Convert a JPEG image into an "avatar style" circle image

main.go

package main

import (
	"bufio"
	"image"
	"image/color"
	"image/draw"
	_ "image/jpeg"
	"image/png"
	"os"
)

type circle struct {
	p image.Point
	r int
}

func (c *circle) ColorModel() color.Model {
	return color.AlphaModel
}

func (c *circle) Bounds() image.Rectangle {
	return image.Rect(c.p.X-c.r, c.p.Y-c.r, c.p.X+c.r, c.p.Y+c.r)
}

func (c *circle) At(x, y int) color.Color {
	xx, yy, rr := float64(x-c.p.X)+0.5, float64(y-c.p.Y)+0.5, float64(c.r)
	if xx*xx+yy*yy < rr*rr {
		return color.Alpha{255}
	}
	return color.Alpha{0}
}

// applyCircleMask creates a new image from a round shape within the original
func applyCircleMask(source image.Image, origin image.Point, r int) image.Image {
	c := &circle{origin, r}
	result := image.NewRGBA(c.Bounds())
	draw.DrawMask(result, source.Bounds(), source, image.Point{}, c, image.Point{}, draw.Over)
	return result
}

func main() {
	// load source
	inFile, err := os.Open("erik.jpeg")
	if err != nil {
		panic(err)
	}
	defer inFile.Close()
	reader := bufio.NewReader(inFile)
	m, _, err := image.Decode(reader)
	if err != nil {
		panic(err)
	}

	// convert
	b := m.Bounds()
	w := (b.Max.X - b.Min.X) / 2
	h := (b.Max.Y - b.Min.Y) / 2
	r := min(w, h)
	m2 := applyCircleMask(m, image.Point{X: w, Y: h}, r)

	// write to dest
	outFile, err := os.Create("out.png")
	if err != nil {
		panic(err)
	}
	defer outFile.Close()
	if err := png.Encode(outFile, m2); err != nil {
		panic(err)
	}
}