docs/SkBlendMode_Overview.bmh

#Topic Blend_Mode_Overview

Describes how destination pixel is replaced with a combination of itself and
source pixel. Blend_Mode may use source, destination, or both. Blend_Mode may
operate on each Color component independently, or may allow all source pixel
components to contribute to one destination pixel component. 

Blend_Mode does not use adjacent pixels to determine the outcome.

Blend_Mode uses source and read destination Alpha to determine written
destination Alpha; both source and destination Alpha may also affect written
destination Color components.

Regardless of how Alpha is encoded in source and destination pixel, nearly all
Color_Types treat it as ranging from zero to one. And, nearly all Blend_Mode
algorithms limit the output so that all results are also zero to one.

Two exceptions are SkBlendMode::kPlus and kRGBA_F16_SkColorType. 

SkBlendMode::kPlus permits computing Alpha and Color component values larger
than one. For Color_Types other than kRGBA_F16_SkColorType, resulting Alpha
and component values are clamped to one.

kRGBA_F16_SkColorType permits values outside the zero to one range. It is up
to the client to ensure that the result is within the range of zero to one,
and therefore well-defined.

#Subtopic Porter_Duff
#Alias Porter_Duff ##
#Line # classic color blend algorithms ##

#A Compositing Digital Images # https://graphics.pixar.com/library/Compositing/paper.pdf ##
describes Porter_Duff modes SkBlendMode::kClear through SkBlendMode::kXor.

Drawing a bitmap with transparency using Porter_Duff compositing is free to clear
the destination.

#Illustration 1

Draw geometry with transparency using Porter_Duff compositing does not combine
transparent source pixels, leaving the destination outside the geometry untouched.

#Illustration 2

##

#Subtopic Lighten_Darken
#Line # color blends to lighten or darken result ##

Modes SkBlendMode::kPlus and SkBlendMode::kScreen use
simple arithmetic to lighten or darken the destination. Modes 
SkBlendMode::kOverlay through SkBlendMode::kMultiply use more complicated
algorithms to lighten or darken; sometimes one mode does both, as described by
#A Blend Modes # https://en.wikipedia.org/wiki/Blend_modes ##
.

#Illustration

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#Subtopic Modulate_Blend
#Line # multiply color components ##

SkBlendMode::kModulate is a mashup of SkBlendMode::kSrcATop and SkBlendMode::kMultiply.
It multiplies all components, including Alpha; unlike SkBlendMode::kMultiply, if either
source or destination is transparent, result is transparent. SkBlendMode::kModulate
uses Premultiplied values to compute the product; SkBlendMode::kMultiply uses Unpremultiplied
values to compute the product.

#Illustration

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#Subtopic Color_Blends
#Line # non-separable blend modes ##

Modes SkBlendMode::kHue, SkBlendMode::kSaturation, SkBlendMode::kColor, and
SkBlendMode::kLuminosity convert source and destination pixels using all
components color information, using 
###$
$A non-separable blend modes $ https://www.w3.org/TR/compositing-1/#blendingnonseparable $$
$$$#
.

#Illustration

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#Topic Blend_Mode_Overview ##