main.go

package main

import (
	"flag"
	"image"
	"log"
	"os"

	_ "image/gif"
	_ "image/jpeg"
	_ "image/png"

	"github.com/fogleman/gg"
)

var (
	path = flag.String("image", "", "image file path to draw lines")
	w    = flag.Int("w", 0, "count of width in pieces")
	h    = flag.Int("h", 0, "count of height in pieces")
)

func main() {
	flag.Parse()

	if *path == "" {
		log.Fatal("Must give -image")
	}

	file, err := os.Open(*path)
	if err != nil {
		log.Fatal(err)
	}

	img, _, err := image.Decode(file)
	if err != nil {
		log.Fatal(err)
	}

	b := img.Bounds()

	dc := gg.NewContextForImage(img)
	dc.SetHexColor("FFFF00")

	if 1 < *w {
		pw := float64(b.Dx()) / float64(*w)
		for i := 1; i < *w; i++ {
			x := float64(i) * pw
			dc.DrawLine(x, float64(b.Min.Y), x, float64(b.Max.Y))
			dc.Stroke()
		}
	}

	if 1 < *h {
		ph := float64(b.Dy()) / float64(*h)
		for i := 1; i < *h; i++ {
			y := float64(i) * ph
			dc.DrawLine(float64(b.Min.X), y, float64(b.Max.X), y)
			dc.Stroke()
		}
	}

	dc.SavePNG("out.png")
}