fix: implement custom compute_normals for pygfx

pygfx doesn't have a compute_normals function, so we implement
our own vertex normal computation from positions and face indices.

src/fluency/rendering/pygfx_renderer.py

@@ -17,6 +17,36 @@ from fluency.rendering.base import (
 )
 
 
+def compute_normals(vertices: np.ndarray, faces: np.ndarray) -> np.ndarray:
+    """Compute vertex normals from positions and face indices."""
+    vertices = np.asarray(vertices, dtype=np.float32)
+    faces = np.asarray(faces, dtype=np.int32)
+
+    normals = np.zeros_like(vertices)
+
+    for face in faces:
+        v0, v1, v2 = vertices[face[0]], vertices[face[1]], vertices[face[2]]
+
+        edge1 = v1 - v0
+        edge2 = v2 - v0
+
+        face_normal = np.cross(edge1, edge2)
+
+        length = np.linalg.norm(face_normal)
+        if length > 1e-10:
+            face_normal = face_normal / length
+
+        normals[face[0]] += face_normal
+        normals[face[1]] += face_normal
+        normals[face[2]] += face_normal
+
+    lengths = np.linalg.norm(normals, axis=1, keepdims=True)
+    lengths = np.where(lengths > 1e-10, lengths, 1)
+    normals = normals / lengths
+
+    return normals.astype(np.float32)
+
+
 @dataclass
 class PygfxRenderObject(RenderObject):
     """pygfx render object wrapper."""
@@ -128,10 +158,9 @@ class PygfxRenderer(Renderer):
         vertices = np.asarray(vertices, dtype=np.float32)
         faces = np.asarray(faces, dtype=np.int32)
 
-        geometry = gfx.Geometry(positions=vertices, indices=faces)
+        normals = compute_normals(vertices, faces)
 
-        normals = gfx.compute_normals(geometry)
+        geometry = gfx.Geometry(positions=vertices, indices=faces, normals=normals)
-        geometry.normals = normals
 
         material = gfx.MeshPhongMaterial(color=color, flat_shading=False)
 
@@ -252,8 +281,9 @@ class PygfxRenderer(Renderer):
         vertices = np.asarray(vertices, dtype=np.float32)
         faces = np.asarray(faces, dtype=np.int32)
 
-        geometry = gfx.Geometry(positions=vertices, indices=faces)
+        normals = compute_normals(vertices, faces)
-        geometry.normals = gfx.compute_normals(geometry)
+
+        geometry = gfx.Geometry(positions=vertices, indices=faces, normals=normals)
 
         obj.geometry = geometry
         if obj.scene_node is not None: