lib: Add checks to bail out of traversal if BVH is not valid

It's been a pretty common case where for one reason or another, I've
shuffled some code in a way that leaves objects without BVHs. The
traversal now bails out if this is the case, and we emit warnings if
trees are missing, instead of just crashing.

src/lib/accelerators/bvh.c

@@ -390,8 +390,10 @@ static inline struct bvh *build_bvh_generic(
 	void (*get_bbox_and_center)(const void *, unsigned, struct boundingBox *, struct vector *),
 	size_t count)
 {
-	if (count < 1)
-		return calloc(1, sizeof(struct bvh));
+	if (count < 1) {
+		logr(debug, "bvh count < 1\n");
+		return NULL;
+	}
 
 	struct vector *centers = malloc(sizeof(struct vector) * count);
 	struct boundingBox *bboxes = malloc(sizeof(struct boundingBox) * count);
@@ -653,7 +655,14 @@ void destroy_bvh(struct bvh *bvh) {
 void bvh_build_task(void *arg) {
 	block_signals();
 	struct mesh *mesh = (struct mesh *)arg;
+	struct timeval timer = { 0 };
+	timer_start(&timer);
 	mesh->bvh = build_mesh_bvh(mesh);
+	if (mesh->bvh) {
+		logr(debug, "Built BVH for %s, took %lums\n", mesh->name, timer_get_ms(timer));
+	} else {
+		logr(debug, "BVH build FAILED for %s\n", mesh->name);
+	}
 }
 
 // FIXME: Add pthread_cancel() support

src/lib/renderer/instance.c

@@ -156,6 +156,7 @@ static bool intersectMesh(const struct instance *instance, const struct lightRay
 	struct lightRay copy = *ray;
 	tform_ray(&copy, instance->composite.Ainv);
 	struct mesh *mesh = &((struct mesh_arr *)instance->object_arr)->items[instance->object_idx];
+	if (!mesh->bvh) return false;
 	copy.start = vec_add(copy.start, vec_scale(copy.direction, mesh->rayOffset));
 	if (traverse_bottom_level_bvh(mesh, &copy, isect, sampler)) {
 		// Repopulate uv with actual texture mapping
@@ -207,6 +208,12 @@ bool isMesh(const struct instance *instance) {
 
 static void getMeshBBoxAndCenter(const struct instance *instance, struct boundingBox *bbox, struct vector *center) {
 	struct mesh *mesh = &((struct mesh_arr *)instance->object_arr)->items[instance->object_idx];
+	if (!mesh->bvh) {
+		*bbox = (struct boundingBox){ 0 };
+		*center = vec_zero();
+		mesh->rayOffset = 0.0f;
+		return;
+	}
 	*bbox = get_root_bbox(mesh->bvh);
 	tform_bbox(bbox, instance->composite.A);
 	*center = bboxCenter(bbox);