trace from bounding box, not camera

tape-drive/shaders/replacement/trace.rint.glsl

@@ -8,27 +8,68 @@ const float EPSILON = .001;
 
 const uint MAX_STEPS = 50;
 
+// axis aligned, center at the origin, dimensions "box_size"
+// https://iquilezles.org/articles/intersectors/
+vec2 box_intersection(in vec3 ro, in vec3 rd, vec3 box_size)
+{
+    vec3 m = 1.0 / rd; // can precompute if traversing a set of aligned boxes
+    vec3 n = m * ro; // can precompute if traversing a set of aligned boxes
+    vec3 k = abs(m) * box_size;
+    vec3 t1 = -n - k;
+    vec3 t2 = -n + k;
+    float tN = max(max(t1.x, t1.y), t1.z);
+    float tF = min(min(t2.x, t2.y), t2.z);
+    if (tN > tF || tF < 0.0) return vec2(-1.0); // no intersection
+    return vec2(tN, tF);
+}
+
 void main() {
-    if (SPEC_BOUNDING_BOXES) {
-        reportIntersectionEXT(gl_RayTminEXT, 0);
-        return;
-    }
+    Object object = objects.objects[gl_InstanceID];
+
+    vec3 internal_pos = vec3(
+            float((gl_PrimitiveID >> (object.subdivision_log2 * 2)) & ((1 << object.subdivision_log2) - 1))
+                / float(1 << object.subdivision_log2),
+            float((gl_PrimitiveID >> (object.subdivision_log2 * 1)) & ((1 << object.subdivision_log2) - 1))
+                / float(1 << object.subdivision_log2),
+            float((gl_PrimitiveID >> (object.subdivision_log2 * 0)) & ((1 << object.subdivision_log2) - 1))
+                / float(1 << object.subdivision_log2)
+        );
+
+    vec3 low_corner = (((vec3(0.0) / float(1 << object.subdivision_log2)) + internal_pos)
+            * object.dimensions) + object.lowest_corner;
+    vec3 high_corner = (((vec3(1.0) / float(1 << object.subdivision_log2)) + internal_pos)
+            * object.dimensions) + object.lowest_corner;
 
     vec3 raypos = gl_ObjectRayOriginEXT;
     vec3 raydir = gl_ObjectRayDirectionEXT;
 
-    float farplane = gl_RayTmaxEXT;
+    vec3 adjusted_raypos = raypos - ((high_corner + low_corner) / 2.0);
+    vec3 dimensions = high_corner - low_corner;
 
-    vec3 p = raypos;
-    float total_dist = gl_RayTminEXT;
+    vec2 near_far = box_intersection(adjusted_raypos, raydir, dimensions);
+
+    if (near_far.x < 0.) {
+        return;
+    }
+
+    float near_plane = near_far.x;
+    float far_plane = near_far.y;
+
+    if (SPEC_BOUNDING_BOXES) {
+        reportIntersectionEXT(near_plane, 0);
+        return;
+    }
+
+    float total_dist = near_plane;
+    vec3 p = fma(raydir, vec3(total_dist), raypos);
     for (uint i = 0; i < MAX_STEPS; i++) {
         float dist = scene(vec4(p, camera_uniforms.campos_and_time.w));
         total_dist += dist;
         p = fma(raydir, vec3(total_dist), raypos);
         if (abs(dist) < EPSILON) {
-            reportIntersectionEXT(max(total_dist, gl_RayTminEXT), 0);
+            reportIntersectionEXT(max(total_dist, near_plane), 0);
             return;
-        } else if (total_dist > farplane) {
+        } else if (total_dist > far_plane) {
             break;
         }
     }