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perf(ui): hoist loop-invariant per-item math out of the per-pixel loop (#125)

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The cooperative-load section already streams each item into shared memory
once per workgroup, but the per-pixel inner loop then recomputed values
that are constant for the whole item/glyph. The compiler can't hoist them
itself — they read shared memory at a varying index.

Precompute them once per item at load time into new shared slots:

  - ui-images / ui-text / ui-fused (images+text phases): invRectSize =
    1.0/rect.zw, turning the per-pixel `(sp-rect.xy)/rect.zw` vec2 divide
    into a multiply.
  - ui-text / ui-fused (text phase): the SDF AA `band` (a vec2 divide +
    two maxes depending only on the glyph's uv span and rect size).

In ui-fused the new slots (s_inv, s_band) are reused across phases like
the existing s_v* scratch, so the LDS bump is per-category, not summed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
catbot 2026-06-18 13:22:07 +00:00
commit ac9180076c
3 changed files with 45 additions and 20 deletions
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30
shaders/ui-text.comp.glsl
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@ -23,6 +23,13 @@ const float DIST_SCALE = 32.0;
shared vec4 s_rect[UI_CHUNK];
shared vec4 s_uv[UI_CHUNK];
shared vec4 s_color[UI_CHUNK];
// Per-glyph constants precomputed once at cooperative-load time and read by the
// per-pixel inner loop, instead of being recomputed for every pixel × glyph:
// s_invRectSize = 1.0/rect.zw (turns the per-pixel divide into a multiply)
// s_band = the SDF AA scale (a divide + two maxes that depend only on
// the glyph's uv span and rect size — fully loop-invariant)
shared vec2 s_invRectSize[UI_CHUNK];
shared float s_band[UI_CHUNK];
shared uint s_keep[UI_CHUNK];
shared uint s_order[UI_CHUNK];
shared uint s_count;
@ -49,6 +56,15 @@ void main() {
s_rect[lid] = uiGlyphHeap[pc.hdr.itemBuffer].items[idx].rect;
s_uv[lid] = uiGlyphHeap[pc.hdr.itemBuffer].items[idx].uv;
s_color[lid] = uiGlyphHeap[pc.hdr.itemBuffer].items[idx].color;
s_invRectSize[lid] = 1.0 / s_rect[lid].zw;
// Atlas-px per screen-px along this glyph's transform — keeps AA
// crisp at any rendering size. uvSpan * atlasSize / screenSpan,
// with FontAtlas::kAtlasSize = 1024. Constant for the whole glyph.
vec2 uvSpan = s_uv[lid].zw - s_uv[lid].xy;
vec2 atlasPerScreen = (uvSpan * 1024.0) * s_invRectSize[lid];
float scalePx = max(atlasPerScreen.x, atlasPerScreen.y);
// 1-screen-px AA band, expressed in atlas-pixel units of dAtlas.
s_band[lid] = max(scalePx, 0.0001);
keep = uiAabbOverlapsTile(s_rect[lid].xy, s_rect[lid].xy + s_rect[lid].zw,
tileMin, tileMax);
}
@ -73,7 +89,7 @@ void main() {
if (sp.x < lo.x || sp.y < lo.y) continue;
if (sp.x >= hi.x || sp.y >= hi.y) continue;
vec2 t = (sp - s_rect[c].xy) / s_rect[c].zw;
vec2 t = (sp - s_rect[c].xy) * s_invRectSize[c];
vec2 uv = mix(s_uv[c].xy, s_uv[c].zw, t);
// Font slots are push constants — provably dynamically uniform, so no
@ -88,16 +104,8 @@ void main() {
// Distance in atlas-pixels (negative inside the glyph).
float dAtlas = (ON_EDGE - sdf) * DIST_SCALE;
// Atlas-px per screen-px along this glyph's transform — keeps AA crisp
// at any rendering size. uvSpan * atlasSize / screenSpan.
vec2 uvSpan = s_uv[c].zw - s_uv[c].xy;
// FontAtlas::kAtlasSize = 1024.
vec2 atlasPerScreen = (uvSpan * 1024.0) / s_rect[c].zw;
float scalePx = max(atlasPerScreen.x, atlasPerScreen.y);
// 1-screen-px AA band, expressed in atlas-pixel units of dAtlas.
float band = max(scalePx, 0.0001);
float a = clamp(0.5 - dAtlas / band, 0.0, 1.0);
// band (the AA scale) was precomputed once per glyph at load.
float a = clamp(0.5 - dAtlas / s_band[c], 0.0, 1.0);
if (a <= 0.0) continue;
vec4 col = s_color[c];