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decode_example_test.go
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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 (
"encoding/base64"
"fmt"
"image"
"log"
"strings"
// 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_decodeConfig() {
reader := base64.NewDecoder(base64.StdEncoding, strings.NewReader(data))
config, format, err := image.DecodeConfig(reader)
if err != nil {
log.Fatal(err)
}
fmt.Println("Width:", config.Width, "Height:", config.Height, "Format:", format)
}
func Example() {
// Decode the JPEG data. If reading from file, create a reader with
//
// reader, err := os.Open("testdata/video-001.q50.420.jpeg")
// if err != nil {
// log.Fatal(err)
// }
// defer reader.Close()
reader := base64.NewDecoder(base64.StdEncoding, strings.NewReader(data))
m, _, err := image.Decode(reader)
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: 364 790 7242 0
// 0x1000-0x1fff: 645 2967 1039 0
// 0x2000-0x2fff: 1072 2299 979 0
// 0x3000-0x3fff: 820 2266 980 0
// 0x4000-0x4fff: 537 1305 541 0
// 0x5000-0x5fff: 319 962 261 0
// 0x6000-0x6fff: 322 375 177 0
// 0x7000-0x7fff: 601 279 214 0
// 0x8000-0x8fff: 3478 227 273 0
// 0x9000-0x9fff: 2260 234 329 0
// 0xa000-0xafff: 921 282 373 0
// 0xb000-0xbfff: 321 335 397 0
// 0xc000-0xcfff: 229 388 298 0
// 0xd000-0xdfff: 260 414 277 0
// 0xe000-0xefff: 516 428 298 0
// 0xf000-0xffff: 2785 1899 1772 15450
}
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