Is loading .obj file with .mtl files still supported?

[leleju]

In mitsuba documentation 0.5.0, it states that obj files .mtl files can be automatically loaded. I currently have a obj file where some parts of it are red and some parts are blue (all using the simplest diffuse model). I opened the obj file with windows 3d viewer and it looks fine.

However, if I simply add the obj in mitsuba as:

<shape type="obj" id="myshape">
    <string name="filename" value="myshape.obj"/>
</shape>

Then it renders the whole obj as a gray diffuse surface. If I do the same thing as stated in the documentation:

<shape type="obj">
    <string name="filename" value="myShape.obj"/>
    <bsdf name="Glass" type="dielectric">
          <float name="intIOR" value="1.5"/>
    </bsdf>
    <bsdf name="Water" type="dielectric">
          <float name="intIOR" value="1.333"/>
    </bsdf>
</shape>

then I get the error that each obj can only be assigned with a single bsdf. Does that mean that I need to divide my obj files into parts, and assign a different bsdf to each one instead of simply using the .mtl file?

I'm currently using python 3.12 and mitsuba 3.5.

[merlinND]

Hello @leleju,

Indeed, Mitsuba 3 does not account for the .mtl files. The recommended workflow to create scenes is to use the mitsuba-blender Blender add-on, which will automatically split meshes by material and export a correct XML file.

Pull requests adding support for MTL files may be welcome (@wjakob and @njroussel to confirm?).

[sirkrisp]

Hi, this functionality would be really helpful, e.g. for working with ShapeNet where I cannot use Blender to manually convert all objects.

[sirkrisp]

Generated this code with Gemini, seems to work

import trimesh
import numpy as np
import os
import xml.etree.ElementTree as ET
from xml.dom import minidom

def obj_to_mitsuba_xml_trimesh(obj_path: str, output_dir: str = 'meshes') -> str:
	"""
	Loads an .obj file using the high-performance Trimesh library, splits the mesh
	by material, and generates a Mitsuba 3 XML scene string.

	Args:
		obj_path (str): Path to the input .obj file.
		output_dir (str): Directory to save the split .obj mesh files.

	Returns:
		str: A formatted XML string for a Mitsuba 3 scene.
	"""
	print("Loading mesh with Trimesh...")
	# 1. Load the entire scene with Trimesh in one go.
	# This is extremely fast and handles all parsing internally.
	# It returns a list of meshes if the OBJ has multiple materials.
	scene = trimesh.load(obj_path, process=False, split_object=True)

	# If the OBJ has only one material, it might load as a single mesh.
	# We can get the geometry-per-material from the scene graph.
	geometries = list(scene.geometry.values())

	os.makedirs(output_dir, exist_ok=True)
	base_name = os.path.splitext(os.path.basename(obj_path))[0]

	# 2. Build the XML Scene
	xml_scene = ET.Element('scene', version='3.0.0')

	print(f"Found {len(geometries)} material groups. Processing...")
	for i, mesh in enumerate(geometries):
		# Trimesh stores material properties in mesh.visual.material
		material = mesh.visual.material
		
		# Use a default name if none is found
		mat_name = material.name if hasattr(material, 'name') and material.name else f"material_{i}"
		
		# 3. Create BSDF from material properties
		bsdf = ET.SubElement(xml_scene, 'bsdf', type='roughplastic', id=mat_name)
		
		# Diffuse color (Kd)
		kd = material.diffuse[:3] / 255.0 if hasattr(material, 'diffuse') else np.array([0.7, 0.7, 0.7])
		ET.SubElement(bsdf, 'rgb', name='diffuse_reflectance', value=f'{kd[0]:.4f} {kd[1]:.4f} {kd[2]:.4f}')
		
		# Specular & Roughness from Shininess (Ns)
		ns = material.Ns if hasattr(material, 'Ns') else 10.0
		ks_color = material.specular[:3] / 255.0 if hasattr(material, 'specular') else np.array([0.0, 0.0, 0.0])
		
		ET.SubElement(bsdf, 'rgb', name='specular_reflectance', value=f'{ks_color[0]:.4f} {ks_color[1]:.4f} {ks_color[2]:.4f}')
		
		ns = max(ns, 1e-4)
		alpha = (2 / (ns + 2)) ** 0.5
		ET.SubElement(bsdf, 'float', name='alpha', value=str(alpha))

		# 4. Export the split mesh and add the shape to the XML
		part_filename = f"{base_name}_{mat_name}.obj"
		part_filepath = os.path.join(output_dir, part_filename)
		
		print(f"Exporting part: {part_filepath}")
		mesh.export(part_filepath)
		
		shape = ET.SubElement(xml_scene, 'shape', type='obj')
		ET.SubElement(shape, 'string', name='filename', value=part_filepath)
		ET.SubElement(shape, 'ref', id=mat_name)
		
	# Emitter
	ET.SubElement(xml_scene, 'emitter', type='constant')

	# 5. Convert XML tree to a formatted string
	rough_string = ET.tostring(xml_scene, 'utf-8')
	reparsed = minidom.parseString(rough_string)
	return reparsed.toprettyxml(indent="  ")