debugging Mitsuba xml scene import into Blender

[tadz-io]

I'm following Mitsuba's caustic rendering tutorial: https://mitsuba.readthedocs.io/en/latest/src/inverse_rendering/caustics_optimization.html . Now i wanted to export the scene described in the tutorial as a xml file and import it into Blender to visualize the setup. The scene is a python dict (see tutorial for further details):

scene = {
    'type': 'scene',
    'sensor': sensor,
    'integrator': integrator,
    # Glass BSDF
    'simple-glass': {
        'type': 'dielectric',
        'id': 'simple-glass-bsdf',
        'ext_ior': 'air',
        'int_ior': 1.5,
        'specular_reflectance': { 'type': 'spectrum', 'value': 0 },
    },
    'white-bsdf': {
        'type': 'diffuse',
        'id': 'white-bsdf',
        'reflectance': { 'type': 'rgb', 'value': (1, 1, 1) },
    },
    'black-bsdf': {
        'type': 'diffuse',
        'id': 'black-bsdf',
        'reflectance': { 'type': 'spectrum', 'value': 0 },
    },
    # Receiving plane
    'receiving-plane': {
        'type': 'obj',
        'id': 'receiving-plane',
        'filename': 'meshes/rectangle.obj',
        'to_world': \
            mi.ScalarTransform4f.look_at(
                target=[0, 1, 0],
                origin=[0, -7, 0],
                up=[0, 0, 1]
            ).scale((5, 5, 5)),
        'bsdf': {'type': 'ref', 'id': 'white-bsdf'},
    },
    # Glass slab, excluding the 'exit' face (added separately below)
    'slab': {
        'type': 'obj',
        'id': 'slab',
        'filename': 'meshes/slab.obj',
        'to_world': mi.ScalarTransform4f.rotate(axis=(1, 0, 0), angle=90),
        'bsdf': {'type': 'ref', 'id': 'simple-glass'},
    },
    # Glass rectangle, to be optimized
    'lens': {
        'type': 'ply',
        'id': 'lens',
        'filename': lens_fname,
        'to_world': mi.ScalarTransform4f.rotate(axis=(1, 0, 0), angle=90),
        'bsdf': {'type': 'ref', 'id': 'simple-glass'},
    },

    # Directional area emitter placed behind the glass slab
    'focused-emitter-shape': {
        'type': 'obj',
        'filename': 'meshes/rectangle.obj',
        'to_world': mi.ScalarTransform4f.look_at(
            target=[0, 0, 0],
            origin=[0, 5, 0],
            up=[0, 0, 1]
        ),
        'bsdf': {'type': 'ref', 'id': 'black-bsdf'},
        'focused-emitter': emitter,
    },
}

i have exported the dict to a xml using Mitsuba:

import mitsuba as mi
mi.xml.dict_to_xml(scene, filename='caustic_optimization.xml')

I'm running into several issues, some of which i could solve by manually editing the xml file. However, for the more complex issues i would like to debug the python code of mitsuba-blender . I'm using VSC, i have cloned this repo and created a virtual environment using python 3.11. However installing bpy gives me the following error:

pip install bpy       
ERROR: Ignored the following versions that require a different python version: 2.82.1 Requires-Python >=3.7, <3.8
ERROR: Could not find a version that satisfies the requirement bpy (from versions: none)
ERROR: No matching distribution found for bpy

Downgrading python to 3.7.x won't solve it because of other dependency issues... Long story short: how can i setup an environment to run some tests on importing mitsuba xml's into Blender?

I'm using Blender 3.6

[bathal1]

Hi,

Installing bpy via pip will not help you. You need to use blender's python, which already has bpy installed (since it's blender's python API).

For development, you can clone the repo and make a symlink to the mitsuba-blender folder in the directory containing add-ons for blender, e.g. for linux: ~/.config/blender/3.6/scripts/addons. Alternatively you could use this VSCode extension.

[tadz-io]

Thanks for the quick response. Much appreciated!

Yes i tried using Blender's python interpreter as well. I installed pytest and tried running the tests. But all tests fail and many of them complain that bpy.ops.import_scene.mitsuba can't be found. Example below:

./tests/test_compare.py::test_round_trip[scenes/test1.xml] Failed: [undefined]AttributeError: Calling operator "bpy.ops.import_scene.mitsuba" error, could not be found
xml_scene = 'scenes/test1.xml'
resource_resolver = <fixtures.ResourceResolver object at 0x12611fbb0>
mitsuba_scene_ztest = <fixtures.MitsubaRenderTester object at 0x12611faf0>

Blender's python interpreter is using bpy 4.0.0. I tried upgrading to 4.1.0 but no luck:

/Applications/Blender.app/Contents/Resources/3.6/python/bin/python3.10 -m pip install bpy==4.1.0  
Defaulting to user installation because normal site-packages is not writeable
ERROR: Ignored the following versions that require a different python version: 2.82.1 Requires-Python >=3.7, <3.8
ERROR: Could not find a version that satisfies the requirement bpy==4.1.0 (from versions: 3.4.0, 3.5.0, 3.6.0, 4.0.0)
ERROR: No matching distribution found for bpy==4.1.0

Any idea? I will try the symlink or vsc extension for development. thanks for the tip

[merlinND]

Hello @tadz-io,

I have very limited experience with Blender extensions, but based on Calling operator "bpy.ops.import_scene.mitsuba" error, could not be found: are you sure that the extension is really installed and enabled?
You can check by creating and exporting a simple scene (e.g. just the default cube).

As @bathal1 said, trying to install bpy with pip will not help, you should use the bpy version that comes bundled with your Blender version.

[tadz-io]

@merlinND sorry, i should have been more explicit. I got it to work now by using VSC's blender extension as suggested by @bathal1 , and running it from my local mitsuba-blender repo. I actually had to the delete the installed mitsuba-blender extension in blender for it work (or rename it). Thanks for the input though