render3d/overlay: the font atlas kept as its coverage, one byte a texel

The atlas is white glyphs on alpha and the overlay reads only the coverage,
but it was uploaded RGBA8: Maroon Lake's 3072 x 2688 atlas was 43 MB of Metal
memory with its mips, the largest single thing in the launcher window.
overlay_font now keeps the alpha alone as R8 (ov_font_tex; any other PNG
shape is uploaded as before) and overlay.frag reads .r - the white fallback
texture reads 1 there as it did from .a.

Launcher window, graphics regions: the atlas 43.0 -> 11.5 MB; process
footprint at 25 s 402 -> 378 MB. The headless launcher shot is identical to
the unchanged build's (0 pixels differ; two unchanged runs differ from each
other by 1.69%, and the new one by the same against them).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.render3d/overlay.ludic

@@ -67,6 +67,29 @@ function overlay_init(render3d_st: mut Render3dState, font_dir: string) -> bool
   return true
 }
 
+# The atlas is white glyphs on alpha, and the overlay reads only the coverage: it is kept as one
+# byte a texel (R8). As RGBA a 3072 x 2688 atlas was 44 MB with its mips, the most of any UI texture.
+@alloc_ok("loading a font: a load, not a frame")
+function ov_font_tex(render3d_st: mut Render3dState, path: string) -> int {
+  let px = png_decode(render3d_st, path)
+  if px == null { return 0 }
+  if render3d_st.tex_channels != 4 or render3d_st.tex_depth != 8 {
+    let id0 = tex_upload(render3d_st, px, false, true)
+    free(px)
+    tex_note_size(render3d_st, id0)
+    return id0
+  }
+  let n = render3d_st.tex_w * render3d_st.tex_h
+  let a = bytes(n)
+  for i in 0 .. n { a[i] = px[i * 4 + 3] }
+  free(px)
+  render3d_st.tex_channels = 1
+  let id = tex_upload(render3d_st, a, false, true)
+  free(a)
+  tex_note_size(render3d_st, id)
+  return id
+}
+
 # Load (or swap to) the font atlas in `font_dir`: font.png and font.json. A game calls it
 # again to change fonts at run time - a language whose script the default atlas does not
 # carry brings its own. Returns false, and keeps the font it had, if there is none there.
@@ -74,7 +97,7 @@ function overlay_init(render3d_st: mut Render3dState, font_dir: string) -> bool
 function overlay_font(render3d_st: mut Render3dState, font_dir: string) -> bool {
   let meta = Fs.read_text(font_dir + "/font.json")
   if meta == null { return false }
-  let tex = tex_load_ex(render3d_st, font_dir + "/font.png", false, 0)
+  let tex = ov_font_tex(render3d_st, font_dir + "/font.png")
   if tex == 0 { return false }
   let j = Json.parse(meta)
   render3d_st.ov_font_cols = jint(j, "cols", 16);  render3d_st.ov_font_rows = jint(j, "rows", 6)

packages/ludic.render3d/shaders/overlay.frag

@@ -1,4 +1,4 @@
-// 2D overlay: an image texture, the font atlas (white glyphs on alpha) or a flat colour, times a
+// 2D overlay: an image texture, the font atlas (glyph coverage, R8) or a flat colour, times a
 // colour. Which one is the vertex's own: v carries 4 x the mode (0 image, 1 font, 2 flat) on top of
 // its real coordinate, so text, panels and an image's quads share a draw (overlay.ludic, ov_use_tex).
 in vec2 v_uv;
@@ -27,7 +27,7 @@ void main() {
   if (mode > 1.5) {
     o_color = v_col;
   } else if (mode > 0.5) {
-    o_color = vec4(v_col.rgb, v_col.a * texture(u_font, uv).a);
+    o_color = vec4(v_col.rgb, v_col.a * texture(u_font, uv).r);   // the atlas is its coverage alone (R8)
   } else {
     vec4 t = texture(u_tex, uv);
     o_color = vec4(v_col.rgb * t.rgb, v_col.a * t.a);