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examples/blur: fix the blur filter example to be a real box blur. (#2000)

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Also add a note that this is not how games should implement a blur if avoidable.

Krita's blur and the original blur:
https://rm.cloudns.org/img/uploaded/29c0418f25bf56b802f746d2f269e215.png
The original image and the new blur (Krita's): 
https://rm.cloudns.org/img/uploaded/bd04828baf6c01fcf7301a6672a4c406.png

Note how Ebiten's blur has almost no vertical component.

Note how after the change it looks basically like Krita's.
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divVerent 2022-02-23 13:15:52 -05:00 committed by GitHub
parent 62229e82a6
commit 99aef33970
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1 changed files with 26 additions and 4 deletions
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30
examples/blur/main.go
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@ -49,17 +49,39 @@ func (g *Game) Draw(screen *ebiten.Image) {
// Box blur (7x7)
// https://en.wikipedia.org/wiki/Box_blur
//
// Note that this is a fixed function implementation of a box blur - more
// efficiency can be gained by using a separable blur
// (blurring horizontally and vertically separately, or for large blurs,
// even multiple horizontal or vertical passes), ideally combined with
// doing the summing up in a fragment shader (Kage can be used here).
//
// So this implementation only serves to demonstrate use of alpha blending.
layers := 0
for j := -3; j <= 3; j++ {
for i := -3; i <= 3; i++ {
op := &ebiten.DrawImageOptions{}
op.GeoM.Translate(float64(i), 244+float64(j))
// Alpha scale should be 1.0/49.0, but accumulating 1/49 49 times doesn't reach to 1, because
// the final color is affected by the destination alpha when CompositeModeSourceOver is used.
// This is a blur based on the CompositerModeSourceOver composition mode,
// which is basically (GL_ONE, GL_ONE_MINUS_SRC_ALPHA). ColorM acts
// on unpremultiplied colors, but all Ebiten internal colors are
// premultiplied, meaning this mode is regular alpha blending,
// computing each destination pixel as srcPix * alpha + dstPix * (1 - alpha).
//
// This means that the final color is affected by the destination color when CompositeModeSourceOver is used.
// This composite mode is the default mode. See how this is calculated at the doc:
// https://pkg.go.dev/github.com/hajimehoshi/ebiten/v2#CompositeMode
//
// Use a higher value than 1.0/49.0. Here, 1.0/25.0 here to get a reasonable result.
op.ColorM.Scale(1, 1, 1, 1.0/25.0)
// So if using the same alpha every time, the end result will sure be biased towards the last layer.
//
// Correct averaging works based on
// Let A_n := (a_1 + ... + a_n) / n
// A_{n+1} = (a_1 + ... + a_{n+1}) / (n + 1)
// A_{n+1} = (n * A_n + a_{n+1)) / (n + 1)
// A_{n+1} = A_n * (1 - 1/(n+1)) + a_{n+1} * 1/(n+1)
// which is precisely what an alpha blend with alpha 1/(n+1) does.
layers++
op.ColorM.Scale(1, 1, 1, 1.0/float64(layers))
screen.DrawImage(gophersImage, op)
}
}