craft-mesh: ignore empty OBJ material assignments

* Ignore empty OBJ material assignments

* Preserve empty material resets in OBJ writes

Co-authored-by: NetterSilver <133535268+Silvertree2010@users.noreply.github.com>

---------

Co-authored-by: NetterSilver <133535268+Silvertree2010@users.noreply.github.com>

crates/mesh/src/obj.rs

@@ -236,6 +236,13 @@ impl Reader<'_> {
 
     fn use_material(&mut self, name: &str) -> Result<()> {
         self.slot = None;
+        if name.trim().is_empty() {
+            // A bare `usemtl` selects no named material. In particular, do not create a
+            // material with an empty name: callers can map the missing material to their own
+            // default, and writers need not emit a meaningless empty material name.
+            self.material = None;
+            return Ok(());
+        }
         self.material = match self.material_ids.get(name) {
             Some(i) => Some(*i),
             None if self.scene.materials.len() >= self.limits.max_materials => {
@@ -518,6 +525,7 @@ pub fn write(scene: &Scene, mtllib: Option<&str>, limits: &Limits) -> Result<Str
     // OBJ indices run on across objects.
     let (mut vbase, mut tbase, mut nbase) = (1u64, 1u64, 1u64);
     let mut smoothing = 0;
+    let mut current_material: Option<Option<usize>> = None;
     for (index, world) in scene.instances(limits)? {
         let Some(mesh) = scene.meshes.get(index) else { continue };
         mesh.validate()?;
@@ -546,8 +554,17 @@ pub fn write(scene: &Scene, mtllib: Option<&str>, limits: &Limits) -> Result<Str
             if !part.name.trim().is_empty() {
                 let _ = writeln!(text, "g {}", part.name.trim().replace(|c: char| c.is_control(), " "));
             }
-            if let Some(m) = part.material.and_then(|m| materials.get(m)) {
-                let _ = writeln!(text, "usemtl {m}");
+            if current_material != Some(part.material) {
+                match part.material.and_then(|m| materials.get(m)) {
+                    Some(m) => {
+                        let _ = writeln!(text, "usemtl {m}");
+                    }
+                    None if current_material.is_some() => {
+                        let _ = writeln!(text, "usemtl");
+                    }
+                    None => {}
+                }
+                current_material = Some(part.material);
             }
             if part.smoothing != smoothing {
                 smoothing = part.smoothing;

crates/mesh/src/tests/obj.rs

@@ -125,6 +125,30 @@ fn objects_groups_quads_and_negative_indices() {
     assert_eq!((s.meshes.len(), s.meshes[0].name.as_str(), s.meshes[0].parts.len()), (1, "", 2));
 }
 
+#[test]
+fn bare_usemtl_does_not_create_an_empty_named_material() {
+    for line in ["usemtl", "usemtl "] {
+        let src = format!("v 0 0 0\nv 1 0 0\nv 0 1 0\n{line}\nf 1 2 3\n");
+        let scene = read(src.as_bytes(), &Limits::default(), &mut |_| None).unwrap();
+        assert!(scene.materials.is_empty(), "{line:?}: {:?}", scene.materials);
+        assert_eq!(scene.meshes.iter().map(Mesh::triangle_count).sum::<usize>(), 1);
+        assert_eq!(scene.meshes[0].parts[0].material, None);
+        scene.validate().unwrap();
+    }
+}
+
+#[test]
+fn writing_a_bare_usemtl_preserves_a_material_reset() {
+    let src = b"v 0 0 0\nv 1 0 0\nv 0 1 0\nv 1 1 0\nusemtl Named\nf 1 2 3\nusemtl\nf 2 4 3\n";
+    let scene = read(src, &Limits::default(), &mut no_external).unwrap();
+    assert_eq!(scene.meshes[0].parts.iter().map(|p| p.material).collect::<Vec<_>>(), [Some(0), None]);
+
+    let text = write(&scene, None, &Limits::default()).unwrap();
+    assert!(text.contains("usemtl Named\n") && text.contains("usemtl\nf 2 4 3\n"), "{text}");
+    let roundtrip = read(text.as_bytes(), &Limits::default(), &mut no_external).unwrap();
+    assert_eq!(roundtrip.meshes[0].parts.iter().map(|p| p.material).collect::<Vec<_>>(), [Some(0), None]);
+}
+
 #[test]
 fn corner_normals_and_smoothing_groups() {
     let vertices = "v 0 0 0\nv 1 0 0\nv 0 1 0\nv 0 -0.8660254037844386 0.5\n";