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Diffstat (limited to 'src/image/jpeg/idct.go')
| -rw-r--r-- | src/image/jpeg/idct.go | 192 |
1 files changed, 192 insertions, 0 deletions
diff --git a/src/image/jpeg/idct.go b/src/image/jpeg/idct.go new file mode 100644 index 000000000..46fcaecb7 --- /dev/null +++ b/src/image/jpeg/idct.go @@ -0,0 +1,192 @@ +// 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 + // If all the AC components are zero, then the IDCT is trivial. + if src[y8+1] == 0 && src[y8+2] == 0 && src[y8+3] == 0 && + src[y8+4] == 0 && src[y8+5] == 0 && src[y8+6] == 0 && src[y8+7] == 0 { + dc := src[y8+0] << 3 + src[y8+0] = dc + src[y8+1] = dc + src[y8+2] = dc + src[y8+3] = dc + src[y8+4] = dc + src[y8+5] = dc + src[y8+6] = dc + src[y8+7] = dc + continue + } + + // Prescale. + x0 := (src[y8+0] << 11) + 128 + x1 := src[y8+4] << 11 + x2 := src[y8+6] + x3 := src[y8+2] + x4 := src[y8+1] + x5 := src[y8+7] + x6 := src[y8+5] + x7 := src[y8+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. + src[y8+0] = (x7 + x1) >> 8 + src[y8+1] = (x3 + x2) >> 8 + src[y8+2] = (x0 + x4) >> 8 + src[y8+3] = (x8 + x6) >> 8 + src[y8+4] = (x8 - x6) >> 8 + src[y8+5] = (x0 - x4) >> 8 + src[y8+6] = (x3 - x2) >> 8 + src[y8+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. + + // Prescale. + y0 := (src[8*0+x] << 8) + 8192 + y1 := src[8*4+x] << 8 + y2 := src[8*6+x] + y3 := src[8*2+x] + y4 := src[8*1+x] + y5 := src[8*7+x] + y6 := src[8*5+x] + y7 := src[8*3+x] + + // 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. + src[8*0+x] = (y7 + y1) >> 14 + src[8*1+x] = (y3 + y2) >> 14 + src[8*2+x] = (y0 + y4) >> 14 + src[8*3+x] = (y8 + y6) >> 14 + src[8*4+x] = (y8 - y6) >> 14 + src[8*5+x] = (y0 - y4) >> 14 + src[8*6+x] = (y3 - y2) >> 14 + src[8*7+x] = (y7 - y1) >> 14 + } +} |