Fix fallback metallic texture in Filament renderer.

Set fallback_textures_[mjTEXROLE_METALLIC] to fallback_white_ rather
than fallback_black_, so that materials defining metallicness without an
explicit metallic texture map do not have their metallic factor zeroed
out by the texture multiplier in pbr.mat.

PiperOrigin-RevId: 975852073
Change-Id: I366f10891e1b249655f921ca23adc2d3b40b8946

Author: havess

doc/changelog.rst

@@ -57,6 +57,16 @@ Bug fixes
 
 - Rewrote the Plane-Mesh collider to fix bugs and improve performance. Fixes :issue:`3524`.
 
+Rendering
+^^^^^^^^^
+
+.. admonition:: Breaking API changes
+   :class: attention
+
+   - For materials with physically-based rendering parameters that do *not* provide a metallic texture the fallback is
+     now white. Previously the fallback was black, which caused scalar metallic values to be discarded when multiplied
+     in the PBR shader.
+
 Version 3.12.0 (August 20, 2026)
 --------------------------------
 

src/render/filament/core/object_manager.cc

@@ -126,7 +126,7 @@ ObjectManager::ObjectManager(filament::Engine* engine) : engine_(engine) {
   fallback_textures_[mjTEXROLE_OPACITY] = fallback_white_;
   fallback_textures_[mjTEXROLE_OCCLUSION] = fallback_white_;
   fallback_textures_[mjTEXROLE_ROUGHNESS] = fallback_white_;
-  fallback_textures_[mjTEXROLE_METALLIC] = fallback_black_;
+  fallback_textures_[mjTEXROLE_METALLIC] = fallback_white_;
   fallback_textures_[mjTEXROLE_NORMAL] = fallback_normal_;
   fallback_textures_[mjTEXROLE_EMISSIVE] = fallback_white_;
   fallback_textures_[mjTEXROLE_ORM] = fallback_orm_;

test/render/testdata/emissive_bloom.xml

@@ -5,7 +5,7 @@
 
     <material name="floor" texture="dark" texrepeat="6 6" texuniform="true" reflectance="0.4" metallic="0.5" roughness="0.2"/>
     <material name="wall" rgba="0.13 0.13 0.16 1" metallic="0.05" roughness="0.85"/>
-    <material name="chrome" rgba="0.95 0.95 0.95 1" metallic="1.0" roughness="0.05" emission="1.0"/>
+    <material name="white" rgba="0.95 0.95 0.95 1" metallic="0.0" roughness="0.05" emission="1.0"/>
   </asset>
 
   <custom>
@@ -26,21 +26,21 @@
     <geom name="backwall" type="box" pos="0 1.6 0.9" size="3 0.05 0.9" material="wall"/>
 
     <light type="point" pos="-2.0 -0.2 0.40" diffuse="1.00 0.20 0.20" intensity="180000" range="3" bulbradius="120"/>
-    <geom type="sphere" pos="-2.0 0.20 0.18" size="0.18" material="chrome"/>
+    <geom type="sphere" pos="-2.0 0.20 0.18" size="0.18" material="white"/>
 
     <light type="point" pos="-1.2 -0.2 0.40" diffuse="1.00 0.50 0.10" intensity="180000" range="3" bulbradius="120"/>
-    <geom type="sphere" pos="-1.2 0.20 0.18" size="0.18" material="chrome"/>
+    <geom type="sphere" pos="-1.2 0.20 0.18" size="0.18" material="white"/>
 
     <light type="point" pos="-0.4 -0.2 0.40" diffuse="1.00 0.95 0.30" intensity="180000" range="3" bulbradius="120"/>
-    <geom type="sphere" pos="-0.4 0.20 0.18" size="0.18" material="chrome"/>
+    <geom type="sphere" pos="-0.4 0.20 0.18" size="0.18" material="white"/>
 
     <light type="point" pos=" 0.4 -0.2 0.40" diffuse="0.20 1.00 0.40" intensity="180000" range="3" bulbradius="120"/>
-    <geom type="sphere" pos=" 0.4 0.20 0.18" size="0.18" material="chrome"/>
+    <geom type="sphere" pos=" 0.4 0.20 0.18" size="0.18" material="white"/>
 
     <light type="point" pos=" 1.2 -0.2 0.40" diffuse="0.30 0.55 1.00" intensity="180000" range="3" bulbradius="120"/>
-    <geom type="sphere" pos=" 1.2 0.20 0.18" size="0.18" material="chrome"/>
+    <geom type="sphere" pos=" 1.2 0.20 0.18" size="0.18" material="white"/>
 
     <light type="point" pos=" 2.0 -0.2 0.40" diffuse="1.00 0.30 0.95" intensity="180000" range="3" bulbradius="120"/>
-    <geom type="sphere" pos=" 2.0 0.20 0.18" size="0.18" material="chrome"/>
+    <geom type="sphere" pos=" 2.0 0.20 0.18" size="0.18" material="white"/>
   </worldbody>
 </mujoco>

test/render/testdata/reflection.xml

@@ -25,7 +25,7 @@
     <!-- packed occlusion(R)/roughness(G)/metallic(B): smooth copper -->
     <texture name="orm" type="2d" builtin="flat" rgb1="1 .3 1" width="8" height="8"/>
     <material name="floor"  texture="grid" texrepeat="3 3" texuniform="true" reflectance=".5"/>
-    <material name="mirror" rgba=".03 .03 .04 1" reflectance=".9" metallic="1" roughness=".04"/>
+    <material name="mirror" rgba=".03 .03 .04 1" reflectance=".9" metallic="0" roughness=".04"/>
     <material name="copper_mirror" rgba=".95 .64 .38 1" reflectance=".9">
       <layer texture="orm" role="orm"/>
     </material>