macos fix

1 files changed, 15 insertions, 5 deletions

diff --git a/shaders/ui.frag.glsl b/shaders/ui.frag.glsl
index ddc1cde0..69cb63b9 100644
--- a/shaders/ui.frag.glsl
+++ b/shaders/ui.frag.glsl
@@ -1,7 +1,10 @@
 #version 450
 
 // Sample the glyph atlas (R = coverage 0..1) and mix the cell's bg up to its fg
-// by that coverage. Fully opaque: the swapchain is SDR, no blending needed.
+// by that coverage. Output is PREMULTIPLIED: an opaque cell is unchanged, and a
+// cell whose background is the see-through ground (config.gui_transparent, a
+// theme with no bg of its own) contributes only its ink, at the ink's own
+// coverage, so the compositor keeps the rest.
 
 layout(set = 2, binding = 0) uniform sampler2D u_atlas;
 
@@ -26,13 +29,20 @@ float cellNoise(uint serial, uint col, uint row) {
 void main() {
     float a = texture(u_atlas, v_uv).r;
     bool oldLayer = (v_effect.x & 0x80000000u) != 0u;
-    uint effect = v_effect.x & 0x7fffffffu;
+    // 0 where the cell's background IS the ground and the ground is nothing.
+    // src/gui/gui.zig sets that bit; the two flags above it are why the effect
+    // id is masked with 0x3fffffff here and in ui.vert.glsl.
+    float bgA = (v_effect.x & 0x40000000u) != 0u ? 0.0 : 1.0;
+    uint effect = v_effect.x & 0x3fffffffu;
     float progress = clamp(uintBitsToFloat(v_effect.y), 0.0, 1.0);
     uint cellCol = v_effect.w & 0xffffu;
     uint cellRow = v_effect.w >> 16;
     float noise = cellNoise(v_effect.z, cellCol, cellRow);
 
-    vec3 cell = mix(v_bg, v_fg, a);
+    // Ink over ground, both premultiplied: v_fg*a + (v_bg*bgA)*(1-a), which is
+    // mix() with the ground already premultiplied. At bgA = 1 that is exactly
+    // the mix(v_bg, v_fg, a) at alpha 1 this shader always wrote.
+    vec4 cell = vec4(mix(v_bg * bgA, v_fg, a), mix(bgA, 1.0, a));
 
     // 3 = data dissolve. Changed cells retain the frozen old cell until their
     // stable threshold, then atomically become the new cell. Exact endpoints
@@ -40,11 +50,11 @@ void main() {
     if (effect == 3u) {
         bool reveal = progress >= 1.0 || (progress > 0.0 && noise < progress);
         if (oldLayer ? reveal : !reveal) discard;
-        o_col = vec4(cell, 1.0);
+        o_col = cell;
         return;
     }
 
     // FreeType's hinted grayscale bitmap is already the intended coverage.
     // Preserve it exactly; weight-changing gamma hacks blur small stems.
-    o_col = vec4(cell, 1.0);
+    o_col = cell;
 }