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// Copyright 2012 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.
// This example demonstrates decoding a JPEG image and examining its pixels.
package image_test
import (
"fmt"
"image"
"log"
"os"
// Package image/jpeg is not used explicitly in the code below,
// but is imported for its initialization side-effect, which allows
// image.Decode to understand JPEG formatted images. Uncomment these
// two lines to also understand GIF and PNG images:
// _ "image/gif"
// _ "image/png"
_ "image/jpeg"
)
func Example() {
// Open the file.
file, err := os.Open("testdata/video-001.jpeg")
if err != nil {
log.Fatal(err)
}
defer file.Close()
// Decode the image.
m, _, err := image.Decode(file)
if err != nil {
log.Fatal(err)
}
bounds := m.Bounds()
// Calculate a 16-bin histogram for m's red, green, blue and alpha components.
//
// An image's bounds do not necessarily start at (0, 0), so the two loops start
// at bounds.Min.Y and bounds.Min.X. Looping over Y first and X second is more
// likely to result in better memory access patterns than X first and Y second.
var histogram [16][4]int
for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
for x := bounds.Min.X; x < bounds.Max.X; x++ {
r, g, b, a := m.At(x, y).RGBA()
// A color's RGBA method returns values in the range [0, 65535].
// Shifting by 12 reduces this to the range [0, 15].
histogram[r>>12][0]++
histogram[g>>12][1]++
histogram[b>>12][2]++
histogram[a>>12][3]++
}
}
// Print the results.
fmt.Printf("%-14s %6s %6s %6s %6s\n", "bin", "red", "green", "blue", "alpha")
for i, x := range histogram {
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])
}
// Output:
// bin red green blue alpha
// 0x0000-0x0fff: 471 819 7596 0
// 0x1000-0x1fff: 576 2892 726 0
// 0x2000-0x2fff: 1038 2330 943 0
// 0x3000-0x3fff: 883 2321 1014 0
// 0x4000-0x4fff: 501 1295 525 0
// 0x5000-0x5fff: 302 962 242 0
// 0x6000-0x6fff: 219 358 150 0
// 0x7000-0x7fff: 352 281 192 0
// 0x8000-0x8fff: 3688 216 246 0
// 0x9000-0x9fff: 2277 237 283 0
// 0xa000-0xafff: 971 254 357 0
// 0xb000-0xbfff: 317 306 429 0
// 0xc000-0xcfff: 203 402 401 0
// 0xd000-0xdfff: 256 394 241 0
// 0xe000-0xefff: 378 343 173 0
// 0xf000-0xffff: 3018 2040 1932 15450
}
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