examples: Add an example "stars" (#1679)

examples/stars/main.go

@@ -0,0 +1,115 @@
+// Copyright 2021 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.
+
+//go:build example
+// +build example
+
+package main
+
+import (
+	"image/color"
+	"log"
+	"math/rand"
+	"time"
+
+	"github.com/hajimehoshi/ebiten/v2"
+	"github.com/hajimehoshi/ebiten/v2/ebitenutil"
+)
+
+const (
+	screenWidth  = 640
+	screenHeight = 480
+	scale        = 64
+	starsNum     = 1024
+)
+
+func abs(a int) int {
+	if a < 0 {
+		return -a
+	}
+	return a
+}
+
+type Star struct {
+	fromx, fromy, tox, toy, brightness float64
+}
+
+func (s *Star) Init() {
+	s.tox = rand.Float64() * screenWidth * scale
+	s.fromx = s.tox
+	s.toy = rand.Float64() * screenHeight * scale
+	s.fromy = s.toy
+	s.brightness = rand.Float64() * 0xff
+}
+
+func (s *Star) Update(x, y float64) {
+	s.fromx = s.tox
+	s.fromy = s.toy
+	s.tox += (s.tox - x) / 32
+	s.toy += (s.toy - y) / 32
+	s.brightness += 1
+	if 0xff < s.brightness {
+		s.brightness = 0xff
+	}
+	if s.fromx < 0 || screenWidth*scale < s.fromx || s.fromy < 0 || screenHeight*scale < s.fromy {
+		s.Init()
+	}
+}
+
+func (s *Star) Draw(screen *ebiten.Image) {
+	color := color.RGBA{uint8(0xbb * s.brightness / 0xff),
+		uint8(0xdd * s.brightness / 0xff),
+		uint8(0xff * s.brightness / 0xff),
+		0xff}
+	ebitenutil.DrawLine(screen, s.fromx/scale, s.fromy/scale, s.tox/scale, s.toy/scale, color)
+}
+
+type Game struct {
+	stars [starsNum]Star
+}
+
+func NewGame() *Game {
+	g := &Game{}
+	for i := 0; i < starsNum; i++ {
+		g.stars[i].Init()
+	}
+	return g
+}
+
+func (g *Game) Update() error {
+	x, y := ebiten.CursorPosition()
+	for i := 0; i < starsNum; i++ {
+		g.stars[i].Update(float64(x*scale), float64(y*scale))
+	}
+	return nil
+}
+
+func (g *Game) Draw(screen *ebiten.Image) {
+	for i := 0; i < starsNum; i++ {
+		g.stars[i].Draw(screen)
+	}
+}
+
+func (g *Game) Layout(outsideWidth, outsideHeight int) (int, int) {
+	return screenWidth, screenHeight
+}
+
+func main() {
+	rand.Seed(time.Now().UnixNano())
+	ebiten.SetWindowSize(screenWidth*2, screenHeight*2)
+	ebiten.SetWindowTitle("Stars (Ebiten Demo)")
+	if err := ebiten.RunGame(NewGame()); err != nil {
+		log.Fatal(err)
+	}
+}