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examples/masking: Use source-in instead of source-out for code readability

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Hajime Hoshi 2018-01-29 02:53:34 +09:00
parent 023315c256
commit d6769659eb
1 changed files with 13 additions and 15 deletions
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28
examples/masking/main.go
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@ -65,12 +65,11 @@ func init() {
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))))
// Alphas around the center are 0 and values outside of the circle are 0xff.
b := uint8(max(0, min(0xff, int(3*d*0xff/r)-2*0xff)))
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)
}
@ -100,25 +99,24 @@ func update(screen *ebiten.Image) error {
return nil
}
maskedFgImage.Clear()
// Reset the maskedFgImage.
maskedFgImage.Fill(color.White)
op := &ebiten.DrawImageOptions{}
op.CompositeMode = ebiten.CompositeModeCopy
op.GeoM.Translate(float64(spotLightX), float64(spotLightY))
maskedFgImage.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 (maskedFgImage). With source-out, the destination
// image values are not used at the result image.
// Use 'source-in' composite mode so that the source image (fgImage) is used but the alpha
// is determined by the destination image (maskedFgImage).
//
// 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 maskedFgImage draws the source image
// with a hole that shape is spotLightImage.
// The result image is the source image with the destination alpha. In maskedFgImage, alpha
// values in the hole is 0 and alpha values in other places are 0xff. As a result, the
// maskedFgImage draws the source image with a hole that shape is spotLightImage. Note that
// RGB values in the destination image are ignored.
//
// See also https://www.w3.org/TR/compositing-1/#porterduffcompositingoperators_srcout.
// See also https://www.w3.org/TR/compositing-1/#porterduffcompositingoperators_srcin.
op = &ebiten.DrawImageOptions{}
op.CompositeMode = ebiten.CompositeModeSourceOut
op.CompositeMode = ebiten.CompositeModeSourceIn
maskedFgImage.DrawImage(fgImage, op)
screen.Fill(color.RGBA{0x00, 0x00, 0x80, 0xff})