Fix #130829: Incorrect render result with light trees

The original report stumbled upon this issue with a more tricky
configuration when light linking is combined with light tress.
However, the actual contributing factor was a mesh with emission
shader which is not assigned to any triangles. This triggered a
bug in the BoundBox::transformed() which converted non-valid bounds
to bounds by performing per-corner growing.

Additionally fix incorrect handling of shared nodes which only
worked for leaf nodes. This was due to the fact how the measure
was accumulated: it is possible that add() is called with an empty
measure.

Pull Request: https://projects.blender.org/blender/blender/pulls/134699

intern/cycles/scene/light_tree.h

@@ -157,6 +157,12 @@ struct LightTreeLightLink {
 
   void add(const LightTreeLightLink &other)
   {
+    /* other.set_membership is zero when expanding with an empty bucket: in this case there is no
+     * need to mark node as not shareable. */
+    if (other.set_membership == 0) {
+      return;
+    }
+
     if (set_membership == 0) {
       set_membership = other.set_membership;
       shareable = other.shareable;
@@ -418,7 +424,7 @@ class LightTree {
     return make_unique<LightTreeNode>(measure, bit_trial);
   }
 
-  size_t num_emitters()
+  size_t num_emitters() const
   {
     return emitters_.size();
   }

intern/cycles/test/CMakeLists.txt

@@ -33,6 +33,7 @@ set(SRC
   kernel_camera_projection_test.cpp
   render_graph_finalize_test.cpp
   util_aligned_malloc_test.cpp
+  util_boundbox_test.cpp
   util_ies_test.cpp
   util_math_test.cpp
   util_math_fast_test.cpp

intern/cycles/test/util_boundbox_test.cpp

@@ -0,0 +1,30 @@
+/* SPDX-FileCopyrightText: 2011-2025 Blender Foundation
+ *
+ * SPDX-License-Identifier: Apache-2.0 */
+
+#include "util/boundbox.h"
+
+#include "testing/testing.h"
+
+#include "util/transform.h"
+
+CCL_NAMESPACE_BEGIN
+
+TEST(BoundBox, transformed)
+{
+  {
+    const Transform tfm = transform_translate(make_float3(1, 2, 3));
+    const BoundBox orig_bounds(make_float3(-2, -3, -4), make_float3(3, 4, 5));
+    const BoundBox transformed_bounds = orig_bounds.transformed(&tfm);
+    EXPECT_LE(len(transformed_bounds.min - make_float3(-1, -1, -1)), 1e-6f);
+    EXPECT_LE(len(transformed_bounds.max - make_float3(4, 6, 8)), 1e-6f);
+  }
+
+  /* Non-valid boundbox should result in non-valid after transform. */
+  {
+    const Transform tfm = transform_scale(make_float3(1, 1, 1));
+    EXPECT_FALSE(BoundBox(BoundBox::empty).transformed(&tfm).valid());
+  }
+}
+
+CCL_NAMESPACE_END

intern/cycles/util/boundbox.h

@@ -135,6 +135,12 @@ class BoundBox {
   {
     BoundBox result = BoundBox::empty;
 
+    /* Avoid transforming empty bounding box since the code below performs per-corner transform
+     * which will convert non-valid bounds to valid. */
+    if (!valid()) {
+      return result;
+    }
+
     for (int i = 0; i < 8; i++) {
       float3 p;