perf: improve 3d ray marching performance

CLAUDE.md

@@ -204,6 +204,13 @@ sample per pixel. When AA is on, `fs_refine` reads that texture and runs the
 reads the refined texture. PNG export (`fs_color`) still supersamples every
 pixel.
 
+The 3D view (`colorize.wgsl::ray_marching`) sphere-traces the DE height
+field straight from the data texture. It's cheap: rays start on the z = 0
+plane, and most hit within a few steps (about 4 on average). A min-height
+mip pyramid (quadtree height-field tracing) was tried and measured about 3×
+slower, because it needs about 12 costlier steps per ray. Don't reintroduce it.
+Orbiting the camera only re-runs the colourise pass, never iteration.
+
 While the user is actively panning/zooming, the app renders downscaled with
 AA off (`INTERACT_DOWNSCALE`) and snaps back to full resolution once input
 settles (`INTERACT_SETTLE`).

src/app.rs

@@ -2127,9 +2127,12 @@ impl FractalApp {
         if self.rendering_mode == 2 {
             if let Some(mt) = multi_touch {
                 let t = mt.translation_delta;
+                // Orbiting only moves the camera, which the colourise pass
+                // handles alone, so (like mouse-drag orbiting) it doesn't count
+                // as interaction: that would drop to the low-res pass and
+                // re-iterate the fractal twice.
                 if t.x != 0.0 || t.y != 0.0 {
                     self.camera.rotate(-t.x * ROT_SENS, -t.y * ROT_SENS);
-                    interacted = true;
                 }
                 if mt.zoom_delta != 1.0 {
                     let off = mt.center_pos - rect.center();

src/shaders/colorize.wgsl

@@ -68,6 +68,9 @@ fn fs_main(@builtin(position) pos: vec4<f32>) -> @location(0) vec4<f32> {
     }
 }
 
+// Colour of rays that miss the fractal's footprint.
+const RAY_MISS: vec4<f32> = vec4<f32>(1.0, 0.0, 0.0, 1.0);
+
 // Per-frame constants of the raymarch, computed once per pixel in
 // `ray_marching` rather than on each of the up-to-100 `sdf` steps.
 struct MarchConsts {
@@ -126,27 +129,43 @@ fn ray_marching(pos: vec4<f32>) -> vec4<f32> {
     let ray_origin = world_pos.xyz;
     let ray_dir = u.camera_direction;
 
-    let z_intersect = ray_origin.z / ray_dir.z;
-    var p = ray_origin - ray_dir * z_intersect;
-    var i = 0u;
-    var dist = 0.0;
+    // Start where the ray crosses z = 0, the topmost possible surface (the
+    // camera pitch is clamped short of ±90°, so ray_dir.z > 0).
+    let start = ray_origin - ray_dir * (ray_origin.z / ray_dir.z);
 
-    let dist_threshold = 0.000001;
-    while i < 100u {
-        let from_origin = p - ray_origin;
-        if dot(from_origin, from_origin) > 9. {
-            break;
-        }
-        dist = sdf(p, k);
-        if dist < dist_threshold {
-            break;
-        }
-        p += dist * ray_dir;
-        i += 1u;
+    // The terrain only exists over the footprint x in [0, aspect],
+    // y in [0, 1]: clip the ray's xy to it up front, so rays that miss it cost
+    // nothing and the rest start marching at its edge. A huge finite 1/d on
+    // an axis the ray doesn't move along (top-down, during the 2D <-> 3D
+    // transition) keeps the slab maths finite.
+    let inv = select(1.0 / ray_dir.xy, vec2<f32>(1e30), abs(ray_dir.xy) < vec2<f32>(1e-20));
+    let ta = -start.xy * inv;
+    let tb = (vec2<f32>(aspect_ratio, 1.0) - start.xy) * inv;
+    let t_leave = min(max(ta.x, tb.x), max(ta.y, tb.y));
+    var t = max(max(min(ta.x, tb.x), min(ta.y, tb.y)), 0.0);
+    if t >= t_leave {
+        return RAY_MISS;
     }
 
+    // About 1/50 of a texel at typical sizes: tighter only adds steps
+    // without visibly moving the hit.
+    let dist_threshold = 0.00001;
+    var hit = false;
+    for (var i = 0u; i < 100u; i++) {
+        let dist = sdf(start + t * ray_dir, k);
         if dist < dist_threshold {
-        return shadow_fragment(p.xy * k.to_texel);
+            hit = true;
+            break;
         }
-    return vec4<f32>(1., 0., 0., 1.);
+        t += dist;
+        // Past the footprint's far edge: nothing left to hit.
+        if t >= t_leave {
+            break;
+        }
+    }
+    // Shade outside the loop, so its registers don't weigh on the march.
+    if !hit {
+        return RAY_MISS;
+    }
+    return shadow_fragment((start.xy + t * ray_dir.xy) * k.to_texel);
 }