diff --git a/flow-state/src/scenes/shader/horizon-lines.js b/flow-state/src/scenes/shader/horizon-lines.js
index 209c22a..08ea428 100644
--- a/flow-state/src/scenes/shader/horizon-lines.js
+++ b/flow-state/src/scenes/shader/horizon-lines.js
@@ -39,7 +39,10 @@ vec4 scene(vec2 uv, vec2 p) {
     float offset = sin(p.x * 2.2 + t * 2.0) * u_bend * envelope;
 
     vec3 col = pal(0) * 0.06;
-    float total = 0.0;
+
+    // One pixel, in the scene units p is measured in. p spans [-1, 1] vertically
+    // regardless of output size, so this is the exact height of a pixel.
+    float px = 2.0 / u_resolution.y;
 
     for (int i = 0; i < 40; i++) {
         if (float(i) >= u_count) break;
@@ -49,12 +52,28 @@ vec4 scene(vec2 uv, vec2 p) {
         float y = slot + offset * (0.4 + fract(fi * 0.37));
         float d = abs(p.y - y);
 
-        float line = smoothstep(u_thickness * (0.5 + u_sigLine), 0.0, d);
+        // Analytic coverage rather than a smoothstep to zero.
+        //
+        // The width here is genuinely sub-pixel: 0.008 scene units against a
+        // 0.028-unit pixel at 720p is a line under a third of a pixel wide, and
+        // the range goes down to a fourteenth. Ramping from full to nothing
+        // across a third of a pixel is a near-vertical cliff, and a cliff turns
+        // any float wobble in d into a whole byte of colour — which is what
+        // made this the one scene in the library that failed the determinism
+        // gate, intermittently, at 2/255.
+        //
+        // Coverage fixes the cause rather than the symptom. The transition
+        // always spans exactly one pixel, so nothing sits on a discontinuity;
+        // and a line thinner than a pixel now comes out DIM rather than being
+        // drawn at full brightness wherever a pixel centre happens to land on
+        // it, which is both correct and the end of the shimmer it used to have.
+        // Width stays in scene units, so resolution independence is unchanged.
+        float w = u_thickness * (0.5 + u_sigLine);
+        float line = sat((w - d) / px + 0.5);
         float halo = exp(-d * 26.0) * u_glow;
 
         vec3 c = pal(i);
         col += c * (line + halo * 0.55);
-        total += line;
     }
 
     // Keep the far edges dark so the lines read as a subject, not wallpaper.