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128 lines
7.1 KiB
128 lines
7.1 KiB
7 months ago
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---
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title: 'SkBlendMode Overview'
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linkTitle: 'SkBlendMode Overview'
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weight: 260
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---
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Describes how destination <a href='undocumented#Pixel'>pixel</a> is replaced
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with a combination of itself and source <a href='undocumented#Pixel'>pixel</a>.
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<a href='#Blend_Mode'>Blend_Mode</a> may use source, destination, or both.
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<a href='#Blend_Mode'>Blend_Mode</a> may operate on each
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<a href='https://api.skia.org/SkColor_8h.html'>Color</a> component
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independently, or may allow all source <a href='undocumented#Pixel'>pixel</a>
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components to contribute to one destination
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<a href='undocumented#Pixel'>pixel</a> component.
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<a href='#Blend_Mode'>Blend_Mode</a> does not use adjacent pixels to determine
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the outcome.
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<a href='#Blend_Mode'>Blend_Mode</a> uses source and read destination
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<a href='https://api.skia.org/SkColor_8h.html#a918cf5a3a68406ac8107f6be48fb906e'>Alpha</a>
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to determine written destination
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<a href='https://api.skia.org/SkColor_8h.html#a918cf5a3a68406ac8107f6be48fb906e'>Alpha</a>;
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both source and destination
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<a href='https://api.skia.org/SkColor_8h.html#a918cf5a3a68406ac8107f6be48fb906e'>Alpha</a>
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may also affect written destination
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<a href='https://api.skia.org/SkColor_8h.html'>Color</a> components.
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Regardless of how
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<a href='https://api.skia.org/SkColor_8h.html#a918cf5a3a68406ac8107f6be48fb906e'>Alpha</a>
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is encoded in source and destination <a href='undocumented#Pixel'>pixel</a>,
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nearly all <a href='#Image_Info_Color_Type'>Color_Types</a> treat it as ranging
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from zero to one. And, nearly all <a href='#Blend_Mode'>Blend_Mode</a>
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algorithms limit the output so that all results are also zero to one.
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Two exceptions are
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kPlus'>kPlus</a>
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and
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<a href='https://api.skia.org/SkImageInfo_8h.html#a9ac0b62b3d2c6c7e1a80db557243f93e'>kRGBA_F16_SkColorType</a>.
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kPlus'>kPlus</a>
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permits computing
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<a href='https://api.skia.org/SkColor_8h.html#a918cf5a3a68406ac8107f6be48fb906e'>Alpha</a>
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and <a href='https://api.skia.org/SkColor_8h.html'>Color</a> component values
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larger than one. For <a href='#Image_Info_Color_Type'>Color_Types</a> other than
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<a href='https://api.skia.org/SkImageInfo_8h.html#a9ac0b62b3d2c6c7e1a80db557243f93e'>kRGBA_F16_SkColorType</a>,
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resulting
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<a href='https://api.skia.org/SkColor_8h.html#a918cf5a3a68406ac8107f6be48fb906e'>Alpha</a>
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and component values are clamped to one.
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<a href='https://api.skia.org/SkImageInfo_8h.html#a9ac0b62b3d2c6c7e1a80db557243f93e'>kRGBA_F16_SkColorType</a>
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permits values outside the zero to one range. It is up to the client to ensure
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that the result is within the range of zero to one, and therefore well-defined.
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<a name='Porter_Duff'></a>
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<a href='https://graphics.pixar.com/library/Compositing/paper.pdf'>Compositing
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Digital Images</a></a> describes
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<a href='#Blend_Mode_Overview_Porter_Duff'>Porter_Duff</a> modes
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kClear'>kClear</a>
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through
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kXor'>kXor</a>.
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Drawing a <a href='https://api.skia.org/classSkBitmap.html'>bitmap</a> with
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transparency using <a href='#Blend_Mode_Overview_Porter_Duff'>Porter_Duff</a>
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compositing is free to clear the destination.
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Draw geometry with transparency using
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<a href='#Blend_Mode_Overview_Porter_Duff'>Porter_Duff</a> compositing does not
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combine transparent source pixels, leaving the destination outside the geometry
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untouched.
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<a name='Lighten_Darken'></a>
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Modes
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kPlus'>kPlus</a>
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and
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kScreen'>kScreen</a>
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use simple arithmetic to lighten or darken the destination. Modes
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kOverlay'>kOverlay</a>
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through
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kMultiply'>kMultiply</a>
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use more complicated algorithms to lighten or darken; sometimes one mode does
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both, as described by <a href='https://en.wikipedia.org/wiki/Blend_modes'>Blend
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Modes</a></a> .
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<a name='Modulate_Blend'></a>
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kModulate'>kModulate</a>
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is a mashup of
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kSrcATop'>kSrcATop</a>
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and
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kMultiply'>kMultiply</a>.
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It multiplies all components, including
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<a href='https://api.skia.org/SkColor_8h.html#a918cf5a3a68406ac8107f6be48fb906e'>Alpha</a>;
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unlike
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kMultiply'>kMultiply</a>,
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if either source or destination is transparent, result is transparent.
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kModulate'>kModulate</a>
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uses <a href='undocumented#Premultiply'>Premultiplied</a> values to compute the
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product;
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kMultiply'>kMultiply</a>
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uses <a href='undocumented#Unpremultiply'>Unpremultiplied</a> values to compute
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the product.
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<a name='Color_Blends'></a>
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Modes
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kHue'>kHue</a>,
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kSaturation'>kSaturation</a>,
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kColor'>kColor</a>,
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and
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<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kLuminosity'>kLuminosity</a>
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convert source and destination pixels using all components
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<a href='https://api.skia.org/SkColor_8h.html'>color</a> information, using
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<a href='https://www.w3.org/TR/compositing-1/#blendingnonseparable'>non-separable
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blend modes</a></a> .
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