Correctly apply frame transform to light direction.

PiperOrigin-RevId: 665340267
Change-Id: Ida16ddbf3a057ffd3e260b71180a45acbf749931

src/user/user_objects.cc

@@ -3222,11 +3222,14 @@ void mjCLight::CopyFromSpec() {
 void mjCLight::Compile(void) {
   CopyFromSpec();
 
-  double quat[4]= {1, 0, 0, 0};
-
   // frame
   if (frame) {
-    mjuu_frameaccumChild(frame->pos, frame->quat, pos, quat);
+    // apply frame transform to pos, qunit is unused
+    double qunit[4]= {1, 0, 0, 0};
+    mjuu_frameaccumChild(frame->pos, frame->quat, pos, qunit);
+
+    // rotate dir
+    mjuu_rotVecQuat(dir, dir, frame->quat);
   }
 
   // normalize direction, make sure it is not zero

test/user/user_objects_test.cc

@@ -2086,7 +2086,6 @@ TEST_F(MujocoTest, Frame) {
       </body>
     </worldbody>
   </mujoco>
-
   )";
   constexpr mjtNum eps = 1e-14;
   std::array<char, 1024> error;
@@ -2142,6 +2141,34 @@ TEST_F(MujocoTest, Frame) {
   mj_deleteData(d);
 }
 
+TEST_F(MujocoTest, FrameTransformsLight) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <worldbody>
+      <frame euler="0 45 0" pos="0 0 1">
+        <light pos="-1 0 0" dir="1 0 -1"/>
+      </frame>
+    </worldbody>
+  </mujoco>
+  )";
+  std::array<char, 1024> error;
+  mjModel* m = LoadModelFromString(xml, error.data(), error.size());
+  EXPECT_THAT(m, testing::NotNull()) << error.data();
+  EXPECT_EQ(m->nlight, 1);
+
+  constexpr mjtNum eps = 1e-14;
+  EXPECT_NEAR(m->light_pos[0], -mju_sqrt(.5), eps);
+  EXPECT_NEAR(m->light_pos[1], 0, eps);
+  EXPECT_NEAR(m->light_pos[2], 1 + mju_sqrt(.5), eps);
+
+  EXPECT_NEAR(m->light_dir[0], 0, eps);
+  EXPECT_NEAR(m->light_dir[1], 0, eps);
+  EXPECT_NEAR(m->light_dir[2], -1, eps);
+
+  mj_deleteModel(m);
+}
+
+
 // ------------- test bvh ------------------------------------------------------
 TEST_F(MujocoTest, RobustBVH) {
   static constexpr char xml1[] = R"(