Midpoint circle algorithm using Jeska's algorithm

jeska.go

package jeska

import (
	"image"
	"image/color"
)

func CircleOutline(img *image.RGBA, r int, c color.RGBA) {
	t1 := r / 16 // Nicer looking circle when initialized.
	x := r
	y := 0
	for x >= y {
		img.Set(x, y, c)
		img.Set(x, -y, c)
		img.Set(-x, y, c)
		img.Set(-x, -y, c)
		img.Set(y, x, c)
		img.Set(y, -x, c)
		img.Set(-y, x, c)
		img.Set(-y, -x, c)
		y++
		t1 += y
		t2 := t1 - x
		if t2 >= 0 {
			t1 = t2
			x--
		}
	}
}

func CircleFill(img *image.RGBA, r int, c color.RGBA) {
	t1 := r / 16 // Nicer looking circle when initialized.
	x := r
	y := 0
	for x >= y {
		for ix := -x; ix <= x; ix++ {
			img.Set(ix, y, c)
			img.Set(ix, -y, c)
			img.Set(y, ix, c)
			img.Set(-y, ix, c)
		}
		y++
		t1 += y
		t2 := t1 - x
		if t2 >= 0 {
			t1 = t2
			x--
		}
	}
}

A generalized version available at my next gist: https://gist.github.com/soypat/1253c460cac3a5e0c92862b879a02577