Concatenate image files in .png vertically

main.go

package main

import (
	"flag"
	"fmt"
	"image"
	"image/draw"
	"image/png"
	"log"
	"math"
	"os"
)

func main() {
	outfn := flag.String("out", "out.png", "filename to output")
	flag.Parse()

	imgs := make([]image.Image, 0)
	for _, e := range flag.Args() {
		f, err := os.Open(e)
		if err != nil {
			log.Fatalf("%v", err)
		}
		defer f.Close()

		i, _, err := image.Decode(f)
		if err != nil {
			log.Fatalf("%v", err)
		}
		imgs = append(imgs, i)
	}

	var w int
	var h int
	for _, e := range imgs {
		fmt.Printf("%v\n", e.Bounds())
		w = int(math.Max(float64(w), float64(e.Bounds().Dx())))
		h += e.Bounds().Dy()
	}

	r := image.Rectangle{image.Pt(0, 0), image.Pt(w, h)}
	out := image.NewRGBA(r)
	fmt.Printf("%v\n", r)

	var y int
	for _, e := range imgs {
		dr := image.Rectangle{image.Pt(0, y), image.Pt(w, y+e.Bounds().Dy())}
		draw.Draw(out, dr, e, image.Pt(0, 0), draw.Src)
		y += e.Bounds().Dy()
		fmt.Printf("y: %v\n", y)
	}

	outf, _ := os.Create(*outfn)
	defer outf.Close()
	err := png.Encode(outf, out)
	if err != nil {
		log.Fatalf("%v", err)
	}
}