Add example 'masking' (#39)

examples/_resources/images/license.md

@@ -28,14 +28,26 @@ Email: info@9031.com
 http://www.sozai-page.com/02_sozai/b/b04/b04_002/b04_002.html
 ```
 
+## fiveyears.jpg
+
+```
+The Go gopher was designed by Renee French. (http://reneefrench.blogspot.com/)
+The design is licensed under the Creative Commons 3.0 Attributions license.
+Read this article for more details: https://blog.golang.org/gopher
+```
+
 ## gophers.jpg
 
 ```
 http://blog.golang.org/go-programming-language-turns-two
+
+Photograph by Chris Nokleberg
+the Creative Commons Attribution 3.0 License
 ```
 
 ## Other image files
 
 ```
-Creative Commons Attribution 3.0 License
+Copyright 2014 Hajime Hoshi
+the Creative Commons Attribution 3.0 License
 ```

examples/masking/main.go

@@ -0,0 +1,141 @@
+// Copyright 2016 Hajime Hoshi
+//
+// 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 (
+	"image"
+	"image/color"
+	_ "image/jpeg"
+	"log"
+	"math"
+
+	"github.com/hajimehoshi/ebiten"
+	"github.com/hajimehoshi/ebiten/ebitenutil"
+)
+
+const (
+	screenWidth  = 320
+	screenHeight = 240
+)
+
+var (
+	gophersImage   *ebiten.Image
+	fiveyearsImage *ebiten.Image
+	maskImage      *ebiten.Image
+	spotLightImage *ebiten.Image
+	spotLightX     = 0
+	spotLightY     = 0
+	spotLightVX    = 1
+	spotLightVY    = 1
+)
+
+func update(screen *ebiten.Image) error {
+	spotLightX += spotLightVX
+	spotLightY += spotLightVY
+	if spotLightX < 0 {
+		spotLightX = -spotLightX
+		spotLightVX = -spotLightVX
+	}
+	if spotLightY < 0 {
+		spotLightY = -spotLightY
+		spotLightVY = -spotLightVY
+	}
+	w, h := spotLightImage.Size()
+	maxX, maxY := screenWidth-w, screenHeight-h
+	if maxX < spotLightX {
+		spotLightX = -spotLightX + 2*maxX
+		spotLightVX = -spotLightVX
+	}
+	if maxY < spotLightY {
+		spotLightY = -spotLightY + 2*maxY
+		spotLightVY = -spotLightVY
+	}
+
+	if err := maskImage.Clear(); err != nil {
+		return err
+	}
+
+	op := &ebiten.DrawImageOptions{}
+	op.GeoM.Translate(float64(spotLightX), float64(spotLightY))
+	if err := maskImage.DrawImage(spotLightImage, op); err != nil {
+		return err
+	}
+
+	op = &ebiten.DrawImageOptions{}
+	op.CompositionMode = ebiten.CompositionModeSourceOut
+	if err := maskImage.DrawImage(fiveyearsImage, op); err != nil {
+		return err
+	}
+
+	if err := screen.Fill(color.RGBA{0x00, 0x00, 0x80, 0xff}); err != nil {
+		return err
+	}
+	if err := screen.DrawImage(gophersImage, &ebiten.DrawImageOptions{}); err != nil {
+		return err
+	}
+	if err := screen.DrawImage(maskImage, &ebiten.DrawImageOptions{}); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func max(a, b int) int {
+	if a < b {
+		return b
+	}
+	return a
+}
+
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}
+
+func main() {
+	var err error
+	gophersImage, _, err = ebitenutil.NewImageFromFile("_resources/images/gophers.jpg", ebiten.FilterNearest)
+	if err != nil {
+		log.Fatal(err)
+	}
+	fiveyearsImage, _, err = ebitenutil.NewImageFromFile("_resources/images/fiveyears.jpg", ebiten.FilterNearest)
+	if err != nil {
+		log.Fatal(err)
+	}
+	maskImage, err = ebiten.NewImage(screenWidth, screenHeight, ebiten.FilterNearest)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	as := image.Point{128, 128}
+	a := image.NewAlpha(image.Rectangle{image.ZP, as})
+	for j := 0; j < as.Y; j++ {
+		for i := 0; i < as.X; i++ {
+			r := as.X / 2
+			d := math.Sqrt(float64((i-r)*(i-r) + (j-r)*(j-r)))
+			b := uint8(max(0, min(0xff, 3*0xff-int(d*3*0xff)/r)))
+			a.SetAlpha(i, j, color.Alpha{b})
+		}
+	}
+	spotLightImage, err = ebiten.NewImageFromImage(a, ebiten.FilterNearest)
+	if err != nil {
+		log.Fatal(err)
+	}
+	if err := ebiten.Run(update, screenWidth, screenHeight, 2, "Masking (Ebiten Demo)"); err != nil {
+		log.Fatal(err)
+	}
+}

graphics.go

@@ -39,7 +39,19 @@ func glFilter(c *opengl.Context, filter Filter) opengl.Filter {
 // CompositionMode represents Porter-Duff composition mode.
 type CompositionMode int
 
+// Note: This name convention follow CSS compositing: https://drafts.fxtf.org/compositing-2/
+
 const (
-	CompositionModeSourceOver CompositionMode = CompositionMode(opengl.CompositionModeSourceOver) // regular alpha blending
+	CompositionModeSourceOver      CompositionMode = CompositionMode(opengl.CompositionModeSourceOver) // regular alpha blending
-	CompositionModeLighter                    = CompositionMode(opengl.CompositionModeLighter)    // sum of source and destination (a.k.a. 'plus' or 'additive')
+	CompositionModeClear                           = CompositionMode(opengl.CompositionModeClear)
+	CompositionModeCopy                            = CompositionMode(opengl.CompositionModeCopy)
+	CompositionModeDestination                     = CompositionMode(opengl.CompositionModeDestination)
+	CompositionModeSourceIn                        = CompositionMode(opengl.CompositionModeSourceIn)
+	CompositionModeDestinationIn                   = CompositionMode(opengl.CompositionModeDestinationIn)
+	CompositionModeSourceOut                       = CompositionMode(opengl.CompositionModeSourceOut)
+	CompositionModeDestinationOut                  = CompositionMode(opengl.CompositionModeDestinationOut)
+	CompositionModeSourceAtop                      = CompositionMode(opengl.CompositionModeSourceAtop)
+	CompositionModeDestinationAtop                 = CompositionMode(opengl.CompositionModeDestinationAtop)
+	CompositionModeXor                             = CompositionMode(opengl.CompositionModeXor)
+	CompositionModeLighter                         = CompositionMode(opengl.CompositionModeLighter) // sum of source and destination (a.k.a. 'plus' or 'additive')
 )

internal/graphics/opengl/types.go

@@ -47,8 +47,8 @@ const (
 	CompositionModeSourceOver CompositionMode = iota // This value must be 0 (= initial value)
 	CompositionModeClear
 	CompositionModeCopy
-	CompositionModeDesination
+	CompositionModeDestination
-	CompositionModeDesinationOver
+	CompositionModeDestinationOver
 	CompositionModeSourceIn
 	CompositionModeDestinationIn
 	CompositionModeSourceOut
@@ -68,10 +68,10 @@ func (c *Context) operations(mode CompositionMode) (src operation, dst operation
 		return c.zero, c.zero
 	case CompositionModeCopy:
 		return c.one, c.zero
-	case CompositionModeDesination:
+	case CompositionModeDestination:
 		return c.zero, c.one
-	case CompositionModeDesinationOver:
+	case CompositionModeDestinationOver:
-		return c.one, c.oneMinusDstAlpha
+		return c.oneMinusDstAlpha, c.one
 	case CompositionModeSourceIn:
 		return c.dstAlpha, c.zero
 	case CompositionModeDestinationIn: