Tweak default lighting and specularity for non-PBR scenes.

Most "classic" objects looked washed out. To "revert" to the old look, you can add the following to your XML files:

```
<custom>
    <numeric name="filament.phong.specular_multiplier" data="0.1"/>
    <numeric name="filament.fallback.scene_light_intensity" data="100000.0"/>
    <numeric name="filament.fallback.environment_light_intensity" data="10000.0"/>
</custom>
```

PiperOrigin-RevId: 869725829
Change-Id: Icab0cedaa27915400497745db886c393709c14b1

src/experimental/filament/filament/object_manager.h

@@ -133,7 +133,7 @@ class ObjectManager {
   filament::Texture* fallback_normal_ = nullptr;
   filament::Texture* fallback_orm_ = nullptr;
   filament::IndirectLight* fallback_indirect_light_ = nullptr;
-  float specular_multiplier_ = 0.6f;
+  float specular_multiplier_ = 0.2f;
   float shininess_multiplier_ = 0.1f;
   float emissive_multiplier_ = 0.3f;
 };

src/experimental/filament/filament/scene_view.cc

@@ -166,6 +166,16 @@ SceneView::SceneView(filament::Engine* engine, ObjectManager* object_mgr)
       m, "filament.fog.inScatteringSize", fog_opts.inScatteringSize);
   views_[kNormalIndex]->setFogOptions(fog_opts);
 
+  fallback_head_light_intensity_ =
+      ReadElement(m, "filament.fallback.head_light_intensity",
+                  fallback_head_light_intensity_);
+  fallback_scene_light_intensity_ =
+      ReadElement(m, "filament.fallback.scene_light_intensity",
+                  fallback_scene_light_intensity_);
+  fallback_environment_light_intensity_ =
+      ReadElement(m, "filament.fallback.environment_light_intensity",
+                  fallback_environment_light_intensity_);
+
   // Create an empty/black indirect light to ensure that the skybox is oriented
   // to respect mujoco's Z-up convention.
   scene_->setIndirectLight(
@@ -313,18 +323,12 @@ void SceneView::PrepareLights() {
   // dealing with a "classic renderer" scene. In this case, let's add a
   // default environment light and set the light intensity ourselves.
   if (total_light_intensity == 0.0f) {
-    // Headlight is not required for Filament and often confusing, disable it by
-    // default.
-    constexpr float kHeadlightIntensityCandela = 0.f;
-    constexpr float kTotalSceneLightIntensityCandela = 100'000.f;
-    constexpr float kFallbackEnvironmentLightIntensityCandela = 10'000.f;
-
-    SetFallbackEnvironmentLight(kFallbackEnvironmentLightIntensityCandela);
-    const float intensity = kTotalSceneLightIntensityCandela / lights_.size();
+    SetFallbackEnvironmentLight(fallback_environment_light_intensity_);
+    const float intensity = fallback_scene_light_intensity_ / lights_.size();
     for (auto& light : lights_) {
       if (light) {
-        light->SetIntensity(light->IsHeadlight() ? kHeadlightIntensityCandela
-                                                : intensity);
+        light->SetIntensity(
+            light->IsHeadlight() ? fallback_head_light_intensity_ : intensity);
       }
     }
   }

src/experimental/filament/filament/scene_view.h

@@ -100,6 +100,9 @@ class SceneView {
   ColorGradingOptions color_grading_options_;
   DrawMode active_mode_ = DrawMode::kNumDrawModes;
   float aspect_ratio_ = 1.0f;
+  float fallback_head_light_intensity_ = 0.f;
+  float fallback_scene_light_intensity_ = 80'000.f;
+  float fallback_environment_light_intensity_ = 5'000.f;
 };
 
 }  // namespace mujoco