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examples/masking: Add comments

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This commit is contained in:
Hajime Hoshi 2018-01-29 01:59:46 +09:00
parent ac936f31b9
commit 9730af7976
1 changed files with 48 additions and 26 deletions
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74
examples/masking/main.go
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@ -33,8 +33,8 @@ const (
)
var (
gophersImage *ebiten.Image
fiveyearsImage *ebiten.Image
bgImage *ebiten.Image
fgImage *ebiten.Image
maskImage *ebiten.Image
spotLightImage *ebiten.Image
spotLightX = 0
@ -43,6 +43,37 @@ var (
spotLightVY = 1
)
func init() {
var err error
bgImage, _, err = ebitenutil.NewImageFromFile(ebitenutil.JoinStringsIntoFilePath("_resources", "images", "gophers.jpg"), ebiten.FilterNearest)
if err != nil {
log.Fatal(err)
}
fgImage, _, err = ebitenutil.NewImageFromFile(ebitenutil.JoinStringsIntoFilePath("_resources", "images", "fiveyears.jpg"), ebiten.FilterNearest)
if err != nil {
log.Fatal(err)
}
maskImage, _ = ebiten.NewImage(screenWidth, screenHeight, ebiten.FilterNearest)
// Initialize the spot light image.
const r = 64
alphas := image.Point{r * 2, r * 2}
a := image.NewAlpha(image.Rectangle{image.ZP, alphas})
for j := 0; j < alphas.Y; j++ {
for i := 0; i < alphas.X; i++ {
// d is the distance between (i, j) and the (circle) center.
d := math.Sqrt(float64((i-r)*(i-r) + (j-r)*(j-r)))
// Alpha at the center is 0xff and the outside of the circle is 0.
b := uint8(max(0, min(0xff, 3*(0xff-int(d*0xff)/r))))
a.SetAlpha(i, j, color.Alpha{b})
}
}
// Note that alpha values matter an other RGB values don't matter in the spot light image.
spotLightImage, _ = ebiten.NewImageFromImage(a, ebiten.FilterNearest)
}
func update(screen *ebiten.Image) error {
spotLightX += spotLightVX
spotLightY += spotLightVY
@ -64,21 +95,34 @@ func update(screen *ebiten.Image) error {
spotLightY = -spotLightY + 2*maxY
spotLightVY = -spotLightVY
}
if ebiten.IsRunningSlowly() {
return nil
}
maskImage.Clear()
op := &ebiten.DrawImageOptions{}
op.GeoM.Translate(float64(spotLightX), float64(spotLightY))
maskImage.DrawImage(spotLightImage, op)
// Use 'source-out' composite mode so that the source image (fgImage) is used but the alpha
// is determined by the destination image (maskImage). With source-out, the destination image
// values are not used at the result image.
//
// If the alpha value of the destination is 0xff, the source at the point is not adopted.
// In the opposite way, if the alpha value of the destination is 0, the source at the point
// is fully adopted. As alpha values outside of the spot light image are 0, the source values
// are fully adopted there. As a result, the maskImage draws the source image with a hole
// that shape is spotLightImage.
//
// See also https://www.w3.org/TR/compositing-1/#porterduffcompositingoperators_srcout.
op = &ebiten.DrawImageOptions{}
op.CompositeMode = ebiten.CompositeModeSourceOut
maskImage.DrawImage(fiveyearsImage, op)
maskImage.DrawImage(fgImage, op)
screen.Fill(color.RGBA{0x00, 0x00, 0x80, 0xff})
screen.DrawImage(gophersImage, &ebiten.DrawImageOptions{})
screen.DrawImage(bgImage, &ebiten.DrawImageOptions{})
screen.DrawImage(maskImage, &ebiten.DrawImageOptions{})
return nil
@ -99,28 +143,6 @@ func min(a, b int) int {
}
func main() {
var err error
gophersImage, _, err = ebitenutil.NewImageFromFile(ebitenutil.JoinStringsIntoFilePath("_resources", "images", "gophers.jpg"), ebiten.FilterNearest)
if err != nil {
log.Fatal(err)
}
fiveyearsImage, _, err = ebitenutil.NewImageFromFile(ebitenutil.JoinStringsIntoFilePath("_resources", "images", "fiveyears.jpg"), ebiten.FilterNearest)
if err != nil {
log.Fatal(err)
}
maskImage, _ = ebiten.NewImage(screenWidth, screenHeight, ebiten.FilterNearest)
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, _ = ebiten.NewImageFromImage(a, ebiten.FilterNearest)
if err := ebiten.Run(update, screenWidth, screenHeight, 2, "Masking (Ebiten Demo)"); err != nil {
log.Fatal(err)
}