Weird wave band animation

lissajous.go

package main

import (
	"image"
	"image/color"
	"image/gif"
	"io"
	"math"
	"math/rand"
	"os"
)

var fgColor = color.RGBA{0x00, 0xff, 0x00, 0xff}
var bgColor = color.Black

var palette = []color.Color{bgColor, fgColor}

const (
	bgIndex = 0 // first color in palette
	fgIndex = 1 // next color in palette
)

func main() {
	lissajous(os.Stdout)
}

func lissajous(out io.Writer) {
	const (
		cycles = 3 // number of complete x osc revolutions
		res = 0.001 // angular resolution
		size = 100 // image canvas covers [-size .. +size]
		nframes = 64 // number of animation frames
		delay = 8 // delay between frames in 10ms units
	)

	freq := rand.Float64() * 6.0 // relative frequency of y osc
	anim := gif.GIF{LoopCount: nframes}
	phase := 1.0 // phase difference
	
	for i := 0; i < nframes; i++ {
		rect := image.Rect(0, 0, 2 * size + 1, 2 * size + 1)
		img := image.NewPaletted(rect, palette)
		for t := 0.0; t < cycles * 2 * math.Pi; t += res {
			x := math.Tan(t)
			y := math.Tan(t * freq + phase)
			img.SetColorIndex(size + int(x*size+0.5), size + int(y*size+0.5), fgIndex)
		}
		phase += 0.1
		anim.Delay = append(anim.Delay, delay)
		anim.Image = append(anim.Image, img)
	}
	gif.EncodeAll(out, &anim) // Ignoring encoding errors
}