Generalized circle midpoint algorithm adapted for drawing multiple shapes

jeska2.go

package main

import (
	"image"
	"image/color"
	"image/png"
	"os"
)

func main() {
	red := color.RGBA{R: 255, A: 255}
	img := image.NewRGBA(image.Rect(-100, -100, 100, 100))
	r := 80
	for i := 1; i < r; i++ {
		Outline(img, i, red, nextTest)
	}
	// jeskoFill(img, r, red)
	fp, _ := os.Create("circle.png")
	defer fp.Close()
	png.Encode(fp, img)
}

func nextCircle(x, y, t1 int) (xnext, ynext, t1next int) {
	y++
	t1 += y
	t2 := t1 - x
	if t2 >= 0 {
		t1 = t2
		x--
	}
	return x, y, t1
}

func Outline(img *image.RGBA, r int, c color.RGBA, next func(x, y, t1 int) (xnext, ynext, t1next int)) {
	t1 := r / 16 // Initialize for nicer looking shapes (circle case).
	x := r
	y := 0
	for x >= y {
		img.SetRGBA(x, y, c)
		img.SetRGBA(x, -y, c)
		img.SetRGBA(-x, y, c)
		img.SetRGBA(-x, -y, c)
		img.SetRGBA(y, x, c)
		img.SetRGBA(y, -x, c)
		img.SetRGBA(-y, x, c)
		img.SetRGBA(-y, -x, c)
		xprev := x
		yprev := y
		x, y, t1 = next(x, y, t1)
		if xprev == x && yprev == y {
			panic("unchanging x and y")
		}
	}
}

func Fill(img *image.RGBA, r int, c color.RGBA, next func(x, y, t1 int) (xnext, ynext, t1next int)) {
	t1 := r / 16 // Initialize for nicer looking shapes (circle case).
	x := r
	y := 0
	for x >= y {
		for ix := -x; ix <= x; ix++ { // TODO: optimization: draw only when y value changes from greatest x found.
			img.SetRGBA(ix, y, c)
			img.SetRGBA(ix, -y, c)
			img.SetRGBA(y, ix, c)
			img.SetRGBA(-y, ix, c)
		}
		xprev := x
		yprev := y
		x, y, t1 = next(x, y, t1)
		if xprev == x && yprev == y {
			panic("unchanging x and y")
		}
	}
}

func nextTest(x, y, t1 int) (xnext, ynext, t1next int) {
	y++
	t1 += y * x
	t2 := t1 - x
	if t2 >= 0 {
		t1 = t2
		x -= y
	}
	return x, y, t1
}

func nextParabolaStar(x, y, t1 int) (xnext, ynext, t1next int) {
	y++
	t1 += y * x
	t2 := t1 - x
	if t2 >= 0 {
		t1 = t2
		x -= y
	}
	return x, y, t1
}

func next4Star(x, y, t1 int) (xnext, ynext, t1next int) {
	y++
	t1 += y * x * 2
	t2 := t1 - x
	if t2 >= 0 {
		t1 = t2
		x -= 2 * t2 / t1
	}
	return x, y, t1
}

func nextParabolaFractal(x, y, t1 int) (xnext, ynext, t1next int) {
	y++
	x *= 100
	x /= 90
	t1 += y
	t2 := t1 - x
	if t2 >= 0 {
		t1 = t2
		x -= 1
	}
	return x, y, t1
}

func nextChamfered4Star(x, y, t1 int) (xnext, ynext, t1next int) {
	y++
	t1 += y
	t2 := t1 - x
	if t2 >= 0 {
		t1 = t2 + x
		x -= 2
	}
	return x, y, t1
}

func nextClover(x, y, t1 int) (xnext, ynext, t1next int) {
	y++
	t1 += y
	t2 := t1 - x
	if t2 >= 0 {
		t1 = t2
		x -= 2
	}
	return x, y, t1
}

func nextSquaredCircle(x, y, t1 int) (xnext, ynext, t1next int) {
	y++
	t1 += y
	t2 := t1 - x
	if t2 >= 0 {
		t1 = t2 - y
		x--
	}
	return x, y, t1
}

func nextChamferedSquareDiamond(x, y, t1 int) (xnext, ynext, t1next int) {
	y++
	t1 += y
	t2 := t1 - x
	if t2 >= 0 {
		t1 = t2 + x
		x--
	}
	return x, y, t1
}

func nextRoundedSquareDiamond(x, y, t1 int) (xnext, ynext, t1next int) {
	y++
	t1 += y
	t2 := t1 - x/2
	if t2 >= 0 {
		t1 = t2
		x--
	}
	return x, y, t1
}

func nextSquareDiamond(x, y, t1 int) (xnext, ynext, t1next int) {
	y++
	t1 += y
	t2 := t1
	if t2 >= 0 {
		t1 = t2 + x
		x--
	}
	return x, y, t1
}

func nextExpl(x, y, t1 int) (xnext, ynext, t1next int) {
	ynext = y + 1
	t1next = t1 + y
	t2 := t1 - x
	if t2 >= 0 {
		t1next = t2
		xnext = x + 1
	} else {
		xnext = x
	}
	return xnext, ynext, t1next
}