package main

import (
	"image"
	"image/color"
)

func convertToGray(path string) error {
	src, err := decodeImage(path)
	if err != nil {
		return err
	}
	bounds := src.Bounds()
	dst := image.NewRGBA(bounds)
	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
		for x := bounds.Min.X; x < bounds.Max.X; x++ {
			c := src.At(x, y)
			rgba := color.NRGBAModel.Convert(c).(color.NRGBA)
			if rgba.A > 0 {
				gray := color.GrayModel.Convert(c).(color.Gray)
				rgba.R, rgba.G, rgba.B = gray.Y, gray.Y, gray.Y
				dst.Set(x, y, rgba)
			} else {
				dst.Set(x, y, c)
			}
		}
	}
	return encodePNG(path, dst)
}