1 SkBlendMode Overview 2 === 3 Describes how destination <a href='undocumented#Pixel'>pixel</a> is replaced with a combination of itself and 4 source <a href='undocumented#Pixel'>pixel</a>. <a href='#Blend_Mode'>Blend_Mode</a> may use source, destination, or both. <a href='#Blend_Mode'>Blend_Mode</a> may 5 operate on each <a href='SkColor_Reference#Color'>Color</a> component independently, or may allow all source <a href='undocumented#Pixel'>pixel</a> 6 components to contribute to one destination <a href='undocumented#Pixel'>pixel</a> component. 7 8 <a href='#Blend_Mode'>Blend_Mode</a> does not use adjacent pixels to determine the outcome. 9 10 <a href='#Blend_Mode'>Blend_Mode</a> uses source and read destination <a href='SkColor_Reference#Alpha'>Alpha</a> to determine written 11 destination <a href='SkColor_Reference#Alpha'>Alpha</a>; both source and destination <a href='SkColor_Reference#Alpha'>Alpha</a> may also affect written 12 destination <a href='SkColor_Reference#Color'>Color</a> components. 13 14 Regardless of how <a href='SkColor_Reference#Alpha'>Alpha</a> is encoded in source and destination <a href='undocumented#Pixel'>pixel</a>, nearly all 15 <a href='#Image_Info_Color_Type'>Color_Types</a> treat it as ranging from zero to one. And, nearly all <a href='#Blend_Mode'>Blend_Mode</a> 16 algorithms limit the output so that all results are also zero to one. 17 18 Two exceptions are <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kPlus'>kPlus</a> and <a href='SkImageInfo_Reference#kRGBA_F16_SkColorType'>kRGBA_F16_SkColorType</a>. 19 20 <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kPlus'>kPlus</a> permits computing <a href='SkColor_Reference#Alpha'>Alpha</a> and <a href='SkColor_Reference#Color'>Color</a> component values larger 21 than one. For <a href='#Image_Info_Color_Type'>Color_Types</a> other than <a href='SkImageInfo_Reference#kRGBA_F16_SkColorType'>kRGBA_F16_SkColorType</a>, resulting <a href='SkColor_Reference#Alpha'>Alpha</a> 22 and component values are clamped to one. 23 24 <a href='SkImageInfo_Reference#kRGBA_F16_SkColorType'>kRGBA_F16_SkColorType</a> permits values outside the zero to one range. It is up 25 to the client to ensure that the result is within the range of zero to one, 26 and therefore well-defined. 27 28 <a name='Porter_Duff'></a> 29 30 <a href='https://graphics.pixar.com/library/Compositing/paper.pdf'>Compositing Digital Images</a></a> describes <a href='#Blend_Mode_Overview_Porter_Duff'>Porter_Duff</a> modes <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kClear'>kClear</a> through <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kXor'>kXor</a>. 31 32 Drawing a <a href='SkBitmap_Reference#Bitmap'>bitmap</a> with transparency using <a href='#Blend_Mode_Overview_Porter_Duff'>Porter_Duff</a> compositing is free to clear 33 the destination. 34 35  36 37 Draw geometry with transparency using <a href='#Blend_Mode_Overview_Porter_Duff'>Porter_Duff</a> compositing does not combine 38 transparent source pixels, leaving the destination outside the geometry untouched. 39 40  41 42 <a name='Lighten_Darken'></a> 43 44 Modes <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kPlus'>kPlus</a> and <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kScreen'>kScreen</a> use 45 simple arithmetic to lighten or darken the destination. Modes 46 <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kOverlay'>kOverlay</a> through <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kMultiply'>kMultiply</a> use more complicated 47 algorithms to lighten or darken; sometimes one mode does both, as described by 48 <a href='https://en.wikipedia.org/wiki/Blend_modes'>Blend Modes</a></a> . 49 50  51 52 <a name='Modulate_Blend'></a> 53 54 <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kModulate'>kModulate</a> is a mashup of <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kSrcATop'>kSrcATop</a> and <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kMultiply'>kMultiply</a>. 55 It multiplies all components, including <a href='SkColor_Reference#Alpha'>Alpha</a>; unlike <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kMultiply'>kMultiply</a>, if either 56 source or destination is transparent, result is transparent. <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kModulate'>kModulate</a> 57 uses <a href='undocumented#Premultiply'>Premultiplied</a> values to compute the product; <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kMultiply'>kMultiply</a> uses <a href='undocumented#Unpremultiply'>Unpremultiplied</a> 58 values to compute the product. 59 60  61 62 <a name='Color_Blends'></a> 63 64 Modes <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kHue'>kHue</a>, <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kSaturation'>kSaturation</a>, <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kColor'>kColor</a>, and 65 <a href='SkBlendMode_Reference#SkBlendMode'>SkBlendMode</a>::<a href='#SkBlendMode_kLuminosity'>kLuminosity</a> convert source and destination pixels using all 66 components <a href='SkColor_Reference#Color'>color</a> information, using 67 <a href='https://www.w3.org/TR/compositing-1/#blendingnonseparable'>non-separable blend modes</a></a> . 68 69  70 71
