when getMaterialInstance/setMaterialInstance are called, if entity is not renderable, return false

if no entity is passed, iterate through all entities and use the first renderable entity

thermion_dart/lib/src/bindings/src/thermion_dart_ffi.g.dart

@@ -2876,9 +2876,9 @@ external bool GltfResourceLoader_loadResources(
 );
 
 @ffi.Native<
-    ffi.Void Function(ffi.Pointer<TRenderableManager>, EntityId, ffi.Int,
+    ffi.Bool Function(ffi.Pointer<TRenderableManager>, EntityId, ffi.Int,
         ffi.Pointer<TMaterialInstance>)>(isLeaf: true)
-external void RenderableManager_setMaterialInstanceAt(
+external bool RenderableManager_setMaterialInstanceAt(
   ffi.Pointer<TRenderableManager> tRenderableManager,
   int entityId,
   int primitiveIndex,
@@ -2894,6 +2894,13 @@ external ffi.Pointer<TMaterialInstance> RenderableManager_getMaterialInstanceAt(
   int primitiveIndex,
 );
 
+@ffi.Native<ffi.Size Function(ffi.Pointer<TRenderableManager>, EntityId)>(
+    isLeaf: true)
+external int RenderableManager_getPrimitiveCount(
+  ffi.Pointer<TRenderableManager> tRenderableManager,
+  int entityId,
+);
+
 @ffi.Native<ffi.Bool Function(ffi.Pointer<TRenderableManager>, EntityId)>(
     isLeaf: true)
 external bool RenderableManager_isRenderable(

thermion_dart/lib/src/filament/src/implementation/background_image.dart

@@ -128,13 +128,7 @@ class BackgroundImage extends ThermionAsset {
   Future setCastShadows(bool castShadows) {
     // TODO: implement setCastShadows
     throw UnimplementedError();
-  }
-
-  @override
-  Future setMaterialInstanceAt(covariant MaterialInstance instance) {
-    // TODO: implement setMaterialInstanceAt
-    throw UnimplementedError();
-  }
+  }  
 
   @override
   Future setReceiveShadows(bool castShadows) {

thermion_dart/lib/src/filament/src/implementation/ffi_asset.dart

@@ -444,9 +444,29 @@ class FFIAsset extends ThermionAsset {
   @override
   Future<MaterialInstance> getMaterialInstanceAt(
       {ThermionEntity? entity, int index = 0}) async {
-    entity ??= this.entity;
+    
+    if (entity == null) {
+      if (RenderableManager_isRenderable(app.renderableManager, this.entity)) {
+        entity ??= this.entity;
+      } else {
+        for (final child in await getChildEntities()) {
+          if (RenderableManager_isRenderable(app.renderableManager, child)) {
+            entity = child;
+            break;
+          }
+        }
+      }
+    }
+
+    if (entity == null) {
+      throw Exception("Failed to find renderable entity");
+    }
+
     var ptr = RenderableManager_getMaterialInstanceAt(
         Engine_getRenderableManager(app.engine), entity, 0);
+    if (ptr == nullptr) {
+      throw Exception("Failed to get material instance for asset");
+    }
     return FFIMaterialInstance(ptr, app);
   }
 
@@ -454,12 +474,34 @@ class FFIAsset extends ThermionAsset {
   ///
   ///
   @override
-  Future setMaterialInstanceAt(FFIMaterialInstance instance) async {
-    var childEntities = await getChildEntities();
-    final entities = <ThermionEntity>[entity, ...childEntities];
-    for (final entity in entities) {
-      RenderableManager_setMaterialInstanceAt(
-          Engine_getRenderableManager(app.engine), entity, 0, instance.pointer);
+  Future setMaterialInstanceAt(FFIMaterialInstance instance,
+      {int? entity = null, int primitiveIndex = 0}) async {
+
+     if (entity == null) {
+      if (RenderableManager_isRenderable(app.renderableManager, this.entity)) {
+        entity ??= this.entity;
+      } else {
+        for (final child in await getChildEntities()) {
+          if (RenderableManager_isRenderable(app.renderableManager, child)) {
+            entity = child;
+            break;
+          }
+        }
+      }
+    }
+
+    if (entity == null) {
+      throw Exception("Failed to find renderable entity");
+    }
+    
+    if (!RenderableManager_setMaterialInstanceAt(
+          Engine_getRenderableManager(app.engine),
+          entity,
+          primitiveIndex,
+          instance.pointer)) {
+        _logger.warning(
+            "Failed to set material instance for entity $entity at primitive index ${primitiveIndex}");
+      
     }
   }
 

thermion_dart/lib/src/filament/src/interface/asset.dart

@@ -51,7 +51,10 @@ abstract class ThermionAsset {
   ///
   ///
   ///
-  Future setMaterialInstanceAt(covariant MaterialInstance instance);
+  Future setMaterialInstanceAt(covariant MaterialInstance instance,
+      {int? entity = null, int primitiveIndex = 0}) {
+    throw UnimplementedError();
+  }
 
   ///
   /// Renders an outline around [entity] with the given color.