Add examples/mandelbrot

Fixes #360

examples/mandelbrot/main.go

@@ -0,0 +1,102 @@
+// Copyright 2017 The Ebiten Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build example
+
+package main
+
+import (
+	"fmt"
+	"log"
+	"math"
+
+	"github.com/hajimehoshi/ebiten"
+	"github.com/hajimehoshi/ebiten/ebitenutil"
+)
+
+const (
+	screenWidth  = 640
+	screenHeight = 640
+	maxIt        = 128
+)
+
+var (
+	offscreen    *ebiten.Image
+	offscreenPix []byte
+	palette      [maxIt]byte
+)
+
+func init() {
+	offscreen, _ = ebiten.NewImage(screenWidth, screenHeight, ebiten.FilterNearest)
+	offscreenPix = make([]byte, screenWidth*screenHeight*4)
+	for i := range palette {
+		c := byte(math.Sqrt(float64(i)/float64(len(palette))) * 0xff)
+		palette[i] = c
+	}
+}
+
+func color(it int) (r, g, b byte) {
+	if it == maxIt {
+		return 0, 0, 0
+	}
+	c := palette[it]
+	return c, c, c
+}
+
+func updateOffscreen(centerX, centerY, size float64) {
+	for j := 0; j < screenHeight; j++ {
+		for i := 0; i < screenHeight; i++ {
+			x := float64(i)*size/screenWidth - size/2 + centerX
+			y := (screenHeight-float64(j))*size/screenHeight - size/2 + centerY
+			c := complex(x, y)
+			z := c
+			it := 0
+			for ; it < maxIt; it++ {
+				nz := z*z + c
+				if real(nz)*real(nz)+imag(nz)*imag(nz) > 4 {
+					break
+				}
+				z = nz
+			}
+			r, g, b := color(it)
+			p := 4 * (i + j*screenWidth)
+			offscreenPix[p] = r
+			offscreenPix[p+1] = g
+			offscreenPix[p+2] = b
+			offscreenPix[p+3] = 0xff
+		}
+	}
+	offscreen.ReplacePixels(offscreenPix)
+}
+
+func init() {
+	// Now it is not feasible to call updateOffscreen every frame due to performance.
+	updateOffscreen(-0.75, 0.25, 2)
+}
+
+func update(screen *ebiten.Image) error {
+	if ebiten.IsRunningSlowly() {
+		return nil
+	}
+
+	screen.DrawImage(offscreen, nil)
+	ebitenutil.DebugPrint(screen, fmt.Sprintf("FPS: %0.2f", ebiten.CurrentFPS()))
+	return nil
+}
+
+func main() {
+	if err := ebiten.Run(update, screenWidth, screenHeight, 1, "Mandelbrot (Ebiten Demo)"); err != nil {
+		log.Fatal(err)
+	}
+}