Compute mean luma of image files

mean_luma.go

package main

import (
	"fmt"
	"image"
	"image/color"
	_ "image/jpeg"
	"os"
)

func MeanLuma(img image.Image) (mean float64) {
	bounds := img.Bounds()
	count := 0
	for x := bounds.Min.X; x < bounds.Max.X; x++ {
		for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
			count++
			pixel := color.YCbCrModel.Convert(img.At(x, y)).(color.YCbCr)
			luma := float64(pixel.Y)
			mean += (luma - mean) / float64(count)
		}
	}

	return
}

func LoadImageFile(path string) (image.Image, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, err
	}
	defer f.Close()

	img, _, err := image.Decode(f)
	if err != nil {
		return nil, err
	}

	return img, nil
}

func main() {
	for _, arg := range os.Args[1:] {
		img, err := LoadImageFile(arg)
		if err != nil {
			fmt.Printf("Failed to load %q: %v\n", arg, err)
			continue
		}

		fmt.Printf("%s: mean Luma: %v\n", arg, MeanLuma(img))
	}
}