tour.golang.org＃36 pic.Show实现的功能

Question

In tour.golang.org , exercice 36 , what this statement pic.Show(Pic) supposed to do?

package main

import "code.google.com/p/go-tour/pic"

func Pic(dx, dy int) [][]uint8 {
    var p = make([]([]uint8), dy)
    for i := 0; i < len(p); i++ {
        p[i] = make([]uint8, dx)
        for j := 0; j < len(p[i]); j++ {
            p[i][j] = uint8((i + j) / 2)
        }
    }
    return p
}

func main() {
    pic.Show(Pic)
}

when I run the example, it just print some characters on the standard output, isn’t supposed to show some picture?

Answer 1

pic.Show() creates the image and encodes it as base64. Here is the code:

func Show(f func(int, int) [][]uint8) {
        const (
                dx = 256
                dy = 256
        )
        data := f(dx, dy)
        m := image.NewNRGBA(image.Rect(0, 0, dx, dy))
        for y := 0; y < dy; y++ {
                for x := 0; x < dx; x++ {
                        v := data[y][x]
                        i := y*m.Stride + x*4
                        m.Pix[i] = v
                        m.Pix[i+1] = v
                        m.Pix[i+2] = 255
                        m.Pix[i+3] = 255
                }
        }
        ShowImage(m)
}

func ShowImage(m image.Image) {
        var buf bytes.Buffer
        err := png.Encode(&buf, m)
        if err != nil {
                panic(err)
        }
        enc := base64.StdEncoding.EncodeToString(buf.Bytes())
        fmt.Println("IMAGE:" + enc)
}

Funny thing is it ends up printing the string "IMAGE:" followed by the base64 encoded string. Seems that play.golang.org parses the output converting this to an img tag. Try the following example: http://play.golang.org/p/RZPqp164eS