ebiten/examples/clip/main.go

// Copyright 2020 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.

package main

import (
	"bytes"
	"image"
	_ "image/jpeg"
	"log"
	"math"

	"github.com/hajimehoshi/ebiten/v2"
	"github.com/hajimehoshi/ebiten/v2/examples/resources/images"
)

const (
	screenWidth  = 640
	screenHeight = 480
)

var (
	gophersImage *ebiten.Image
)

type Game struct {
	count int
}

func (g *Game) Update() error {
	g.count++
	return nil
}

func (g *Game) Draw(screen *ebiten.Image) {
	w, h := gophersImage.Bounds().Dx(), gophersImage.Bounds().Dy()
	op := &ebiten.DrawImageOptions{}

	// Move the image's center to the screen's upper-left corner.
	// This is a preparation for rotating. When geometry matrices are applied,
	// the origin point is the upper-left corner.
	op.GeoM.Translate(-float64(w)/2, -float64(h)/2)

	// Rotate the image. As a result, the anchor point of this rotate is
	// the center of the image.
	op.GeoM.Rotate(float64(g.count%360) * 2 * math.Pi / 360)

	// Move the image to the screen's center.
	op.GeoM.Translate(screenWidth/2, screenHeight/2)

	// Use SubImage to clip the rendering region.
	const size = 60
	r := image.Rect(320-size, 240-size, 320+size, 240+size)
	screen.SubImage(r).(*ebiten.Image).DrawImage(gophersImage, op)
}

func (g *Game) Layout(outsideWidth, outsideHeight int) (int, int) {
	return screenWidth, screenHeight
}

func main() {
	// Decode an image from the image file's byte slice.
	img, _, err := image.Decode(bytes.NewReader(images.Gophers_jpg))
	if err != nil {
		log.Fatal(err)
	}
	gophersImage = ebiten.NewImageFromImage(img)

	ebiten.SetWindowSize(screenWidth, screenHeight)
	ebiten.SetWindowTitle("Clip (Ebitengine Demo)")
	if err := ebiten.RunGame(&Game{}); err != nil {
		log.Fatal(err)
	}
}