2057384 - Smooth WebRender box-shadow blur and edge during a transition

Status: RESOLVED FIXED

(Core :: Graphics: WebRender, task)

Comment 1

[Glenn Watson [:gw]]

Attachment: Bug 2057384 - Smooth WebRender box-shadow blur and edge during a transition

Two related quantizations made an animated box-shadow (a CSS transition of the
blur radius and/or offset) look choppy rather than interpolating smoothly:

1. The blur mask sigma was rounded to the nearest integer before being used as
   the blur std-dev and as part of the mask cache key. A blur transition
   therefore stepped between only ~3 discrete cached masks, so the blur
   appeared to pop in near the end of the transition. Keep the sigma at full
   sub-pixel precision, computed at the final mask resolution (after both the
   MAX_BLUR_STD_DEVIATION downscale and the to_cache_size safety cap); the mask
   cache key still buckets it to 1/60px via Au so static shadows still dedupe.

2. The nine-patch shader mapped UV=1.0 to the integer atlas allocation edge
   (round(alloc_size * content_scale)). The sub-texel difference between the
   true content extent and its rounded allocation shifted the nine-patch
   registration whenever the mask size stepped as the blur animated, producing
   a small (~0.3px) non-monotonic jitter of the shadow edge. Pass the true
   device-space content extent to the shader and map UV=1.0 to it, rounding the
   atlas allocation up so the content edge always lies inside the entry (mapping
   to a point outside the entry would sample a neighbouring atlas allocation).

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/e4182afaac1e
https://hg.mozilla.org/integration/autoland/rev/c2d47a8efce9
Smooth WebRender box-shadow blur and edge during a transition r=gfx-reviewers,nical

Comment 3

[Cosmin Sabou [:CosminS]]

https://hg.mozilla.org/mozilla-central/rev/c2d47a8efce9