Refining Pixel Snapping for Split-Axis Sticky Compositing
Blink28624 Helmi

Refining Pixel Snapping for Split-Axis Sticky Compositing

These sources detail a technical implementation for pixel snapping in a web browser's compositor, specifically addressing how sticky positioning handles subpixel offsets across split axes. The primary goal is to ensure that elements remain crisp and stable during scrolling by rounding translation offsets to integer pixels on the compositor thread. To achieve this, the system uses a pixel_snap_offset to track fractional remainders, which prevents visual jitter caused by rounding mismatches between the main thread and the compositor. The documentation proposes a refactored approach where this offset is partitioned per axis, allowing separate transform nodes to manage horizontal and vertical snapping independently. By aligning the Blink-side matrix rounding with the compositor’s logic, the system maintains a consistent "baseline" behavior that is easier to reason about and test. Ultimately, this ensures that subpixel-correct placement is preserved in raster space while the movement itself remains perfectly aligned to the pixel grid.

Jaksot(497)

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 Syys 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 Syys 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 Syys 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 Syys 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 Syys 202542min

blink::PaintArtifact

blink::PaintArtifact

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

21 Syys 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 Syys 202541min

blink::PaintController

blink::PaintController

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

21 Syys 202533min