Diff image compression

main.go

package main

import (
	"compress/gzip"
	"fmt"
	"image/jpeg"
	"log"
	"os"
)

var ImageFile string = "image.jpeg"

func main() {
	rgbData, diffStreams := GetBitmapData()
	fmt.Println("gzip(rgb) =", MeasureGzipSize(rgbData), "gzip(diff) =",
		MeasureGzipSize(diffStreams), "RGB data length", len(rgbData))
}

func GetBitmapData() (rgbData []byte, diffStreams []byte) {
	file, err := os.Open("image.jpeg")
	if err != nil {
		log.Fatal("Failed to open " + ImageFile)
	}
	defer file.Close()

	image, err := jpeg.Decode(file)
	if err != nil {
		log.Fatal("Failed to decode " + ImageFile)
	}

	width := image.Bounds().Dx()
	height := image.Bounds().Dy()
	rgbData = make([]byte, 3*width*height)
	for y := 0; y < height; y++ {
		for x := 0; x < width; x++ {
			r, g, b, _ := image.At(x+image.Bounds().Min.X, y+image.Bounds().Min.Y).RGBA()
			index := (y*width + x) * 3
			rgbData[index] = byte(r >> 8)
			rgbData[index+1] = byte(g >> 8)
			rgbData[index+2] = byte(b >> 8)
		}
	}

	diffStreams = make([]byte, 3*width*height)
	var lastR, lastG, lastB byte
	for y := 0; y < height; y++ {
		for x := 0; x < width; x++ {
			r, g, b, _ := image.At(x+image.Bounds().Min.X, y+image.Bounds().Min.Y).RGBA()
			redIndex := y*width + x
			greenIndex := y*width + x + width*height
			blueIndex := y*width + x + 2*width*height
			diffStreams[redIndex] = byte(r>>8) - lastR
			diffStreams[greenIndex] = byte(g>>8) - lastG
			diffStreams[blueIndex] = byte(b>>8) - lastB
			lastR = byte(r >> 8)
			lastG = byte(r >> 8)
			lastB = byte(r >> 8)
		}
	}

	return
}

func MeasureGzipSize(data []byte) int {
	counter := &CounterWriter{}
	writer, _ := gzip.NewWriterLevel(counter, gzip.BestCompression)
	count, err := writer.Write(data)
	if err != nil || count < len(data) {
		log.Fatal("failed to compress in one pass")
	}
	if err := writer.Flush(); err != nil {
		log.Fatal("got error:", err)
	}
	return counter.Count
}

type CounterWriter struct {
	Count int
}

func (c *CounterWriter) Write(b []byte) (n int, err error) {
	c.Count += len(b)
	return len(b), nil
}