66 lines
1.3 KiB
Go
66 lines
1.3 KiB
Go
package pixel
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import "errors"
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// ComposeTarget is a BasicTarget capable of Porter-Duff composition.
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type ComposeTarget interface {
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BasicTarget
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// SetComposeMethod sets a Porter-Duff composition method to be used.
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SetComposeMethod(ComposeMethod)
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}
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// ComposeMethod is a Porter-Duff composition method.
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type ComposeMethod int
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// Here's the list of all available Porter-Duff composition methods. Use ComposeOver for the basic
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// alpha blending.
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const (
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ComposeOver ComposeMethod = iota
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ComposeIn
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ComposeOut
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ComposeAtop
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ComposeRover
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ComposeRin
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ComposeRout
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ComposeRatop
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ComposeXor
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ComposePlus
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ComposeCopy
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)
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// Compose composes two colors together according to the ComposeMethod. A is the foreground, B is
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// the background.
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func (cm ComposeMethod) Compose(a, b RGBA) RGBA {
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var fa, fb float64
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switch cm {
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case ComposeOver:
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fa, fb = 1, 1-a.A
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case ComposeIn:
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fa, fb = b.A, 0
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case ComposeOut:
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fa, fb = 1-b.A, 0
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case ComposeAtop:
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fa, fb = b.A, 1-a.A
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case ComposeRover:
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fa, fb = 1-b.A, 1
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case ComposeRin:
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fa, fb = 0, a.A
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case ComposeRout:
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fa, fb = 0, 1-a.A
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case ComposeRatop:
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fa, fb = 1-b.A, a.A
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case ComposeXor:
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fa, fb = 1-b.A, 1-a.A
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case ComposePlus:
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fa, fb = 1, 1
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case ComposeCopy:
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fa, fb = 1, 0
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default:
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panic(errors.New("Compose: invalid ComposeMethod"))
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}
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return a.Mul(Alpha(fa)).Add(b.Mul(Alpha(fb)))
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}
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