| // 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++ { |
| x0 := b[y*8+0] |
| x1 := b[y*8+1] |
| x2 := b[y*8+2] |
| x3 := b[y*8+3] |
| x4 := b[y*8+4] |
| x5 := b[y*8+5] |
| x6 := b[y*8+6] |
| x7 := b[y*8+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 |
| |
| b[y*8+0] = (tmp10 + tmp11 - 8*centerJSample) << pass1Bits |
| b[y*8+4] = (tmp10 - tmp11) << pass1Bits |
| z1 := (tmp12 + tmp13) * fix_0_541196100 |
| z1 += 1 << (constBits - pass1Bits - 1) |
| b[y*8+2] = (z1 + tmp12*fix_0_765366865) >> (constBits - pass1Bits) |
| b[y*8+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 = tmp0 * fix_1_501321110 |
| tmp1 = tmp1 * fix_3_072711026 |
| tmp2 = tmp2 * fix_2_053119869 |
| tmp3 = tmp3 * fix_0_298631336 |
| tmp10 = tmp10 * -fix_0_899976223 |
| tmp11 = tmp11 * -fix_2_562915447 |
| tmp12 = tmp12 * -fix_0_390180644 |
| tmp13 = tmp13 * -fix_1_961570560 |
| |
| tmp12 += z1 |
| tmp13 += z1 |
| b[y*8+1] = (tmp0 + tmp10 + tmp12) >> (constBits - pass1Bits) |
| b[y*8+3] = (tmp1 + tmp11 + tmp13) >> (constBits - pass1Bits) |
| b[y*8+5] = (tmp2 + tmp11 + tmp12) >> (constBits - pass1Bits) |
| b[y*8+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 = tmp0 * fix_1_501321110 |
| tmp1 = tmp1 * fix_3_072711026 |
| tmp2 = tmp2 * fix_2_053119869 |
| tmp3 = tmp3 * fix_0_298631336 |
| tmp10 = tmp10 * -fix_0_899976223 |
| tmp11 = tmp11 * -fix_2_562915447 |
| tmp12 = tmp12 * -fix_0_390180644 |
| tmp13 = 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) |
| } |
| } |
