// 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 } }