examples: Add doom fire algorithm example (#808)

examples/doomfire/main.go

@@ -0,0 +1,139 @@
+// Copyright 2019 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 jsgo
+
+package main
+
+import (
+	"image/color"
+	"log"
+	"math/rand"
+	"time"
+
+	"github.com/hajimehoshi/ebiten"
+)
+
+const (
+	screenWidth  = 100
+	screenHeight = 50
+	screenSize   = screenWidth * screenHeight
+)
+
+var (
+	pixels      = make([]byte, screenSize*4)
+	firePixels  = make([]byte, screenSize)
+	firePalette = []color.RGBA{
+		{R: 7, G: 7, B: 7, A: 255},       //  0
+		{R: 31, G: 7, B: 7, A: 255},      //  1
+		{R: 47, G: 15, B: 7, A: 255},     //  2
+		{R: 71, G: 15, B: 7, A: 255},     //  3
+		{R: 87, G: 23, B: 7, A: 255},     //  4
+		{R: 103, G: 31, B: 7, A: 255},    //  5
+		{R: 119, G: 31, B: 7, A: 255},    //  6
+		{R: 143, G: 39, B: 7, A: 255},    //  7
+		{R: 159, G: 47, B: 7, A: 255},    //  8
+		{R: 175, G: 63, B: 7, A: 255},    //  9
+		{R: 191, G: 71, B: 7, A: 255},    // 10
+		{R: 199, G: 71, B: 7, A: 255},    // 11
+		{R: 223, G: 79, B: 7, A: 255},    // 12
+		{R: 223, G: 87, B: 7, A: 255},    // 13
+		{R: 223, G: 87, B: 7, A: 255},    // 14
+		{R: 215, G: 95, B: 7, A: 255},    // 15
+		{R: 215, G: 95, B: 7, A: 255},    // 16
+		{R: 215, G: 103, B: 15, A: 255},  // 17
+		{R: 207, G: 111, B: 15, A: 255},  // 18
+		{R: 207, G: 119, B: 15, A: 255},  // 19
+		{R: 207, G: 127, B: 15, A: 255},  // 20
+		{R: 207, G: 135, B: 23, A: 255},  // 21
+		{R: 199, G: 135, B: 23, A: 255},  // 22
+		{R: 199, G: 143, B: 23, A: 255},  // 23
+		{R: 199, G: 151, B: 31, A: 255},  // 24
+		{R: 191, G: 159, B: 31, A: 255},  // 25
+		{R: 191, G: 159, B: 31, A: 255},  // 26
+		{R: 191, G: 167, B: 39, A: 255},  // 27
+		{R: 191, G: 167, B: 39, A: 255},  // 28
+		{R: 191, G: 175, B: 47, A: 255},  // 29
+		{R: 183, G: 175, B: 47, A: 255},  // 30
+		{R: 183, G: 183, B: 47, A: 255},  // 31
+		{R: 183, G: 183, B: 55, A: 255},  // 32
+		{R: 207, G: 207, B: 111, A: 255}, // 33
+		{R: 223, G: 223, B: 159, A: 255}, // 34
+		{R: 239, G: 239, B: 199, A: 255}, // 35
+		{R: 255, G: 255, B: 255, A: 255}, // 36
+	}
+)
+
+func init() {
+	for i := screenSize - screenWidth; i < screenSize; i++ {
+		firePixels[i] = 36
+	}
+}
+
+func updateFirePixels() {
+	for i := 0; i < screenWidth; i++ {
+		for j := 0; j < screenHeight; j++ {
+			idx := i + (screenWidth * j)
+			updateFireIntensityPerPixel(idx)
+		}
+	}
+}
+
+func updateFireIntensityPerPixel(currentPixelIndex int) {
+	below := currentPixelIndex + screenWidth
+	if below >= screenSize {
+		return
+	}
+
+	d := rand.Intn(3)
+	newI := int(firePixels[below]) - d
+	if newI < 0 {
+		newI = 0
+	}
+
+	if currentPixelIndex-d < 0 {
+		return
+	}
+	firePixels[currentPixelIndex-d] = byte(newI)
+}
+
+func renderFire() {
+	for i, v := range firePixels {
+		p := firePalette[v]
+		pixels[i*4] = p.R
+		pixels[i*4+1] = p.G
+		pixels[i*4+2] = p.B
+		pixels[i*4+3] = p.A
+	}
+}
+
+func update(screen *ebiten.Image) error {
+	updateFirePixels()
+	renderFire()
+
+	if ebiten.IsDrawingSkipped() {
+		return nil
+	}
+
+	screen.ReplacePixels(pixels)
+	return nil
+}
+
+func main() {
+	rand.Seed(time.Now().UnixNano())
+
+	if err := ebiten.Run(update, screenWidth, screenHeight, 6, "Doom Fire (Ebiten Demo)"); err != nil {
+		log.Fatal(err)
+	}
+}