decode_example_test.go

Documentation: image

		 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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