Initial implementation of GeometryMapper

third_party/WebKit/Source/platform/graphics/paint/README.md

 # Platform paint code
 
 This directory contains the implementation of display lists and display
 list-based painting, except for code which requires knowledge of `core/`
 concepts, such as DOM elements and layout objects.
 
 This code is owned by the [paint team][paint-team-site].
 
 Slimming Paint v2 is currently being implemented. Unlike Slimming Paint v1, SPv2
 represents its paint artifact not as a flat display list, but as a list of
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 The [`PaintArtifactCompositor`](PaintArtifactCompositor.h) is responsible for
 consuming the `PaintArtifact` produced by the `PaintController`, and converting
 it into a form suitable for the compositor to consume.
 
 At present, `PaintArtifactCompositor` creates a cc layer tree, with one layer
 for each paint chunk. In the future, it is expected that we will use heuristics
 to combine paint chunks into a smaller number of layers.
 
 The owner of the `PaintArtifactCompositor` (e.g. `WebView`) can then attach its
 root layer to the overall layer hierarchy to be displayed to the user.
+
+## Geometry routines
+
+The [`GeometryMapper`](GeometryMapper.h) is responsible for efficiently computing
+visual and transformed rects of display items in the coordinate space of ancestor
+[`PropertyTreeState`](PropertyTreeState.h)s.
+
+The transformed rect of a display item in an ancestor `PropertyTreeState` is that
+rect, multiplied by the transforms between the display item's `PropertyTreeState`
+and the ancestors, then flattened into 2D.
+
+The visual rect of a display item in an ancestor `PropertyTreeState` is the intersection
+of all of the intermediate clips (transformed in to the ancestor state), with
+the display item's transformed rect.