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_decodeConfig() {
    25  	reader := base64.NewDecoder(base64.StdEncoding, strings.NewReader(data))
    26  	config, format, err := image.DecodeConfig(reader)
    27  	if err != nil {
    28  		log.Fatal(err)
    29  	}
    30  	fmt.Println("Width:", config.Width, "Height:", config.Height, "Format:", format)
    31  }
    32  
    33  func Example() {
    34  	// Decode the JPEG data. If reading from file, create a reader with
    35  	//
    36  	// reader, err := os.Open("testdata/video-001.q50.420.jpeg")
    37  	// if err != nil {
    38  	//     log.Fatal(err)
    39  	// }
    40  	// defer reader.Close()
    41  	reader := base64.NewDecoder(base64.StdEncoding, strings.NewReader(data))
    42  	m, _, err := image.Decode(reader)
    43  	if err != nil {
    44  		log.Fatal(err)
    45  	}
    46  	bounds := m.Bounds()
    47  
    48  	// Calculate a 16-bin histogram for m's red, green, blue and alpha components.
    49  	//
    50  	// An image's bounds do not necessarily start at (0, 0), so the two loops start
    51  	// at bounds.Min.Y and bounds.Min.X. Looping over Y first and X second is more
    52  	// likely to result in better memory access patterns than X first and Y second.
    53  	var histogram [16][4]int
    54  	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
    55  		for x := bounds.Min.X; x < bounds.Max.X; x++ {
    56  			r, g, b, a := m.At(x, y).RGBA()
    57  			// A color's RGBA method returns values in the range [0, 65535].
    58  			// Shifting by 12 reduces this to the range [0, 15].
    59  			histogram[r>>12][0]++
    60  			histogram[g>>12][1]++
    61  			histogram[b>>12][2]++
    62  			histogram[a>>12][3]++
    63  		}
    64  	}
    65  
    66  	// Print the results.
    67  	fmt.Printf("%-14s %6s %6s %6s %6s\n", "bin", "red", "green", "blue", "alpha")
    68  	for i, x := range histogram {
    69  		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])
    70  	}
    71  	// Output:
    72  	// bin               red  green   blue  alpha
    73  	// 0x0000-0x0fff:    364    790   7242      0
    74  	// 0x1000-0x1fff:    645   2967   1039      0
    75  	// 0x2000-0x2fff:   1072   2299    979      0
    76  	// 0x3000-0x3fff:    820   2266    980      0
    77  	// 0x4000-0x4fff:    537   1305    541      0
    78  	// 0x5000-0x5fff:    319    962    261      0
    79  	// 0x6000-0x6fff:    322    375    177      0
    80  	// 0x7000-0x7fff:    601    279    214      0
    81  	// 0x8000-0x8fff:   3478    227    273      0
    82  	// 0x9000-0x9fff:   2260    234    329      0
    83  	// 0xa000-0xafff:    921    282    373      0
    84  	// 0xb000-0xbfff:    321    335    397      0
    85  	// 0xc000-0xcfff:    229    388    298      0
    86  	// 0xd000-0xdfff:    260    414    277      0
    87  	// 0xe000-0xefff:    516    428    298      0
    88  	// 0xf000-0xffff:   2785   1899   1772  15450
    89  }
    90  
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   150
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