90 lines
3.0 KiB
Plaintext
90 lines
3.0 KiB
Plaintext
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#Topic Blend_Mode_Overview
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Describes how destination pixel is replaced with a combination of itself and
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source pixel. Blend_Mode may use source, destination, or both. Blend_Mode may
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operate on each Color component independently, or may allow all source pixel
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components to contribute to one destination pixel component.
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Blend_Mode does not use adjacent pixels to determine the outcome.
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Blend_Mode uses source and read destination Alpha to determine written
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destination Alpha; both source and destination Alpha may also affect written
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destination Color components.
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Regardless of how Alpha is encoded in source and destination pixel, nearly all
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Color_Types treat it as ranging from zero to one. And, nearly all Blend_Mode
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algorithms limit the output so that all results are also zero to one.
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Two exceptions are SkBlendMode::kPlus and kRGBA_F16_SkColorType.
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SkBlendMode::kPlus permits computing Alpha and Color component values larger
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than one. For Color_Types other than kRGBA_F16_SkColorType, resulting Alpha
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and component values are clamped to one.
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kRGBA_F16_SkColorType permits values outside the zero to one range. It is up
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to the client to ensure that the result is within the range of zero to one,
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and therefore well-defined.
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#Subtopic Porter_Duff
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#Alias Porter_Duff ##
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#Line # classic color blend algorithms ##
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#A Compositing Digital Images # https://graphics.pixar.com/library/Compositing/paper.pdf ##
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describes Porter_Duff modes SkBlendMode::kClear through SkBlendMode::kXor.
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Drawing a bitmap with transparency using Porter_Duff compositing is free to clear
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the destination.
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#Illustration 1
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Draw geometry with transparency using Porter_Duff compositing does not combine
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transparent source pixels, leaving the destination outside the geometry untouched.
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#Illustration 2
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##
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#Subtopic Lighten_Darken
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#Line # color blends to lighten or darken result ##
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Modes SkBlendMode::kPlus and SkBlendMode::kScreen use
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simple arithmetic to lighten or darken the destination. Modes
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SkBlendMode::kOverlay through SkBlendMode::kMultiply use more complicated
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algorithms to lighten or darken; sometimes one mode does both, as described by
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#A Blend Modes # https://en.wikipedia.org/wiki/Blend_modes ##
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.
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#Illustration
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##
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#Subtopic Modulate_Blend
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#Line # multiply color components ##
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SkBlendMode::kModulate is a mashup of SkBlendMode::kSrcATop and SkBlendMode::kMultiply.
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It multiplies all components, including Alpha; unlike SkBlendMode::kMultiply, if either
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source or destination is transparent, result is transparent. SkBlendMode::kModulate
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uses Premultiplied values to compute the product; SkBlendMode::kMultiply uses Unpremultiplied
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values to compute the product.
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#Illustration
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##
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#Subtopic Color_Blends
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#Line # non-separable blend modes ##
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Modes SkBlendMode::kHue, SkBlendMode::kSaturation, SkBlendMode::kColor, and
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SkBlendMode::kLuminosity convert source and destination pixels using all
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components color information, using
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###$
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$A non-separable blend modes $ https://www.w3.org/TR/compositing-1/#blendingnonseparable $$
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$$$#
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.
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#Illustration
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##
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#Topic Blend_Mode_Overview ##
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