libgo/go/image/decode_example_test.go

   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:    353    759   7228      0
  65         // 0x1000-0x1fff:    629   2944   1036      0
  66         // 0x2000-0x2fff:   1075   2319    984      0
  67         // 0x3000-0x3fff:    838   2291    988      0
  68         // 0x4000-0x4fff:    540   1302    542      0
  69         // 0x5000-0x5fff:    319    971    263      0
  70         // 0x6000-0x6fff:    316    377    178      0
  71         // 0x7000-0x7fff:    581    280    216      0
  72         // 0x8000-0x8fff:   3457    228    274      0
  73         // 0x9000-0x9fff:   2294    237    334      0
  74         // 0xa000-0xafff:    938    283    370      0
  75         // 0xb000-0xbfff:    322    338    401      0
  76         // 0xc000-0xcfff:    229    386    295      0
  77         // 0xd000-0xdfff:    263    416    281      0
  78         // 0xe000-0xefff:    538    433    312      0
  79         // 0xf000-0xffff:   2758   1886   1748  15450
  80 }
  81
  82 const data = `
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