1 // Copyright 2012 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 // This example demonstrates decoding a JPEG image and examining its pixels. 6 package image_test 7 8 import ( 9 "encoding/base64" 10 "fmt" 11 "image" 12 "log" 13 "strings" 14 15 // Package image/jpeg is not used explicitly in the code below, 16 // but is imported for its initialization side-effect, which allows 17 // image.Decode to understand JPEG formatted images. Uncomment these 18 // two lines to also understand GIF and PNG images: 19 // _ "image/gif" 20 // _ "image/png" 21 _ "image/jpeg" 22 ) 23 24 func Example() { 25 // Decode the JPEG data. If reading from file, create a reader with 26 // 27 // reader, err := os.Open("testdata/video-001.q50.420.jpeg") 28 // if err != nil { 29 // log.Fatal(err) 30 // } 31 // defer reader.Close() 32 reader := base64.NewDecoder(base64.StdEncoding, strings.NewReader(data)) 33 m, _, err := image.Decode(reader) 34 if err != nil { 35 log.Fatal(err) 36 } 37 bounds := m.Bounds() 38 39 // Calculate a 16-bin histogram for m's red, green, blue and alpha components. 40 // 41 // An image's bounds do not necessarily start at (0, 0), so the two loops start 42 // at bounds.Min.Y and bounds.Min.X. Looping over Y first and X second is more 43 // likely to result in better memory access patterns than X first and Y second. 44 var histogram [16][4]int 45 for y := bounds.Min.Y; y < bounds.Max.Y; y++ { 46 for x := bounds.Min.X; x < bounds.Max.X; x++ { 47 r, g, b, a := m.At(x, y).RGBA() 48 // A color's RGBA method returns values in the range [0, 65535]. 49 // Shifting by 12 reduces this to the range [0, 15]. 50 histogram[r>>12][0]++ 51 histogram[g>>12][1]++ 52 histogram[b>>12][2]++ 53 histogram[a>>12][3]++ 54 } 55 } 56 57 // Print the results. 58 fmt.Printf("%-14s %6s %6s %6s %6s\n", "bin", "red", "green", "blue", "alpha") 59 for i, x := range histogram { 60 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]) 61 } 62 // Output: 63 // bin red green blue alpha 64 // 0x0000-0x0fff: 364 790 7242 0 65 // 0x1000-0x1fff: 645 2967 1039 0 66 // 0x2000-0x2fff: 1072 2299 979 0 67 // 0x3000-0x3fff: 820 2266 980 0 68 // 0x4000-0x4fff: 537 1305 541 0 69 // 0x5000-0x5fff: 319 962 261 0 70 // 0x6000-0x6fff: 322 375 177 0 71 // 0x7000-0x7fff: 601 279 214 0 72 // 0x8000-0x8fff: 3478 227 273 0 73 // 0x9000-0x9fff: 2260 234 329 0 74 // 0xa000-0xafff: 921 282 373 0 75 // 0xb000-0xbfff: 321 335 397 0 76 // 0xc000-0xcfff: 229 388 298 0 77 // 0xd000-0xdfff: 260 414 277 0 78 // 0xe000-0xefff: 516 428 298 0 79 // 0xf000-0xffff: 2785 1899 1772 15450 80 } 81 82 const data = ` 83 /9j/4AAQSkZJRgABAQIAHAAcAAD/2wBDABALDA4MChAODQ4SERATGCgaGBYWGDEjJR0oOjM9PDkzODdA 84 SFxOQERXRTc4UG1RV19iZ2hnPk1xeXBkeFxlZ2P/2wBDARESEhgVGC8aGi9jQjhCY2NjY2NjY2NjY2Nj 85 Y2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2P/wAARCABnAJYDASIAAhEBAxEB/8QA 86 HwAAAQUBAQEBAQEAAAAAAAAAAAECAwQFBgcICQoL/8QAtRAAAgEDAwIEAwUFBAQAAAF9AQIDAAQRBRIh 87 MUEGE1FhByJxFDKBkaEII0KxwRVS0fAkM2JyggkKFhcYGRolJicoKSo0NTY3ODk6Q0RFRkdISUpTVFVW 88 V1hZWmNkZWZnaGlqc3R1dnd4eXqDhIWGh4iJipKTlJWWl5iZmqKjpKWmp6ipqrKztLW2t7i5usLDxMXG 89 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