = MULTIPLY(tmp10, FIX_0_541196100) + z5; /* c2-c6 */
 213     z4 = MULTIPLY(tmp12, FIX_1_306562965) + z5; /* c2+c6 */
 214     z3 = MULTIPLY(tmp11, FIX_0_707106781); /* c4 */
 215
 216     z11 = tmp7 + z3;            /* phase 5 */
 217     z13 = tmp7 - z3;
 218
 219     dataptr[DCTSIZE*5] = z13 + z2; /* phase 6 */
 220     dataptr[DCTSIZE*3] = z13 - z2;
 221     dataptr[DCTSIZE*1] = z11 + z4;
 222     dataptr[DCTSIZE*7] = z11 - z4;
 223
 224     dataptr++;                  /* advance pointer to next column */
 225   }
 226 }
 227
 228 #endif /* DCT_IFAST_SUPPORTED */