// Copyright 2009 The Go Authors. All rights reserved. | |

// Use of this source code is governed by a BSD-style | |

// license that can be found in the LICENSE file. | |

package jpeg | |

// This is a Go translation of idct.c from | |

// | |

// http://standards.iso.org/ittf/PubliclyAvailableStandards/ISO_IEC_13818-4_2004_Conformance_Testing/Video/verifier/mpeg2decode_960109.tar.gz | |

// | |

// which carries the following notice: | |

/* Copyright (C) 1996, MPEG Software Simulation Group. All Rights Reserved. */ | |

/* | |

* Disclaimer of Warranty | |

* | |

* These software programs are available to the user without any license fee or | |

* royalty on an "as is" basis. The MPEG Software Simulation Group disclaims | |

* any and all warranties, whether express, implied, or statuary, including any | |

* implied warranties or merchantability or of fitness for a particular | |

* purpose. In no event shall the copyright-holder be liable for any | |

* incidental, punitive, or consequential damages of any kind whatsoever | |

* arising from the use of these programs. | |

* | |

* This disclaimer of warranty extends to the user of these programs and user's | |

* customers, employees, agents, transferees, successors, and assigns. | |

* | |

* The MPEG Software Simulation Group does not represent or warrant that the | |

* programs furnished hereunder are free of infringement of any third-party | |

* patents. | |

* | |

* Commercial implementations of MPEG-1 and MPEG-2 video, including shareware, | |

* are subject to royalty fees to patent holders. Many of these patents are | |

* general enough such that they are unavoidable regardless of implementation | |

* design. | |

* | |

*/ | |

const blockSize = 64 // A DCT block is 8x8. | |

type block [blockSize]int32 | |

const ( | |

w1 = 2841 // 2048*sqrt(2)*cos(1*pi/16) | |

w2 = 2676 // 2048*sqrt(2)*cos(2*pi/16) | |

w3 = 2408 // 2048*sqrt(2)*cos(3*pi/16) | |

w5 = 1609 // 2048*sqrt(2)*cos(5*pi/16) | |

w6 = 1108 // 2048*sqrt(2)*cos(6*pi/16) | |

w7 = 565 // 2048*sqrt(2)*cos(7*pi/16) | |

w1pw7 = w1 + w7 | |

w1mw7 = w1 - w7 | |

w2pw6 = w2 + w6 | |

w2mw6 = w2 - w6 | |

w3pw5 = w3 + w5 | |

w3mw5 = w3 - w5 | |

r2 = 181 // 256/sqrt(2) | |

) | |

// idct performs a 2-D Inverse Discrete Cosine Transformation. | |

// | |

// The input coefficients should already have been multiplied by the | |

// appropriate quantization table. We use fixed-point computation, with the | |

// number of bits for the fractional component varying over the intermediate | |

// stages. | |

// | |

// For more on the actual algorithm, see Z. Wang, "Fast algorithms for the | |

// discrete W transform and for the discrete Fourier transform", IEEE Trans. on | |

// ASSP, Vol. ASSP- 32, pp. 803-816, Aug. 1984. | |

func idct(src *block) { | |

// Horizontal 1-D IDCT. | |

for y := 0; y < 8; y++ { | |

y8 := y * 8 | |

s := src[y8 : y8+8 : y8+8] // Small cap improves performance, see https://golang.org/issue/27857 | |

// If all the AC components are zero, then the IDCT is trivial. | |

if s[1] == 0 && s[2] == 0 && s[3] == 0 && | |

s[4] == 0 && s[5] == 0 && s[6] == 0 && s[7] == 0 { | |

dc := s[0] << 3 | |

s[0] = dc | |

s[1] = dc | |

s[2] = dc | |

s[3] = dc | |

s[4] = dc | |

s[5] = dc | |

s[6] = dc | |

s[7] = dc | |

continue | |

} | |

// Prescale. | |

x0 := (s[0] << 11) + 128 | |

x1 := s[4] << 11 | |

x2 := s[6] | |

x3 := s[2] | |

x4 := s[1] | |

x5 := s[7] | |

x6 := s[5] | |

x7 := s[3] | |

// Stage 1. | |

x8 := w7 * (x4 + x5) | |

x4 = x8 + w1mw7*x4 | |

x5 = x8 - w1pw7*x5 | |

x8 = w3 * (x6 + x7) | |

x6 = x8 - w3mw5*x6 | |

x7 = x8 - w3pw5*x7 | |

// Stage 2. | |

x8 = x0 + x1 | |

x0 -= x1 | |

x1 = w6 * (x3 + x2) | |

x2 = x1 - w2pw6*x2 | |

x3 = x1 + w2mw6*x3 | |

x1 = x4 + x6 | |

x4 -= x6 | |

x6 = x5 + x7 | |

x5 -= x7 | |

// Stage 3. | |

x7 = x8 + x3 | |

x8 -= x3 | |

x3 = x0 + x2 | |

x0 -= x2 | |

x2 = (r2*(x4+x5) + 128) >> 8 | |

x4 = (r2*(x4-x5) + 128) >> 8 | |

// Stage 4. | |

s[0] = (x7 + x1) >> 8 | |

s[1] = (x3 + x2) >> 8 | |

s[2] = (x0 + x4) >> 8 | |

s[3] = (x8 + x6) >> 8 | |

s[4] = (x8 - x6) >> 8 | |

s[5] = (x0 - x4) >> 8 | |

s[6] = (x3 - x2) >> 8 | |

s[7] = (x7 - x1) >> 8 | |

} | |

// Vertical 1-D IDCT. | |

for x := 0; x < 8; x++ { | |

// Similar to the horizontal 1-D IDCT case, if all the AC components are zero, then the IDCT is trivial. | |

// However, after performing the horizontal 1-D IDCT, there are typically non-zero AC components, so | |

// we do not bother to check for the all-zero case. | |

s := src[x : x+57 : x+57] // Small cap improves performance, see https://golang.org/issue/27857 | |

// Prescale. | |

y0 := (s[8*0] << 8) + 8192 | |

y1 := s[8*4] << 8 | |

y2 := s[8*6] | |

y3 := s[8*2] | |

y4 := s[8*1] | |

y5 := s[8*7] | |

y6 := s[8*5] | |

y7 := s[8*3] | |

// Stage 1. | |

y8 := w7*(y4+y5) + 4 | |

y4 = (y8 + w1mw7*y4) >> 3 | |

y5 = (y8 - w1pw7*y5) >> 3 | |

y8 = w3*(y6+y7) + 4 | |

y6 = (y8 - w3mw5*y6) >> 3 | |

y7 = (y8 - w3pw5*y7) >> 3 | |

// Stage 2. | |

y8 = y0 + y1 | |

y0 -= y1 | |

y1 = w6*(y3+y2) + 4 | |

y2 = (y1 - w2pw6*y2) >> 3 | |

y3 = (y1 + w2mw6*y3) >> 3 | |

y1 = y4 + y6 | |

y4 -= y6 | |

y6 = y5 + y7 | |

y5 -= y7 | |

// Stage 3. | |

y7 = y8 + y3 | |

y8 -= y3 | |

y3 = y0 + y2 | |

y0 -= y2 | |

y2 = (r2*(y4+y5) + 128) >> 8 | |

y4 = (r2*(y4-y5) + 128) >> 8 | |

// Stage 4. | |

s[8*0] = (y7 + y1) >> 14 | |

s[8*1] = (y3 + y2) >> 14 | |

s[8*2] = (y0 + y4) >> 14 | |

s[8*3] = (y8 + y6) >> 14 | |

s[8*4] = (y8 - y6) >> 14 | |

s[8*5] = (y0 - y4) >> 14 | |

s[8*6] = (y3 - y2) >> 14 | |

s[8*7] = (y7 - y1) >> 14 | |

} | |

} |