workbench/libs/jpeg/jidctflt.c

   1 /*
   2  * jidctflt.c
   3  *
   4  * Copyright (C) 1994-1998, Thomas G. Lane.
   5  * Modified 2010 by Guido Vollbeding.
   6  * This file is part of the Independent JPEG Group's software.
   7  * For conditions of distribution and use, see the accompanying README file.
   8  *
   9  * This file contains a floating-point implementation of the
  10  * inverse DCT (Discrete Cosine Transform).  In the IJG code, this routine
  11  * must also perform dequantization of the input coefficients.
  12  *
  13  * This implementation should be more accurate than either of the integer
  14  * IDCT implementations.  However, it may not give the same results on all
  15  * machines because of differences in roundoff behavior.  Speed will depend
  16  * on the hardware's floating point capacity.
  17  *
  18  * A 2-D IDCT can be done by 1-D IDCT on each column followed by 1-D IDCT
  19  * on each row (or vice versa, but it's more convenient to emit a row at
  20  * a time).  Direct algorithms are also available, but they are much more
  21  * complex and seem not to be any faster when reduced to code.
  22  *
  23  * This implementation is based on Arai, Agui, and Nakajima's algorithm for
  24  * scaled DCT.  Their original paper (Trans. IEICE E-71(11):1095) is in
  25  * Japanese, but the algorithm is described in the Pennebaker & Mitchell
  26  * JPEG textbook (see REFERENCES section in file README).  The following code
  27  * is based directly on figure 4-8 in P&M.
  28  * While an 8-point DCT cannot be done in less than 11 multiplies, it is
  29  * possible to arrange the computation so that many of the multiplies are
  30  * simple scalings of the final outputs.  These multiplies can then be
  31  * folded into the multiplications or divisions by the JPEG quantization
  32  * table entries.  The AA&N method leaves only 5 multiplies and 29 adds
  33  * to be done in the DCT itself.
  34  * The primary disadvantage of this method is that with a fixed-point
  35  * implementation, accuracy is lost due to imprecise representation of the
  36  * scaled quantization values.  However, that problem does not arise if
  37  * we use floating point arithmetic.
  38  */
  39
  40 #define JPEG_INTERNALS
  41 #include "jinclude.h"
  42 #include "jpeglib.h"
  43 #include "jdct.h"               /* Private declarations for DCT subsystem */
  44
  45 #ifdef DCT_FLOAT_SUPPORTED
  46
  47
  48 /*
  49  * This module is specialized to the case DCTSIZE = 8.
  50  */
  51
  52 #if DCTSIZE != 8
  53   Sorry, this code only copes with 8x8 DCTs. /* deliberate syntax err */
  54 #endif
  55
  56
  57 /* Dequantize a coefficient by multiplying it by the multiplier-table
  58  * entry; produce a float result.
  59  */
  60
  61 #define DEQUANTIZE(coef,quantval)  (((FAST_FLOAT) (coef)) * (quantval))
  62
  63
  64 /*
  65  * Perform dequantization and inverse DCT on one block of coefficients.
  66  */
  67
  68 GLOBAL(void)
  69 jpeg_idct_float (j_decompress_ptr cinfo, jpeg_component_info * compptr,
  70                  JCOEFPTR coef_block,
  71                  JSAMPARRAY output_buf, JDIMENSION output_col)
  72 {
  73   FAST_FLOAT tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7;
  74   FAST_FLOAT tmp10, tmp11, tmp12, tmp13;
  75   FAST_FLOAT z5, z10, z11, z12, z13;
  76   JCOEFPTR inptr;
  77   FLOAT_MULT_TYPE * quantptr;
  78   FAST_FLOAT * wsptr;
  79   JSAMPROW outptr;
  80   JSAMPLE *range_limit = cinfo->sample_range_limit;
  81   int ctr;
  82   FAST_FLOAT workspace[DCTSIZE2]; /* buffers data between passes */
  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 = z5 - z12 * ((FAST_FLOAT) 1.082392200); /* 2*(c2-c6) */
 156     tmp12 = z5 - z10 * ((FAST_FLOAT) 2.613125930); /* 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*3] = tmp3 + tmp4;
 169     wsptr[DCTSIZE*4] = 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
 178   wsptr = workspace;
 179   for (ctr = 0; ctr < DCTSIZE; ctr++) {
 180     outptr = output_buf[ctr] + output_col;
 181     /* Rows of zeroes can be exploited in the same way as we did with columns.
 182      * However, the column calculation has created many nonzero AC terms, so
 183      * the simplification applies less often (typically 5% to 10% of the time).
 184      * And testing floats for zero is relatively expensive, so we don't bother.
 185      */
 186
 187     /* Even part */
 188
 189     /* Apply signed->unsigned and prepare float->int conversion */
 190     z5 = wsptr[0] + ((FAST_FLOAT) CENTERJSAMPLE + (FAST_FLOAT) 0.5);
 191     tmp10 = z5 + wsptr[4];
 192     tmp11 = z5 - wsptr[4];
 193
 194     tmp13 = wsptr[2] + wsptr[6];
 195     tmp12 = (wsptr[2] - wsptr[6]) * ((FAST_FLOAT) 1.414213562) - tmp13;
 196
 197     tmp0 = tmp10 + tmp13;
 198     tmp3 = tmp10 - tmp13;
 199     tmp1 = tmp11 + tmp12;
 200     tmp2 = tmp11 - tmp12;
 201
 202     /* Odd part */
 203
 204     z13 = wsptr[5] + wsptr[3];
 205     z10 = wsptr[5] - wsptr[3];
 206     z11 = wsptr[1] + wsptr[7];
 207     z12 = wsptr[1] - wsptr[7];
 208
 209     tmp7 = z11 + z13;
 210     tmp11 = (z11 - z13) * ((FAST_FLOAT) 1.414213562);
 211
 212     z5 = (z10 + z12) * ((FAST_FLOAT) 1.847759065); /* 2*c2 */
 213     tmp10 = z5 - z12 * ((FAST_FLOAT) 1.082392200); /* 2*(c2-c6) */
 214     tmp12 = z5 - z10 * ((FAST_FLOAT) 2.613125930); /* 2*(c2+c6) */
 215
 216     tmp6 = tmp12 - tmp7;
 217     tmp5 = tmp11 - tmp6;
 218     tmp4 = tmp10 - tmp5;
 219
 220     /* Final output stage: float->int conversion and range-limit */
 221
 222     outptr[0] = range_limit[((int) (tmp0 + tmp7)) & RANGE_MASK];
 223     outptr[7] = range_limit[((int) (tmp0 - tmp7)) & RANGE_MASK];
 224     outptr[1] = range_limit[((int) (tmp1 + tmp6)) & RANGE_MASK];
 225     outptr[6] = range_limit[((int) (tmp1 - tmp6)) & RANGE_MASK];
 226     outptr[2] = range_limit[((int) (tmp2 + tmp5)) & RANGE_MASK];
 227     outptr[5] = range_limit[((int) (tmp2 - tmp5)) & RANGE_MASK];
 228     outptr[3] = range_limit[((int) (tmp3 + tmp4)) & RANGE_MASK];
 229     outptr[4] = range_limit[((int) (tmp3 - tmp4)) & RANGE_MASK];
 230
 231     wsptr += DCTSIZE;           /* advance pointer to next row */
 232   }
 233 }
 234
 235 #endif /* DCT_FLOAT_SUPPORTED */