xref: /third_party/skia/site/docs/user/api/SkBlendMode_Overview.md

---
title: 'SkBlendMode Overview'
linkTitle: 'SkBlendMode Overview'

weight: 260
---

Describes how destination <a href='undocumented#Pixel'>pixel</a> is replaced
with a combination of itself and 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 operate on each
<a href='https://api.skia.org/SkColor_8h.html'>Color</a> component
independently, or may allow all source <a href='undocumented#Pixel'>pixel</a>
components to contribute to one destination
<a href='undocumented#Pixel'>pixel</a> component.

<a href='#Blend_Mode'>Blend_Mode</a> does not use adjacent pixels to determine
the outcome.

<a href='#Blend_Mode'>Blend_Mode</a> uses source and read destination
<a href='https://api.skia.org/SkColor_8h.html#a918cf5a3a68406ac8107f6be48fb906e'>Alpha</a>
to determine written destination
<a href='https://api.skia.org/SkColor_8h.html#a918cf5a3a68406ac8107f6be48fb906e'>Alpha</a>;
both source and destination
<a href='https://api.skia.org/SkColor_8h.html#a918cf5a3a68406ac8107f6be48fb906e'>Alpha</a>
may also affect written destination
<a href='https://api.skia.org/SkColor_8h.html'>Color</a> components.

Regardless of how
<a href='https://api.skia.org/SkColor_8h.html#a918cf5a3a68406ac8107f6be48fb906e'>Alpha</a>
is encoded in source and destination <a href='undocumented#Pixel'>pixel</a>,
nearly all <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>
algorithms limit the output so that all results are also zero to one.

Two exceptions are
<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kPlus'>kPlus</a>
and
<a href='https://api.skia.org/SkImageInfo_8h.html#a9ac0b62b3d2c6c7e1a80db557243f93e'>kRGBA_F16_SkColorType</a>.

<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kPlus'>kPlus</a>
permits computing
<a href='https://api.skia.org/SkColor_8h.html#a918cf5a3a68406ac8107f6be48fb906e'>Alpha</a>
and <a href='https://api.skia.org/SkColor_8h.html'>Color</a> component values
larger than one. For <a href='#Image_Info_Color_Type'>Color_Types</a> other than
<a href='https://api.skia.org/SkImageInfo_8h.html#a9ac0b62b3d2c6c7e1a80db557243f93e'>kRGBA_F16_SkColorType</a>,
resulting
<a href='https://api.skia.org/SkColor_8h.html#a918cf5a3a68406ac8107f6be48fb906e'>Alpha</a>
and component values are clamped to one.

<a href='https://api.skia.org/SkImageInfo_8h.html#a9ac0b62b3d2c6c7e1a80db557243f93e'>kRGBA_F16_SkColorType</a>
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.

<a name='Porter_Duff'></a>

<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='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kClear'>kClear</a>
through
<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kXor'>kXor</a>.

Drawing a <a href='https://api.skia.org/classSkBitmap.html'>bitmap</a> with
transparency using <a href='#Blend_Mode_Overview_Porter_Duff'>Porter_Duff</a>
compositing is free to clear the destination.

![Porter_Duff](https://fiddle.skia.org/i/819903e0bb125385269948474b6c8a84_raster.png)

Draw geometry with transparency using
<a href='#Blend_Mode_Overview_Porter_Duff'>Porter_Duff</a> compositing does not
combine transparent source pixels, leaving the destination outside the geometry
untouched.

![Porter_Duff](https://fiddle.skia.org/i/8f320c1e94e77046e00f7e9e843caa27_raster.png)

<a name='Lighten_Darken'></a>

Modes
<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kPlus'>kPlus</a>
and
<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kScreen'>kScreen</a>
use simple arithmetic to lighten or darken the destination. Modes
<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kOverlay'>kOverlay</a>
through
<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kMultiply'>kMultiply</a>
use more complicated algorithms to lighten or darken; sometimes one mode does
both, as described by <a href='https://en.wikipedia.org/wiki/Blend_modes'>Blend
Modes</a></a> .

![Lighten_Darken](https://fiddle.skia.org/i/23a33fa04cdd0204b2490d05e340f87c_raster.png)

<a name='Modulate_Blend'></a>

<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kModulate'>kModulate</a>
is a mashup of
<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kSrcATop'>kSrcATop</a>
and
<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kMultiply'>kMultiply</a>.
It multiplies all components, including
<a href='https://api.skia.org/SkColor_8h.html#a918cf5a3a68406ac8107f6be48fb906e'>Alpha</a>;
unlike
<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kMultiply'>kMultiply</a>,
if either source or destination is transparent, result is transparent.
<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kModulate'>kModulate</a>
uses <a href='undocumented#Premultiply'>Premultiplied</a> values to compute the
product;
<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kMultiply'>kMultiply</a>
uses <a href='undocumented#Unpremultiply'>Unpremultiplied</a> values to compute
the product.

![Modulate_Blend](https://fiddle.skia.org/i/877f96610ab7638a310432674b04f837_raster.png)

<a name='Color_Blends'></a>

Modes
<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kHue'>kHue</a>,
<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kSaturation'>kSaturation</a>,
<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kColor'>kColor</a>,
and
<a href='https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0'>SkBlendMode</a>::<a href='#SkBlendMode_kLuminosity'>kLuminosity</a>
convert source and destination pixels using all components
<a href='https://api.skia.org/SkColor_8h.html'>color</a> information, using
<a href='https://www.w3.org/TR/compositing-1/#blendingnonseparable'>non-separable
blend modes</a></a> .

![Color_Blends](https://fiddle.skia.org/i/630fe21aea8369b307231f5bcf8b2d50_raster.png)