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Diffstat (limited to 'src/image/jpeg/fdct.go')
| -rw-r--r-- | src/image/jpeg/fdct.go | 192 |
1 files changed, 192 insertions, 0 deletions
diff --git a/src/image/jpeg/fdct.go b/src/image/jpeg/fdct.go new file mode 100644 index 0000000..c7a973e --- /dev/null +++ b/src/image/jpeg/fdct.go @@ -0,0 +1,192 @@ +// Copyright 2011 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 file implements a Forward Discrete Cosine Transformation. + +/* +It is based on the code in jfdctint.c from the Independent JPEG Group, +found at http://www.ijg.org/files/jpegsrc.v8c.tar.gz. + +The "LEGAL ISSUES" section of the README in that archive says: + +In plain English: + +1. We don't promise that this software works. (But if you find any bugs, + please let us know!) +2. You can use this software for whatever you want. You don't have to pay us. +3. You may not pretend that you wrote this software. If you use it in a + program, you must acknowledge somewhere in your documentation that + you've used the IJG code. + +In legalese: + +The authors make NO WARRANTY or representation, either express or implied, +with respect to this software, its quality, accuracy, merchantability, or +fitness for a particular purpose. This software is provided "AS IS", and you, +its user, assume the entire risk as to its quality and accuracy. + +This software is copyright (C) 1991-2011, Thomas G. Lane, Guido Vollbeding. +All Rights Reserved except as specified below. + +Permission is hereby granted to use, copy, modify, and distribute this +software (or portions thereof) for any purpose, without fee, subject to these +conditions: +(1) If any part of the source code for this software is distributed, then this +README file must be included, with this copyright and no-warranty notice +unaltered; and any additions, deletions, or changes to the original files +must be clearly indicated in accompanying documentation. +(2) If only executable code is distributed, then the accompanying +documentation must state that "this software is based in part on the work of +the Independent JPEG Group". +(3) Permission for use of this software is granted only if the user accepts +full responsibility for any undesirable consequences; the authors accept +NO LIABILITY for damages of any kind. + +These conditions apply to any software derived from or based on the IJG code, +not just to the unmodified library. If you use our work, you ought to +acknowledge us. + +Permission is NOT granted for the use of any IJG author's name or company name +in advertising or publicity relating to this software or products derived from +it. This software may be referred to only as "the Independent JPEG Group's +software". + +We specifically permit and encourage the use of this software as the basis of +commercial products, provided that all warranty or liability claims are +assumed by the product vendor. +*/ + +// Trigonometric constants in 13-bit fixed point format. +const ( + fix_0_298631336 = 2446 + fix_0_390180644 = 3196 + fix_0_541196100 = 4433 + fix_0_765366865 = 6270 + fix_0_899976223 = 7373 + fix_1_175875602 = 9633 + fix_1_501321110 = 12299 + fix_1_847759065 = 15137 + fix_1_961570560 = 16069 + fix_2_053119869 = 16819 + fix_2_562915447 = 20995 + fix_3_072711026 = 25172 +) + +const ( + constBits = 13 + pass1Bits = 2 + centerJSample = 128 +) + +// fdct performs a forward DCT on an 8x8 block of coefficients, including a +// level shift. +func fdct(b *block) { + // Pass 1: process rows. + for y := 0; y < 8; y++ { + y8 := y * 8 + s := b[y8 : y8+8 : y8+8] // Small cap improves performance, see https://golang.org/issue/27857 + x0 := s[0] + x1 := s[1] + x2 := s[2] + x3 := s[3] + x4 := s[4] + x5 := s[5] + x6 := s[6] + x7 := s[7] + + tmp0 := x0 + x7 + tmp1 := x1 + x6 + tmp2 := x2 + x5 + tmp3 := x3 + x4 + + tmp10 := tmp0 + tmp3 + tmp12 := tmp0 - tmp3 + tmp11 := tmp1 + tmp2 + tmp13 := tmp1 - tmp2 + + tmp0 = x0 - x7 + tmp1 = x1 - x6 + tmp2 = x2 - x5 + tmp3 = x3 - x4 + + s[0] = (tmp10 + tmp11 - 8*centerJSample) << pass1Bits + s[4] = (tmp10 - tmp11) << pass1Bits + z1 := (tmp12 + tmp13) * fix_0_541196100 + z1 += 1 << (constBits - pass1Bits - 1) + s[2] = (z1 + tmp12*fix_0_765366865) >> (constBits - pass1Bits) + s[6] = (z1 - tmp13*fix_1_847759065) >> (constBits - pass1Bits) + + tmp10 = tmp0 + tmp3 + tmp11 = tmp1 + tmp2 + tmp12 = tmp0 + tmp2 + tmp13 = tmp1 + tmp3 + z1 = (tmp12 + tmp13) * fix_1_175875602 + z1 += 1 << (constBits - pass1Bits - 1) + tmp0 *= fix_1_501321110 + tmp1 *= fix_3_072711026 + tmp2 *= fix_2_053119869 + tmp3 *= fix_0_298631336 + tmp10 *= -fix_0_899976223 + tmp11 *= -fix_2_562915447 + tmp12 *= -fix_0_390180644 + tmp13 *= -fix_1_961570560 + + tmp12 += z1 + tmp13 += z1 + s[1] = (tmp0 + tmp10 + tmp12) >> (constBits - pass1Bits) + s[3] = (tmp1 + tmp11 + tmp13) >> (constBits - pass1Bits) + s[5] = (tmp2 + tmp11 + tmp12) >> (constBits - pass1Bits) + s[7] = (tmp3 + tmp10 + tmp13) >> (constBits - pass1Bits) + } + // Pass 2: process columns. + // We remove pass1Bits scaling, but leave results scaled up by an overall factor of 8. + for x := 0; x < 8; x++ { + tmp0 := b[0*8+x] + b[7*8+x] + tmp1 := b[1*8+x] + b[6*8+x] + tmp2 := b[2*8+x] + b[5*8+x] + tmp3 := b[3*8+x] + b[4*8+x] + + tmp10 := tmp0 + tmp3 + 1<<(pass1Bits-1) + tmp12 := tmp0 - tmp3 + tmp11 := tmp1 + tmp2 + tmp13 := tmp1 - tmp2 + + tmp0 = b[0*8+x] - b[7*8+x] + tmp1 = b[1*8+x] - b[6*8+x] + tmp2 = b[2*8+x] - b[5*8+x] + tmp3 = b[3*8+x] - b[4*8+x] + + b[0*8+x] = (tmp10 + tmp11) >> pass1Bits + b[4*8+x] = (tmp10 - tmp11) >> pass1Bits + + z1 := (tmp12 + tmp13) * fix_0_541196100 + z1 += 1 << (constBits + pass1Bits - 1) + b[2*8+x] = (z1 + tmp12*fix_0_765366865) >> (constBits + pass1Bits) + b[6*8+x] = (z1 - tmp13*fix_1_847759065) >> (constBits + pass1Bits) + + tmp10 = tmp0 + tmp3 + tmp11 = tmp1 + tmp2 + tmp12 = tmp0 + tmp2 + tmp13 = tmp1 + tmp3 + z1 = (tmp12 + tmp13) * fix_1_175875602 + z1 += 1 << (constBits + pass1Bits - 1) + tmp0 *= fix_1_501321110 + tmp1 *= fix_3_072711026 + tmp2 *= fix_2_053119869 + tmp3 *= fix_0_298631336 + tmp10 *= -fix_0_899976223 + tmp11 *= -fix_2_562915447 + tmp12 *= -fix_0_390180644 + tmp13 *= -fix_1_961570560 + + tmp12 += z1 + tmp13 += z1 + b[1*8+x] = (tmp0 + tmp10 + tmp12) >> (constBits + pass1Bits) + b[3*8+x] = (tmp1 + tmp11 + tmp13) >> (constBits + pass1Bits) + b[5*8+x] = (tmp2 + tmp11 + tmp12) >> (constBits + pass1Bits) + b[7*8+x] = (tmp3 + tmp10 + tmp13) >> (constBits + pass1Bits) + } +} |