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