golang png image composition

gistfile1.go

package main

// http://blog.golang.org/go-image-package
// http://d.hatena.ne.jp/taknb2nch/20131231/1388500659
// https://blog.golang.org/go-imagedraw-package
import (
        "flag"
        "fmt"
        "image"
        "image/draw"
        "image/png"
        "os"
)

func main() {
        flag.Parse()
        basename := flag.Arg(0)
        overlapname := flag.Arg(1)
        ifd_base, err := os.Open(basename)
        if err != nil {
                fmt.Fprintln(os.Stderr, "not found:"+basename)
                os.Exit(1)
        }
        ifd_overlap, err := os.Open(overlapname)
        if err != nil {
                fmt.Fprintln(os.Stderr, "not found:"+overlapname)
                os.Exit(1)
        }
        image_base, _ := png.Decode(ifd_base)
        image_overlap, _ := png.Decode(ifd_overlap)
        //
        rect := image_base.Bounds()
        point := image.Point{0, 0}
        image_rgba := image_base.(*image.RGBA)
        draw.Draw(image_rgba, rect, image_overlap, point, 0)
        png.Encode(os.Stdout, image_rgba)
}