obj file loaded as Scene object instead of Trimesh object

[SilvioJin]

Hi,

I'm trying to load a mesh from an .obj file (from ShapeNet dataset) using Trimesh, and then use the repair. fix_winding(mesh) function.

But when I load the mesh, via trimesh.load('/path/to/file.obj') or trimesh.load_mesh('/path/to/file.obj'), the object class returned is Scene, which is incompatible with repair. fix_winding(mesh), only Trimesh object are accepted.

How can I force it to load and return a Trimesh object or parse the Scene object to Trimesh object?

[Mondobot]

I have the same issue. One thing bothers me. How can a load function return a trimesh object or a Scene? When does it return one and when the other? It's really ambiguous. Also, where is the documentation for older versions?

[jackd]

My understanding is that some .obj files specify different sub-meshes. trimesh attempts to recreate this as faithfully as possible. You can always concatenate the meshes together, though not all aspect have concatenation which is supported. If all you care about is the vertices/faces, you can use something like the following:

import trimesh


def as_mesh(scene_or_mesh):
    """
    Convert a possible scene to a mesh.

    If conversion occurs, the returned mesh has only vertex and face data.
    """
    if isinstance(scene_or_mesh, trimesh.Scene):
        if len(scene_or_mesh.geometry) == 0:
            mesh = None  # empty scene
        else:
            # we lose texture information here
            mesh = trimesh.util.concatenate(
                tuple(trimesh.Trimesh(vertices=g.vertices, faces=g.faces)
                    for g in scene_or_mesh.geometry.values()))
    else:
        assert(isinstance(mesh, trimesh.Trimesh))
        mesh = scene_or_mesh
    return mesh

[Mondobot]

Thank you for the answer! For now I just locked the version of trimesh to an older one, but when we'll update I'll use your solution.

[SilvioJin]

Same! Trying a previous version at the moment.
@jackd thanks for your answer!

[mikedh]

Yeah, this was one of the larger API changes in #467, and is an unfortunate consequence of better texture support: meshes with multiple materials get put into different mesh objects. The solution I would recommend (after I fix a bug in texture concatenation, sigh) is:

In [1]: import trimesh

In [2]: m = trimesh.load('bigmodel/airplane/models/model_normalized.obj')

In [3]: m
Out[3]: <trimesh.scene.scene.Scene at 0x7f20b27bf2e8>

In [4]: m.dump?
Signature: m.dump()
Docstring:
Append all meshes in scene to a list of meshes.

Returns
----------
dumped: (n,) list, of Trimesh objects transformed to their
                   location the scene.graph
File:      /dropbox/drop/Dropbox/robotics/trimesh/trimesh/scene/scene.py
Type:      method

In [5]: dump = m.dump()

In [6]: len(dump)
Out[6]: 450

In [7]: dump[0]
Out[7]: <trimesh.base.Trimesh at 0x7f20ea3fc4e0>

# should be a single mesh object
In [8]: concat = dump.sum()

If we had a nice "combine textured meshes" method, we could possibly return the default behavior to return a single mesh, or a have a load kwarg force_mesh which always tried to concatenate multiple meshes in a scene.

[mikedh]

I just pushed a version (3.1.0) that hopefully fixes this in most cases: OBJ models with 0-1 materials will return a single mesh again.

[aluo-x]

Got frustrated w/ PyMesh not building on the cluster, Pytorch3d's mesh loader was too slow, so here's a short snippet using trimesh:

import trimesh
from trimesh.scene.scene import Scene
import logging
logger = logging.getLogger("pywavefront")
logger.setLevel(logging.ERROR)

def custom_load_obj(filename_obj):
    obj_info = trimesh.load(filename_obj, file_type='obj', process=False)
    if type(obj_info) is Scene:
        geo_keys = list(obj_info.geometry.keys())
        total_vert = []
        total_faces = []
        for gk in geo_keys:
            cur_geo = obj_info.geometry[gk]
            cur_vert = cur_geo.vertices.tolist()
            cur_face = np.array(cur_geo.faces.tolist())+len(total_vert)
            total_vert += cur_vert
            total_faces += cur_face.tolist()
        return np.array(total_vert).astype("float32"), np.array(total_faces).astype("int32")
    else:
        return np.array(obj_info.vertices).astype("float32"), np.array(obj_info.faces).astype("int32")

[mikedh]

Nice! There are some convenience methods we added since this issue was closed that also might be helpful:

In [6]: m = trimesh.load('bigmodel/bigben/model.obj')

In [7]: m
Out[7]: <trimesh.Scene(len(geometry)=88)>

In [8]: m = trimesh.load('bigmodel/bigben/model.obj', force='mesh')
concatenating texture: may result in visual artifacts

In [9]: m
Out[9]: <trimesh.Trimesh(vertices.shape=(848, 3), faces.shape=(448, 3))>

[aluo-x]

That's incredibly useful. Since I only need the geometry, I wonder if there is a way to bypass the texture/material concat step.

[mikedh]

Yeah the material concatenation is pretty expensive, we could add Yet Another Keyword Argument™️ to the OBJ loader haha. It still complains about concatenation because it's still loading UV coordinates, but speeds it up by an order of magnitude:

In [7]: %timeit m = trimesh.load('bigmodel/bigben/model.obj', force='mesh', skip_texture=True)
concatenating texture: may result in visual artifacts

123 ms ± 1.16 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)

In [8]: %timeit m = trimesh.load('bigmodel/bigben/model.obj', force='mesh', skip_texture=False)
concatenating texture: may result in visual artifacts

1.9 s ± 27.4 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

Happy to include in the next release if that's useful.

[aluo-x]

I would be super grateful.

[aluo-x]

Also would appreciate a merge of the texture skipping.

[mikedh]

Oh that was merged, I just renamed it to skip_materials from skip_texture before merging to more accurately reflect what it does:

trimesh/trimesh/exchange/obj.py

Lines 26 to 45 in fef8efb

	skip_materials=False,
	**kwargs):
	"""
	Load a Wavefront OBJ file into kwargs for a trimesh.Scene
	object.
	Parameters
	--------------
	file_obj : file like object
	Contains OBJ data
	resolver : trimesh.visual.resolvers.Resolver
	Allow assets such as referenced textures and
	material files to be loaded
	split_object : bool
	Split meshes at each `o` declared in file
	group_material : bool
	Group faces that share the same material
	into the same mesh.
	skip_materials : bool
	Don't load any materials.

[aluo-x]

Nice, there's a lot of flags in there.
Maintaining such a high profile project must be a lot of work! Thanks for the effort.