Module layers 

Opacity layers, snapshot brackets, and the bracket stack they share with clips.

Three of the display list’s commands open a group — a bracket whose matching pop has to undo exactly what it did, and inside which a hoisted clear is confined (see clear). GroupStack is that one stack. It is deliberately one stack and not three: frust_scene records PopClip, PopLayer and PopSnapshot as distinct commands, but a widget tree can emit them unbalanced or interleaved, and three stacks would let one kind of pop lift a bracket another kind still relies on. One stack means the innermost open bracket is always what closes, whichever pop closes it.

Opacity layers

A PushLayer at alpha >= 1.0 composites source-over at full opacity, which is precisely what drawing its contents into the parent does — so it lowers to nothing but its rectangle’s clip, exactly as PushClip does (frust_scene documents the two as the same command with alpha fixed). Below full opacity the layer has to be rendered in isolation and composited as a whole, which is a page and a pass in the scheduler (schedule) — so it lowers to a recorded layer plus that same rectangular clip.

The rectangle goes through the clip stack rather than through LayerProps::clip_path: frust lowers every clip to a scissor or a coverage mask and never to an intermediate texture, and the scheduler refuses a recorded layer carrying a clip path for that exact reason. Setting both would clip the layer twice.

Snapshot brackets

The engine implements no snapshot cache. frust_scene states what a renderer that does not must do instead: paint the body inline, wrapped exactly as if the recorder had emitted push_transform(scale_about(scale, rect.center())) when scale != 1.0 and push_layer(rect, alpha) when alpha < 1.0. SnapshotStack is that emulation.

The scale is applied as a correction composed ahead of every subsequent command’s own transform rather than by rewriting the body’s commands, because the body was recorded in the ordinary composed transform space: the bracket’s presentation parameters are deliberately not baked into it. The correction that reproduces push_transform around a body already carrying transform is transform * scale_about(..) * transform.inverse() — see snapshot_correction.

Only the outermost bracket is honoured, which the display list explicitly permits: an inner bracket’s own alpha and scale are ignored, and its depth is tracked only so the matching pop can be identified.

Structs

Group

One open bracket: what its matching pop has to undo, and the device-space bounds a clear hoisted past it is confined to.

GroupStack

The compiler’s stack of open clip, layer and snapshot brackets.

SnapshotStack

The snapshot bracket’s inline emulation: how deep the walk is inside one, and what the outermost bracket installed.

Enums

LayerLowering

How a PushLayer bracket lowers.

Functions

layer_props

The recorded properties an isolated opacity layer carries.

lower_layer

How a layer at alpha lowers.

snapshot_correction

The correction that reproduces a snapshot bracket’s presentation scale.