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  	"bytes"
    10  	"encoding/base64"
    11  	"fmt"
    12  	"image"
    13  	"log"
    14  	"strings"
    15  
    16  	// Packages image/gif and image/jpeg are not used explicitly in the code
    17  	// below, but are imported for their initialization side-effects, which
    18  	// allow image.Decode and image.DecodeConfig to understand GIF and JPEG
    19  	// formatted images. Uncomment the line below to also understand PNG:
    20  	// _ "image/png"
    21  	_ "image/gif"
    22  	_ "image/jpeg"
    23  )
    24  
    25  func Example_decodeConfig() {
    26  	reader := base64.NewDecoder(base64.StdEncoding, strings.NewReader(data))
    27  	config, format, err := image.DecodeConfig(reader)
    28  	if err != nil {
    29  		log.Fatal(err)
    30  	}
    31  	fmt.Println("Width:", config.Width, "Height:", config.Height, "Format:", format)
    32  }
    33  
    34  // ExampleDecode_untrusted demonstrates decoding an untrusted
    35  // image file in two steps so that unexpectedly large
    36  // memory allocations can be safely avoided.
    37  func ExampleDecode_untrusted() {
    38  	// This GIF data is a valid 1x1 image (layout matches package gif tests).
    39  	gifData := []byte{
    40  		'G', 'I', 'F', '8', '9', 'a',
    41  		1, 0, 1, 0,
    42  		128, 0, 0,
    43  		0, 0, 0, 1, 1, 1,
    44  		0x21, 0xf9, 0x04, 0x00, 0x00, 0x00, 0xff, 0x00,
    45  		0x2c,
    46  		0x00, 0x00, 0x00, 0x00,
    47  		0x01, 0x00, 0x01, 0x00,
    48  		0x00,
    49  		0x02, 0x02, 0x4c, 0x01, 0x00,
    50  		0x3b,
    51  	}
    52  
    53  	cfg, _, err := image.DecodeConfig(bytes.NewReader(gifData))
    54  	if err != nil {
    55  		log.Fatal(err)
    56  	}
    57  
    58  	// Use int64 to avoid overflow on 32-bit platforms.
    59  	const maxPixels = 10000
    60  	if int64(cfg.Width)*int64(cfg.Height) > maxPixels {
    61  		fmt.Println("rejected: dimensions too large")
    62  		return
    63  	}
    64  
    65  	m, _, err := image.Decode(bytes.NewReader(gifData))
    66  	if err != nil {
    67  		log.Fatal(err)
    68  	}
    69  	fmt.Println("decoded", m.Bounds().Dx(), m.Bounds().Dy())
    70  	// Output:
    71  	// decoded 1 1
    72  }
    73  
    74  func Example() {
    75  	// Decode the JPEG data. If reading from file, create a reader with
    76  	//
    77  	// reader, err := os.Open("testdata/video-001.q50.420.jpeg")
    78  	// if err != nil {
    79  	//     log.Fatal(err)
    80  	// }
    81  	// defer reader.Close()
    82  	reader := base64.NewDecoder(base64.StdEncoding, strings.NewReader(data))
    83  	m, _, err := image.Decode(reader)
    84  	if err != nil {
    85  		log.Fatal(err)
    86  	}
    87  	bounds := m.Bounds()
    88  
    89  	// Calculate a 16-bin histogram for m's red, green, blue and alpha components.
    90  	//
    91  	// An image's bounds do not necessarily start at (0, 0), so the two loops start
    92  	// at bounds.Min.Y and bounds.Min.X. Looping over Y first and X second is more
    93  	// likely to result in better memory access patterns than X first and Y second.
    94  	var histogram [16][4]int
    95  	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
    96  		for x := bounds.Min.X; x < bounds.Max.X; x++ {
    97  			r, g, b, a := m.At(x, y).RGBA()
    98  			// A color's RGBA method returns values in the range [0, 65535].
    99  			// Shifting by 12 reduces this to the range [0, 15].
   100  			histogram[r>>12][0]++
   101  			histogram[g>>12][1]++
   102  			histogram[b>>12][2]++
   103  			histogram[a>>12][3]++
   104  		}
   105  	}
   106  
   107  	// Print the results.
   108  	fmt.Printf("%-14s %6s %6s %6s %6s\n", "bin", "red", "green", "blue", "alpha")
   109  	for i, x := range histogram {
   110  		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])
   111  	}
   112  	// Output:
   113  	// bin               red  green   blue  alpha
   114  	// 0x0000-0x0fff:    362    793   7245      0
   115  	// 0x1000-0x1fff:    648   2963   1036      0
   116  	// 0x2000-0x2fff:   1072   2301    977      0
   117  	// 0x3000-0x3fff:    819   2266    982      0
   118  	// 0x4000-0x4fff:    537   1303    541      0
   119  	// 0x5000-0x5fff:    321    964    261      0
   120  	// 0x6000-0x6fff:    321    375    177      0
   121  	// 0x7000-0x7fff:    599    278    213      0
   122  	// 0x8000-0x8fff:   3478    228    275      0
   123  	// 0x9000-0x9fff:   2260    233    328      0
   124  	// 0xa000-0xafff:    921    282    374      0
   125  	// 0xb000-0xbfff:    322    335    395      0
   126  	// 0xc000-0xcfff:    228    388    299      0
   127  	// 0xd000-0xdfff:    261    415    277      0
   128  	// 0xe000-0xefff:    516    423    297      0
   129  	// 0xf000-0xffff:   2785   1903   1773  15450
   130  }
   131  
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   191
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