Canvas
Retained drawing in a styled view: paths, fills and strokes, redrawn only when what they read changes.
canvas(|(width, height)| Canvas) is a normal styled view. It composes with
.child, providers, event handlers, clipping and retained layers. Its callback
tracks reactive reads and runs when a dependency or allocated content size
changes. Empty allocations do not invoke the callback. Drawing is local to and
clipped by the allocated content box. Use ordinary child views for text and
controls that require their own interaction and accessibility nodes.
Path::builder() provides lines, quadratic and cubic curves, circular arcs,
closed contours and affine transforms. Canvas::fill, fill_rule and stroke
accept solid colors, linear gradients and slash patterns. Stroke options include
width, cap, join, miter limit, dash lengths and offset. Angles are radians, with
clockwise rotation in screen coordinates. PathBuilder::build rejects nonfinite
coordinates and paths exceeding 65,536 segments.
The GPU backend rasterizes vectors with tiny-skia at the current display scale and caches their straight-alpha pixels and uploaded texture. Equal commands, size and scale reuse the cache; changed commands invalidate paint without relayout. This is cached vector rasterization, not a GPU tessellation pipeline. It keeps unchanged graphics cheap while complex continuously changing paths still incur CPU rasterization. The software reference uses the same rasterizer.
The raster cache and a single canvas image are each limited to 32 MiB. Commands,
gradient stops and dash arrays have bounded renderer acceptance limits. Invalid
geometry returns a GPU render error; the software reference omits invalid
canvas content. GpuStats::canvas_rasterizations and
DebugCacheStats::canvas_raster_bytes expose the cost. Entries are released when
their scene nodes are removed; least-recently-used pixel entries are evicted
under pressure. Existing renderer texture limits apply separately.
Run cargo run -p zgx-desktop --example canvas for a resizable gradient,
quadratic/cubic curve and patterned rectangle. Regression tests compare changed
content against a fresh frame, exercise isolated layers, verify cache reuse and
check reactive callback disposal.