mirror of
https://github.com/golang/go
synced 2024-11-14 15:00:27 -07:00
c2023a0791
Before, calling the RGBA method of YCbCr color would return red values
in the range [0x0080, 0xff80]. After, the range is [0x0000, 0xffff] and
is consistent with what Gray colors' RGBA method returns. In particular,
pure black, pure white and every Gray color in between are now exactly
representable as a YCbCr color.
This fixes a regression from Go 1.4 (where YCbCr{0x00, 0x80, 0x80} was
no longer equivalent to pure black), introduced by golang.org/cl/8073 in
the Go 1.5 development cycle. In Go 1.4, the +0x80 rounding was not
noticable when Cb == 0x80 && Cr == 0x80, because the YCbCr to RGBA
conversion truncated to 8 bits before multiplying by 0x101, so the
output range was [0x0000, 0xffff].
The TestYCbCrRoundtrip fuzzy-match tolerance grows from 1 to 2 because
the YCbCr to RGB conversion now maps to an ever-so-slightly larger
range, along with the usual imprecision of accumulating rounding errors.
Also s/int/int32/ in ycbcr.go. The conversion shouldn't overflow either
way, as int is always at least 32 bits, but it does make it clearer that
the computation doesn't depend on sizeof(int).
Fixes #11691
Change-Id: I538ca0adf7e040fa96c5bc8b3aef4454535126b9
Reviewed-on: https://go-review.googlesource.com/12220
Reviewed-by: Rob Pike <r@golang.org>
141 lines
7.2 KiB
Go
141 lines
7.2 KiB
Go
// Copyright 2012 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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// This example demonstrates decoding a JPEG image and examining its pixels.
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package image_test
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import (
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"encoding/base64"
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"fmt"
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"image"
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"log"
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"strings"
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// Package image/jpeg is not used explicitly in the code below,
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// but is imported for its initialization side-effect, which allows
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// image.Decode to understand JPEG formatted images. Uncomment these
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// two lines to also understand GIF and PNG images:
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// _ "image/gif"
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// _ "image/png"
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_ "image/jpeg"
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)
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func Example() {
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// Decode the JPEG data. If reading from file, create a reader with
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//
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// reader, err := os.Open("testdata/video-001.q50.420.jpeg")
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// if err != nil {
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// log.Fatal(err)
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// }
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// defer reader.Close()
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reader := base64.NewDecoder(base64.StdEncoding, strings.NewReader(data))
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m, _, err := image.Decode(reader)
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if err != nil {
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log.Fatal(err)
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}
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bounds := m.Bounds()
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// Calculate a 16-bin histogram for m's red, green, blue and alpha components.
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//
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// An image's bounds do not necessarily start at (0, 0), so the two loops start
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// at bounds.Min.Y and bounds.Min.X. Looping over Y first and X second is more
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// likely to result in better memory access patterns than X first and Y second.
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var histogram [16][4]int
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for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
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for x := bounds.Min.X; x < bounds.Max.X; x++ {
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r, g, b, a := m.At(x, y).RGBA()
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// A color's RGBA method returns values in the range [0, 65535].
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// Shifting by 12 reduces this to the range [0, 15].
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histogram[r>>12][0]++
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histogram[g>>12][1]++
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histogram[b>>12][2]++
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histogram[a>>12][3]++
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}
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}
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// Print the results.
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fmt.Printf("%-14s %6s %6s %6s %6s\n", "bin", "red", "green", "blue", "alpha")
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for i, x := range histogram {
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fmt.Printf("0x%04x-0x%04x: %6d %6d %6d %6d\n", i<<12, (i+1)<<12-1, x[0], x[1], x[2], x[3])
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}
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// Output:
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// bin red green blue alpha
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// 0x0000-0x0fff: 364 790 7242 0
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// 0x1000-0x1fff: 645 2967 1039 0
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// 0x2000-0x2fff: 1072 2299 979 0
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// 0x3000-0x3fff: 820 2266 980 0
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// 0x4000-0x4fff: 537 1305 541 0
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// 0x5000-0x5fff: 319 962 261 0
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// 0x6000-0x6fff: 322 375 177 0
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// 0x7000-0x7fff: 601 279 214 0
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// 0x8000-0x8fff: 3478 227 273 0
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// 0x9000-0x9fff: 2260 234 329 0
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// 0xa000-0xafff: 921 282 373 0
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// 0xb000-0xbfff: 321 335 397 0
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// 0xc000-0xcfff: 229 388 298 0
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// 0xd000-0xdfff: 260 414 277 0
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// 0xe000-0xefff: 516 428 298 0
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// 0xf000-0xffff: 2785 1899 1772 15450
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}
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const data = `
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0HwyBXGeRjmrcUhMg2ghezd//rUUVcTKW5s2jZtY/QDaOKKKK8ip8bPRj8KP/9k=
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`
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