libgo/go/image/jpeg/idct.go

   1 // Copyright 2009 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 package jpeg
   6
   7 // This is a Go translation of idct.c from
   8 //
   9 // http://standards.iso.org/ittf/PubliclyAvailableStandards/ISO_IEC_13818-4_2004_Conformance_Testing/Video/verifier/mpeg2decode_960109.tar.gz
  10 //
  11 // which carries the following notice:
  12
  13 /* Copyright (C) 1996, MPEG Software Simulation Group. All Rights Reserved. */
  14
  15 /*
  16  * Disclaimer of Warranty
  17  *
  18  * These software programs are available to the user without any license fee or
  19  * royalty on an "as is" basis.  The MPEG Software Simulation Group disclaims
  20  * any and all warranties, whether express, implied, or statuary, including any
  21  * implied warranties or merchantability or of fitness for a particular
  22  * purpose.  In no event shall the copyright-holder be liable for any
  23  * incidental, punitive, or consequential damages of any kind whatsoever
  24  * arising from the use of these programs.
  25  *
  26  * This disclaimer of warranty extends to the user of these programs and user's
  27  * customers, employees, agents, transferees, successors, and assigns.
  28  *
  29  * The MPEG Software Simulation Group does not represent or warrant that the
  30  * programs furnished hereunder are free of infringement of any third-party
  31  * patents.
  32  *
  33  * Commercial implementations of MPEG-1 and MPEG-2 video, including shareware,
  34  * are subject to royalty fees to patent holders.  Many of these patents are
  35  * general enough such that they are unavoidable regardless of implementation
  36  * design.
  37  *
  38  */
  39
  40 const blockSize = 64 // A DCT block is 8x8.
  41
  42 type block [blockSize]int32
  43
  44 const (
  45         w1 = 2841 // 2048*sqrt(2)*cos(1*pi/16)
  46         w2 = 2676 // 2048*sqrt(2)*cos(2*pi/16)
  47         w3 = 2408 // 2048*sqrt(2)*cos(3*pi/16)
  48         w5 = 1609 // 2048*sqrt(2)*cos(5*pi/16)
  49         w6 = 1108 // 2048*sqrt(2)*cos(6*pi/16)
  50         w7 = 565  // 2048*sqrt(2)*cos(7*pi/16)
  51
  52         w1pw7 = w1 + w7
  53         w1mw7 = w1 - w7
  54         w2pw6 = w2 + w6
  55         w2mw6 = w2 - w6
  56         w3pw5 = w3 + w5
  57         w3mw5 = w3 - w5
  58
  59         r2 = 181 // 256/sqrt(2)
  60 )
  61
  62 // idct performs a 2-D Inverse Discrete Cosine Transformation.
  63 //
  64 // The input coefficients should already have been multiplied by the
  65 // appropriate quantization table. We use fixed-point computation, with the
  66 // number of bits for the fractional component varying over the intermediate
  67 // stages.
  68 //
  69 // For more on the actual algorithm, see Z. Wang, "Fast algorithms for the
  70 // discrete W transform and for the discrete Fourier transform", IEEE Trans. on
  71 // ASSP, Vol. ASSP- 32, pp. 803-816, Aug. 1984.
  72 func idct(src *block) {
  73         // Horizontal 1-D IDCT.
  74         for y := 0; y < 8; y++ {
  75                 y8 := y * 8
  76                 // If all the AC components are zero, then the IDCT is trivial.
  77                 if src[y8+1] == 0 && src[y8+2] == 0 && src[y8+3] == 0 &&
  78                         src[y8+4] == 0 && src[y8+5] == 0 && src[y8+6] == 0 && src[y8+7] == 0 {
  79                         dc := src[y8+0] << 3
  80                         src[y8+0] = dc
  81                         src[y8+1] = dc
  82                         src[y8+2] = dc
  83                         src[y8+3] = dc
  84                         src[y8+4] = dc
  85                         src[y8+5] = dc
  86                         src[y8+6] = dc
  87                         src[y8+7] = dc
  88                         continue
  89                 }
  90
  91                 // Prescale.
  92                 x0 := (src[y8+0] << 11) + 128
  93                 x1 := src[y8+4] << 11
  94                 x2 := src[y8+6]
  95                 x3 := src[y8+2]
  96                 x4 := src[y8+1]
  97                 x5 := src[y8+7]
  98                 x6 := src[y8+5]
  99                 x7 := src[y8+3]
 100
 101                 // Stage 1.
 102                 x8 := w7 * (x4 + x5)
 103                 x4 = x8 + w1mw7*x4
 104                 x5 = x8 - w1pw7*x5
 105                 x8 = w3 * (x6 + x7)
 106                 x6 = x8 - w3mw5*x6
 107                 x7 = x8 - w3pw5*x7
 108
 109                 // Stage 2.
 110                 x8 = x0 + x1
 111                 x0 -= x1
 112                 x1 = w6 * (x3 + x2)
 113                 x2 = x1 - w2pw6*x2
 114                 x3 = x1 + w2mw6*x3
 115                 x1 = x4 + x6
 116                 x4 -= x6
 117                 x6 = x5 + x7
 118                 x5 -= x7
 119
 120                 // Stage 3.
 121                 x7 = x8 + x3
 122                 x8 -= x3
 123                 x3 = x0 + x2
 124                 x0 -= x2
 125                 x2 = (r2*(x4+x5) + 128) >> 8
 126                 x4 = (r2*(x4-x5) + 128) >> 8
 127
 128                 // Stage 4.
 129                 src[y8+0] = (x7 + x1) >> 8
 130                 src[y8+1] = (x3 + x2) >> 8
 131                 src[y8+2] = (x0 + x4) >> 8
 132                 src[y8+3] = (x8 + x6) >> 8
 133                 src[y8+4] = (x8 - x6) >> 8
 134                 src[y8+5] = (x0 - x4) >> 8
 135                 src[y8+6] = (x3 - x2) >> 8
 136                 src[y8+7] = (x7 - x1) >> 8
 137         }
 138
 139         // Vertical 1-D IDCT.
 140         for x := 0; x < 8; x++ {
 141                 // Similar to the horizontal 1-D IDCT case, if all the AC components are zero, then the IDCT is trivial.
 142                 // However, after performing the horizontal 1-D IDCT, there are typically non-zero AC components, so
 143                 // we do not bother to check for the all-zero case.
 144
 145                 // Prescale.
 146                 y0 := (src[8*0+x] << 8) + 8192
 147                 y1 := src[8*4+x] << 8
 148                 y2 := src[8*6+x]
 149                 y3 := src[8*2+x]
 150                 y4 := src[8*1+x]
 151                 y5 := src[8*7+x]
 152                 y6 := src[8*5+x]
 153                 y7 := src[8*3+x]
 154
 155                 // Stage 1.
 156                 y8 := w7*(y4+y5) + 4
 157                 y4 = (y8 + w1mw7*y4) >> 3
 158                 y5 = (y8 - w1pw7*y5) >> 3
 159                 y8 = w3*(y6+y7) + 4
 160                 y6 = (y8 - w3mw5*y6) >> 3
 161                 y7 = (y8 - w3pw5*y7) >> 3
 162
 163                 // Stage 2.
 164                 y8 = y0 + y1
 165                 y0 -= y1
 166                 y1 = w6*(y3+y2) + 4
 167                 y2 = (y1 - w2pw6*y2) >> 3
 168                 y3 = (y1 + w2mw6*y3) >> 3
 169                 y1 = y4 + y6
 170                 y4 -= y6
 171                 y6 = y5 + y7
 172                 y5 -= y7
 173
 174                 // Stage 3.
 175                 y7 = y8 + y3
 176                 y8 -= y3
 177                 y3 = y0 + y2
 178                 y0 -= y2
 179                 y2 = (r2*(y4+y5) + 128) >> 8
 180                 y4 = (r2*(y4-y5) + 128) >> 8
 181
 182                 // Stage 4.
 183                 src[8*0+x] = (y7 + y1) >> 14
 184                 src[8*1+x] = (y3 + y2) >> 14
 185                 src[8*2+x] = (y0 + y4) >> 14
 186                 src[8*3+x] = (y8 + y6) >> 14
 187                 src[8*4+x] = (y8 - y6) >> 14
 188                 src[8*5+x] = (y0 - y4) >> 14
 189                 src[8*6+x] = (y3 - y2) >> 14
 190                 src[8*7+x] = (y7 - y1) >> 14
 191         }
 192 }