cc Transform and Scroll Trees
Blink28628 Sep 2025

cc Transform and Scroll Trees

The source text details the architecture of Chromium's compositor, focusing on the tandem operation of two critical data structures: the Transform Tree and the Scroll Tree. The Transform Tree manages visual coordinate space transformations, including CSS transforms and scroll offsets, with each TransformNode calculating its final position relative to its parent using local matrices and the scroll translation. In parallel, the Scroll Tree, composed of ScrollNodes, handles the logical state of scrollable elements, defining constraints like scrollable bounds, allowed directions, and scroll chaining behavior. Crucially, the sources explain that the two trees are linked via a shared ElementId and that while the Scroll Tree determines how a scroll should happen (bounds and policy), the Transform Tree executes the change by holding and applying the live scroll_offset to visually move the layer content, enabling smooth, composite-threaded scrolling.

Avsnitt(449)

cc::PropertyTrees

cc::PropertyTrees

(TransformTree / ClipTree / EffectTree / ScrollTree) with nodes cc::TransformNode / ClipNode / EffectNode / ScrollNode — The impl‑side property‑tree state that positions, clips, blends, and scrolls co...

22 Sep 202534min

cc::LayerTreeHostImpl

cc::LayerTreeHostImpl

The compositor‑thread twin that owns the active/pending trees, runs impl‑side animations/scroll, and prepares draw. Implements cc::InputHandler for threaded scrolling/pinch.

22 Sep 202546min

main —> impl cc::LayerTreeHost

main —> impl cc::LayerTreeHost

Packages updated layers/property trees; kicks a commit to the compositor thread (via the proxy).

22 Sep 202536min

blink::PropertyTreeManager

blink::PropertyTreeManager

Bridges Blink’s paint property trees into cc property trees: creates/updates cc TransformNode, ClipNode, EffectNode, and ScrollNode (including inner/outer viewport scroll nodes).

22 Sep 202539min

blink::PaintArtifactCompositor

blink::PaintArtifactCompositor

Decides what needs to be composited and prepares the main‑thread representation to feed cc (layers if needed, scrollbar compositing, etc.).

22 Sep 202542min

blink::PaintArtifact

blink::PaintArtifact

The immutable package of paint chunks + display items for this frame, used to drive compositing without re‑painting.

21 Sep 202536min

blink::TransformPaintPropertyNode / ClipPaintPropertyNode / EffectPaintPropertyNode / ScrollPaintPropertyNode

blink::TransformPaintPropertyNode / ClipPaintPropertyNode / EffectPaintPropertyNode / ScrollPaintPropertyNode

The paint property trees captured during painting (transform, clip, effect/opacity, scroll offset) that define visual & coordinate‑space state.

21 Sep 202541min

blink::PaintController

blink::PaintController

Records drawing into a DisplayItemList, segmenting it into PaintChunks keyed by property‑tree state. (Output: a PaintArtifact.)

21 Sep 202533min

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