Viewer: Auto resolution follows the magnification; minified frames are averaged (#417)

Cause: Auto dropped to Half below 75 % magnification, so text between
50 % and 75 % was rendered with fewer pixels than the screen shows and
looked soft. Below 100 % the frame texture was minified with a plain
bilinear sampler and no mip levels (Full at 46 % skipped most frame
pixels), so edges stair-stepped.

Fix:
- Auto renders the pixels the magnification needs, as in After Effects:
  the coarsest whole factor that still gives every screen pixel a
  rendered one (Full above 50 %, Half down to 33.3 %, Third down to
  25 %, Quarter below).
- GPU frames: the display texture carries a full mip chain, each level
  the 2 x 2 mean of the one above (display.wgsl `mip`, same bind group
  layout as `display`), sampled trilinearly with Viewer Zoom Quality
  More Accurate (Faster keeps the nearest pixel of the full-size frame).
- CPU frames (egui-wgpu textures have no mips): uploaded averaged down
  by the whole factor that leaves about one texel per screen pixel, the
  same box average as the texture-limit fit (#201), re-uploaded when the
  magnification calls for another factor; drawn with texture coordinates
  that keep each texel over its pixels when the size isn't a multiple.
  Show Channel / exposure textures follow.

Tests: gpu tests::display_frames_are_mipmapped (level count and level 1
= 2 x 2 means of level 0), viewer_tools
auto_resolution_follows_the_magnification (60 % was Half) and
minified_frames_keep_their_texels_in_place.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/architecture.md

@@ -171,6 +171,11 @@ options (Draft / Fast Previews, shadows, nested switches), so frames rendered un
 options are never shown. When the budget (Settings ▸ Memory & CPU) is full the least recently
 shown frames go first, and frames of an older revision are dropped as soon as the project
 changes. The timeline's green bar counts the frames of exactly what the viewer shows.
+Resolution Auto renders the pixels the magnification needs, as in After Effects (Full above
+50 %, Half down to 33.3 %, Third down to 25 %, then Quarter). Below 100 % the viewer averages
+instead of skipping pixels (Viewer Zoom Quality More Accurate): GPU frames carry a mip chain
+sampled trilinearly, CPU frames go up averaged down by the whole factor that leaves about one
+texel per screen pixel.
 
 A persistent **disk cache** (`render::disk_cache`, Settings ▸ Media & Disk Cache) backs both
 the layer cache and the viewer's RAM preview: layer buffers that were slow to render and every

docs/parity.md

@@ -415,6 +415,10 @@ Fixes from measured reports:
 - Text layers are always rasterised at their on-screen scale, sharp at any Scale as in After
   Effects, and an unscaled layer at a fractional position is no longer rasterised a quarter octave
   too large (#390).
+- The viewer's Auto resolution renders the pixels the magnification needs (Full above 50 %), and
+  frames shown below 100 % are averaged (GPU frames through mip levels, CPU frames by a whole
+  factor) instead of minified bilinearly (#417). Not yet: the viewer resolution and magnification
+  are not remembered between launches.
 
 ## Update: plug-ins and extensions as in After Effects; ease presets become a ScriptUI panel