Display: restore intersection and cusp highlighting

This increases resource use a bit, because we now have to hold two
fragments in memory at once instead of just one. It's still much better
than holding all of the top twelve fragments, though!

app-proto/full-interface/src/inversive.frag

@@ -191,19 +191,44 @@ void main() {
     // composite the sphere fragments
     vec3 color = vec3(0.);
     int layer = layer_cnt - 1;
-    TaggedDepth hit;
+    TaggedDepth hit = top_hits[layer];
-    for (; layer >= layer_threshold; --layer) {
+    Fragment frag_next = sphere_shading(
-        // shade the current fragment
-        hit = top_hits[layer];
-        Fragment frag = sphere_shading(
         sphere_list[hit.id],
         hit.depth * dir,
         hit.dimming * color_list[hit.id]
     );
-        float highlight = highlight_list[hit.id];
+    float highlight_next = highlight_list[hit.id];
+    --layer;
+    for (; layer >= layer_threshold; --layer) {
+        // load the current fragment
+        Fragment frag = frag_next;
+        float highlight = highlight_next;
+        
+        // shade the next fragment
+        hit = top_hits[layer];
+        frag_next = sphere_shading(
+            sphere_list[hit.id],
+            hit.depth * dir,
+            hit.dimming * color_list[hit.id]
+        );
+        highlight_next = highlight_list[hit.id];
+        
+        // highlight intersections
+        float ixn_dist = intersection_dist(frag, frag_next);
+        float max_highlight = max(highlight, highlight_next);
+        float ixn_highlight = 0.5 * max_highlight * (1. - smoothstep(2./3.*ixn_threshold, 1.5*ixn_threshold, ixn_dist));
+        frag.color = mix(frag.color, vec4(1.), ixn_highlight);
+        frag_next.color = mix(frag_next.color, vec4(1.), ixn_highlight);
+        
+        // highlight cusps
+        float cusp_cos = abs(dot(dir, frag.normal));
+        float cusp_threshold = 2.*sqrt(ixn_threshold * sphere_list[hit.id].lt.s);
+        float cusp_highlight = highlight * (1. - smoothstep(2./3.*cusp_threshold, 1.5*cusp_threshold, cusp_cos));
+        frag.color = mix(frag.color, vec4(1.), cusp_highlight);
         
         // composite the current fragment
         color = mix(color, frag.color.rgb, frag.color.a);
     }
+    color = mix(color, frag_next.color.rgb, frag_next.color.a);
     outColor = vec4(sRGB(color), 1.);
 }