1 #Topic Blend_Mode_Overview 2 3 Describes how destination pixel is replaced with a combination of itself and 4 source pixel. Blend_Mode may use source, destination, or both. Blend_Mode may 5 operate on each Color component independently, or may allow all source pixel 6 components to contribute to one destination pixel component. 7 8 Blend_Mode does not use adjacent pixels to determine the outcome. 9 10 Blend_Mode uses source and read destination Alpha to determine written 11 destination Alpha; both source and destination Alpha may also affect written 12 destination Color components. 13 14 Regardless of how Alpha is encoded in source and destination pixel, nearly all 15 Color_Types treat it as ranging from zero to one. And, nearly all Blend_Mode 16 algorithms limit the output so that all results are also zero to one. 17 18 Two exceptions are SkBlendMode::kPlus and kRGBA_F16_SkColorType. 19 20 SkBlendMode::kPlus permits computing Alpha and Color component values larger 21 than one. For Color_Types other than kRGBA_F16_SkColorType, resulting Alpha 22 and component values are clamped to one. 23 24 kRGBA_F16_SkColorType 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 #Subtopic Porter_Duff 29 #Alias Porter_Duff ## 30 #Line # classic color blend algorithms ## 31 32 #A Compositing Digital Images # https://graphics.pixar.com/library/Compositing/paper.pdf ## 33 describes Porter_Duff modes SkBlendMode::kClear through SkBlendMode::kXor. 34 35 Drawing a bitmap with transparency using Porter_Duff compositing is free to clear 36 the destination. 37 38 #Illustration 1 39 40 Draw geometry with transparency using Porter_Duff compositing does not combine 41 transparent source pixels, leaving the destination outside the geometry untouched. 42 43 #Illustration 2 44 45 ## 46 47 #Subtopic Lighten_Darken 48 #Line # color blends to lighten or darken result ## 49 50 Modes SkBlendMode::kPlus and SkBlendMode::kScreen use 51 simple arithmetic to lighten or darken the destination. Modes 52 SkBlendMode::kOverlay through SkBlendMode::kMultiply use more complicated 53 algorithms to lighten or darken; sometimes one mode does both, as described by 54 #A Blend Modes # https://en.wikipedia.org/wiki/Blend_modes ## 55 . 56 57 #Illustration 58 59 ## 60 61 #Subtopic Modulate_Blend 62 #Line # multiply color components ## 63 64 SkBlendMode::kModulate is a mashup of SkBlendMode::kSrcATop and SkBlendMode::kMultiply. 65 It multiplies all components, including Alpha; unlike SkBlendMode::kMultiply, if either 66 source or destination is transparent, result is transparent. SkBlendMode::kModulate 67 uses Premultiplied values to compute the product; SkBlendMode::kMultiply uses Unpremultiplied 68 values to compute the product. 69 70 #Illustration 71 72 ## 73 74 #Subtopic Color_Blends 75 #Line # non-separable blend modes ## 76 77 Modes SkBlendMode::kHue, SkBlendMode::kSaturation, SkBlendMode::kColor, and 78 SkBlendMode::kLuminosity convert source and destination pixels using all 79 components color information, using 80 ###$ 81 $A non-separable blend modes $ https://www.w3.org/TR/compositing-1/#blendingnonseparable $$ 82 $$$# 83 . 84 85 #Illustration 86 87 ## 88 89 #Topic Blend_Mode_Overview ## 90
