Support specifying the name of light associated with lens flares (#2172)

---------

Signed-off-by: Ian Chen <ichen@openrobotics.org>

src/systems/lens_flare/LensFlare.cc

@@ -122,6 +122,11 @@ void LensFlare::Configure(
     this->dataPtr->occlusionSteps = _sdf->Get<uint32_t>("occlusion_steps");
   }
 
+  if (_sdf->HasElement("light_name"))
+  {
+    this->dataPtr->lightName = _sdf->Get<std::string>("light_name");
+  }
+
   // Get Camera Name
   this->dataPtr->cameraName = scopedName(this->dataPtr->entity,
                       _ecm, "::", false);
@@ -187,7 +192,18 @@ void LensFlarePrivate::OnPostRender()
   }
 
   // get light
-  auto light = this->scene->LightByIndex(0);
+  rendering::LightPtr light;
+  if (!this->lightName.empty())
+  {
+    light = this->scene->LightByName(this->lightName);
+  }
+  else if (this->scene->LightCount() > 0)
+  {
+    light = this->scene->LightByIndex(0);
+  }
+  // Light not found. Keep trying in case it's not available in the world yet.
+  if (!light)
+    return;
 
   rendering::RenderEngine *engine = this->camera->Scene()->Engine();
   rendering::RenderPassSystemPtr rpSystem = engine->RenderPassSystem();

src/systems/lens_flare/LensFlare.hh

@@ -46,6 +46,9 @@ namespace systems
   ///   <occlusion_steps> Sets the number of steps to take in
   ///                     each direction to check for occlusions.
   ///                     The default value is set to 10. Use 0 to disable
+  ///   <light_name> Sets the light associated with the lens flares.
+  ///                If not specified. The first light in the scene will
+  ///                be used.
 
   class LensFlare:
     public System,