mandelmapper/render.go

package main

import (
	"image"
	"image/color"
	"image/png"
	"log"
	"math"
	"math/cmplx"
	"net/http"
	"runtime"
	"strconv"
	"strings"
	"sync"

	"deadbeef.codes/steven/mandelmapper/palette"
)

const (
	Size = 128
	//Iterations = (1<<16 - 1) / 256
	Iterations = (1<<16 - 1)
)

var (
	colors []color.RGBA
	queue  = make(chan pixel)
)

type pixel struct {
	out          *image.RGBA
	x, y         int
	tileX, tileY int64
	tileZoom     uint16
	wg           *sync.WaitGroup
}

func init() {
	http.HandleFunc("/mandelbrot/", renderTile)
}

func main() {
	runtime.GOMAXPROCS(runtime.NumCPU())

	for i := 0; i < runtime.GOMAXPROCS(0); i++ {
		go computeThread()
	}

	colorStep := float64(Iterations)
	colors = interpolateColors("Plan9", colorStep)

	log.Fatal(http.ListenAndServe(":6161", nil))
}

func computeThread() {
	for p := range queue {
		val := mandelbrot(
			complex(
				(float64(p.x)/Size+float64(p.tileX))/float64(uint(1<<(p.tileZoom-1))),
				(float64(p.y)/Size+float64(p.tileY))/float64(uint(1<<(p.tileZoom-1))),
			),
		)
		p.out.SetRGBA(p.x, p.y, colors[val])
		p.wg.Done()
	}
}

//interpolateColors accepts a color palette and number of desired colors and builds a slice of colors by interpolating the gaps
func interpolateColors(paletteCode string, numberOfColors float64) []color.RGBA {
	var factor float64
	steps := []float64{}
	cols := []uint32{}
	interpolated := []uint32{}
	interpolatedColors := []color.RGBA{}
	for _, v := range palette.ColorPalettes {
		factor = 1.0 / numberOfColors
		switch v.Keyword {
		case paletteCode:
			if paletteCode != "" {
				for index, col := range v.Colors {
					if col.Step == 0.0 && index != 0 {
						stepRatio := float64(index+1) / float64(len(v.Colors))
						step := float64(int(stepRatio*100)) / 100 // truncate to 2 decimal precision
						steps = append(steps, step)
					} else {
						steps = append(steps, col.Step)
					}
					r, g, b, a := col.Color.RGBA()
					r /= 0xff
					g /= 0xff
					b /= 0xff
					a /= 0xff
					uintColor := uint32(r)<<24 | uint32(g)<<16 | uint32(b)<<8 | uint32(a)
					cols = append(cols, uintColor)
				}
				var min, max, minColor, maxColor float64
				if len(v.Colors) == len(steps) && len(v.Colors) == len(cols) {
					for i := 0.0; i <= 1; i += factor {
						for j := 0; j < len(v.Colors)-1; j++ {
							if i >= steps[j] && i < steps[j+1] {
								min = steps[j]
								max = steps[j+1]
								minColor = float64(cols[j])
								maxColor = float64(cols[j+1])
								uintColor := cosineInterpolation(maxColor, minColor, (i-min)/(max-min))
								interpolated = append(interpolated, uint32(uintColor))
							}
						}
					}
				}
				for _, pixelValue := range interpolated {
					r := pixelValue >> 24 & 0xff
					g := pixelValue >> 16 & 0xff
					b := pixelValue >> 8 & 0xff
					a := 0xff

					interpolatedColors = append(interpolatedColors, color.RGBA{uint8(r), uint8(g), uint8(b), uint8(a)})
				}
			}
		}
	}
	return interpolatedColors
}

func cosineInterpolation(c1, c2, mu float64) float64 {
	mu2 := (1 - math.Cos(mu*math.Pi)) / 2.0
	return c1*(1-mu2) + c2*mu2
}

func linearInterpolation(c1, c2, mu uint32) uint32 {
	return c1*(1-mu) + c2*mu
}

func renderTile(w http.ResponseWriter, r *http.Request) {
	components := strings.Split(r.URL.Path, "/")[1:]

	if len(components) != 4 || components[0] != "mandelbrot" || components[3][len(components[3])-4:] != ".png" {
		w.WriteHeader(http.StatusNotFound)
		return
	}
	components[3] = components[3][:len(components[3])-4]

	tileX, err := strconv.ParseInt(components[2], 10, 64)
	if err != nil {
		w.WriteHeader(http.StatusNotFound)
		return
	}
	tileY, err := strconv.ParseInt(components[3], 10, 64)
	if err != nil {
		w.WriteHeader(http.StatusNotFound)
		return
	}
	tileZoom, err := strconv.ParseUint(components[1], 10, 8)
	if err != nil {
		w.WriteHeader(http.StatusNotFound)
		return
	}

	var wg sync.WaitGroup
	wg.Add(Size * Size)
	img := image.NewRGBA(image.Rect(0, 0, Size, Size))

	for x := 0; x < Size; x++ {
		for y := 0; y < Size; y++ {

			queue <- pixel{img, x, y, tileX - int64(1<<tileZoom-1) - 1, tileY, uint16(tileZoom), &wg}
		}
	}
	wg.Wait()
	w.Header().Set("Content-Type", "image/png")
	png.Encode(w, img)
}

func mandelbrot(c complex128) uint16 {
	var z complex128
	for i := 0; i < Iterations; i++ {
		z = z*z + c
		if cmplx.IsNaN(z) {
			return uint16(i)
		}
	}
	return Iterations
}