[Impeller] Document blending/color management (#34741)
diff --git a/impeller/README.md b/impeller/README.md
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* [How to Enable Metal Validation for Command Line Apps.](docs/metal_validation.md)
* [How Impeller Works Around The Lack of Uniform Buffers in Open GL ES 2.0.](docs/ubo_gles2.md)
* [Guidance for writing efficient shaders](docs/shader_optimization.md)
+* [How color blending works in Impeller](docs/blending.md)
diff --git a/impeller/docs/blending.md b/impeller/docs/blending.md
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+# Color blending
+
+Impeller currently supports the same set of blending operations that
+[Skia](https://api.skia.org/SkBlendMode_8h.html#ad96d76accb8ff5f3eafa29b91f7a25f0)
+supports. Internally, Impeller distinguishes between two different kinds of
+blend modes: Those which can be performed using the raster pipeline blend
+configuration (called "Pipeline Blends"), and those which cannot (called
+"Advanced Blends").
+
+All blend modes conform to the
+[W3C Compositing and Blending recommendation](https://www.w3.org/TR/compositing-1/).
+
+Blend operations are driven by the `Entity::BlendMode` enum. In the Aiks layer,
+all drawing operations conform to the given `Paint::blend_mode`. In the Entities
+layer, all Entities have an associated blend mode, which can be set via
+`Entity::SetBlendMode(Entity::BlendMode)`.
+
+## Glossary of blending terms
+| Term | Definition |
+| --- | --- |
+| Source color | Any color that is output by a fragment shader. |
+| Destination color | The backdrop color in a blend operation. |
+| Premultiplied color | A color that has its alpha multiplied into it. Used for additive blending operations as well as colors presented to a surface. |
+| Porter-Duff alpha composite | One of several operations that add together a source color and destination color, with both the source and destination colors being multiplied by respective alpha factors. |
+| Pipeline blend | A blend mode that Impeller can always implement by using the raster pipeline blend configuration provided by the underlying graphics backend. Most of these are simple _Porter-Duff alpha composites_. |
+| Advanced blend | A blend mode that Impeller computes using a fragment program. |
+
+## Premultiplied colors
+
+In Impeller, all blending _source colors_ are assumed to be _premultiplied_ for
+the purpose of blending. This means that all Entity shaders must output colors
+with premultiplied alpha. In general, these shaders also assume that sampled
+textures and uniform color inputs are premultiplied.
+
+The reason for this is that it enables us to implement all of the _Porter-Duff
+alpha composites_ using the built-in raster pipeline blend configuration offered
+by all major graphics backends.
+
+## Pipeline blends
+
+Most of the pipeline blends are actually _Porter-Duff alpha composites_, which
+add together the source color and destination color -- both the source and
+destination colors are multiplied by an alpha factor which determines the
+behavior of the blend.
+
+Pipeline blends are always cheap and don't require additional draw calls to
+render.
+
+| Pipeline blend |
+| --- |
+| Clear |
+| Source |
+| Destination |
+| SourceOver |
+| DestinationOver |
+| SourceIn |
+| DestinationIn |
+| SourceOut |
+| DestinationOut |
+| SourceATop |
+| DestinationATop |
+| Xor |
+| Plus |
+| Modulate |
+
+## Advanced blends
+
+Advanced blends are blends that Impeller can't always implement using the
+built-in raster pipeline blend configuration offered by graphics backends.
+Instead, they're implemented using special blend shaders that bind the backdrop
+texture in a separate render pass.
+
+Note that all of the advanced blends are _color blends_ rather than _alpha
+composites_, and they can technically be combined with any _pipeline blend_ with
+predictable compositing behavior. However, in order to keep in line with
+Flutter's (and Skia's) current behavior, Impeller uses _Source Over_ compositing
+when rendering all advanced blends.
+
+Advanced blends are expensive when compared to pipeline blends (which are
+essentially free) for the following reasons:
+* For each advanced blend, the current render pass ends because the backdrop
+ texture needs to be sampled.
+* A potentially large texture (the render pass backdrop) is sampled. Although in
+ practice, just the coverage rectangle of the source being blended is actually
+ used.
+* An intermediary texture is allocated for the blend output before being blitted
+ back to the render pass texture.
+
+| Advanced blend |
+| --- |
+| kScreen |
+| Overlay |
+| Darken |
+| Lighten |
+| ColorDodge |
+| ColorBurn |
+| HardLight |
+| SoftLight |
+| Difference |
+| Exclusion |
+| Multiply |
+| Hue |
+| Saturation |
+| Color |
+| Luminosity |
