Fixed "setting an extruder does not change object's color"

src/libslic3r/Model.cpp

@@ -1232,6 +1232,19 @@ void ModelVolume::set_material(t_model_material_id material_id, const ModelMater
         this->object->get_model()->add_material(material_id, material);
 }
 
+// Extract the current extruder ID based on this ModelVolume's config and the parent ModelObject's config.
+int ModelVolume::extruder_id() const
+{
+    int extruder_id = -1;
+    if (this->is_model_part()) {
+        const ConfigOption *opt = this->config.option("extruder");
+        if (opt == nullptr)
+            opt = this->object->config.option("extruder");
+        extruder_id = (opt == nullptr) ? 0 : opt->getInt();
+    }
+    return extruder_id;
+}
+
 #if ENABLE_MODELVOLUME_TRANSFORM
 void ModelVolume::center_geometry()
 {

src/libslic3r/Model.hpp

@@ -312,6 +312,10 @@ public:
     void                set_material_id(t_model_material_id material_id);
     ModelMaterial*      material() const;
     void                set_material(t_model_material_id material_id, const ModelMaterial &material);
+    // Extract the current extruder ID based on this ModelVolume's config and the parent ModelObject's config.
+    // Extruder ID is only valid for FFF. Returns -1 for SLA or if the extruder ID is not applicable (support volumes).
+    int                 extruder_id() const;
+
     // Split this volume, append the result to the object owning this volume.
     // Return the number of volumes created from this one.
     // This is useful to assign different materials to different volumes of an object.

src/slic3r/GUI/3DScene.cpp

@@ -747,18 +747,9 @@ int GLVolumeCollection::load_object_volume(
         { 0.5f, 0.5f, 1.0f, 1.f }
     };
 
-    const ModelVolume *model_volume = model_object->volumes[volume_idx];
+    const ModelVolume   *model_volume = model_object->volumes[volume_idx];
-
+    const int            extruder_id  = model_volume->extruder_id();
-    int extruder_id = -1;
+    const ModelInstance *instance     = model_object->instances[instance_idx];
-    if (model_volume->is_model_part())
-    {
-        const ConfigOption *opt = model_volume->config.option("extruder");
-        if (opt == nullptr)
-            opt = model_object->config.option("extruder");
-        extruder_id = (opt == nullptr) ? 0 : opt->getInt();
-    }
-
-    const ModelInstance *instance = model_object->instances[instance_idx];
 #if ENABLE_MODELVOLUME_TRANSFORM
     const TriangleMesh& mesh = model_volume->mesh;
 #else

src/slic3r/GUI/GLCanvas3D.cpp

@@ -3844,12 +3844,13 @@ void GLCanvas3D::reload_scene(bool refresh_immediately, bool force_full_scene_re
 
     struct ModelVolumeState {
         ModelVolumeState(const GLVolume *volume) : 
-			geometry_id(volume->geometry_id), volume_idx(-1) {}
+			model_volume(nullptr), geometry_id(volume->geometry_id), volume_idx(-1) {}
-		ModelVolumeState(const ModelID &volume_id, const ModelID &instance_id, const GLVolume::CompositeID &composite_id) :
+		ModelVolumeState(const ModelVolume *model_volume, const ModelID &instance_id, const GLVolume::CompositeID &composite_id) :
-			geometry_id(std::make_pair(volume_id.id, instance_id.id)), composite_id(composite_id), volume_idx(-1) {}
+			model_volume(model_volume), geometry_id(std::make_pair(model_volume->id().id, instance_id.id)), composite_id(composite_id), volume_idx(-1) {}
 		ModelVolumeState(const ModelID &volume_id, const ModelID &instance_id) :
-			geometry_id(std::make_pair(volume_id.id, instance_id.id)), volume_idx(-1) {}
+			model_volume(nullptr), geometry_id(std::make_pair(volume_id.id, instance_id.id)), volume_idx(-1) {}
 		bool new_geometry() const { return this->volume_idx == size_t(-1); }
+		const ModelVolume		   *model_volume;
         // ModelID of ModelVolume + ModelID of ModelInstance
         // or timestamp of an SLAPrintObjectStep + ModelID of ModelInstance
         std::pair<size_t, size_t>   geometry_id;
@@ -3888,7 +3889,7 @@ void GLCanvas3D::reload_scene(bool refresh_immediately, bool force_full_scene_re
                 const ModelInstance *model_instance = model_object->instances[instance_idx];
                 for (int volume_idx = 0; volume_idx < (int)model_object->volumes.size(); ++ volume_idx) {
                     const ModelVolume *model_volume = model_object->volumes[volume_idx];
-					model_volume_state.emplace_back(model_volume->id(), model_instance->id(), GLVolume::CompositeID(object_idx, volume_idx, instance_idx));
+					model_volume_state.emplace_back(model_volume, model_instance->id(), GLVolume::CompositeID(object_idx, volume_idx, instance_idx));
                 }
             }
         }
@@ -3941,6 +3942,10 @@ void GLCanvas3D::reload_scene(bool refresh_immediately, bool force_full_scene_re
                 map_glvolume_old_to_new[volume_id] = glvolumes_new.size();
                 mvs->volume_idx = glvolumes_new.size();
                 glvolumes_new.emplace_back(volume);
+                // Update color of the volume based on the current extruder.
+                int extruder_id = mvs->model_volume->extruder_id();
+                if (extruder_id != -1)
+                    volume->extruder_id = extruder_id;
             }
         }
     }