Iterative scene parameter updates | creates | deletes

[georgy95]

I would like to update the scene parameters, without creating a new scene from scratch.

It appears that once the scene is loaded, we can only update the parameters. A full set of good to have operations :

• Add create or append an object e.g. Mesh to the instantiated scene
• I would like to delete already instantiated scene objects (I though this is what keep was for)
• I would like to overwrite objects (logic below)

In the screenshot below, the keep() call does not propagate updates to the scene itself.

As for overwrites:
If my original scene scene = mi.load_dict(mi.cornell_box()) had an root object with key green, I can overwrite it by supplying a new value e.g.

newgreen = {
'type': 'diffuse',
 'reflectance': {'type': 'rgb', 'value': [0.105421, 0.37798, 0.076425]}
}
params["green"] = mi.load_dict(newgreen) # overwrite or create

[njroussel]

Hi @georgy95

Currently, the only modifications that can be applied to an instantiated scene are updates to certain plugin parameters, as described in the tutorial.

The SceneParameters.keep() method is practically syntactic sugar to programmatically identify certain parameters in your scene. It indeed, is not meant to delete objects.

The general recommendation is to instantiate objects (materials, integrators, geometry, ...) separately and only tie them together into a scene when necessary, if you want to avoid reloading a scene from scratch on every "major" update. By "major" update I mean:

• Adding/removing plugins
• Chaning the type of an existing plugin
• Modifying some plugin parameter that is not exposed through mi.traverse()

[georgy95]

It's possible to instantiate integrators and materials on their own and use them. For geometry-shapes, since they reference materials, can't be instantiated into a Mitsuba object on their own (due to material not found error).

Unless you meant to instantiate dictionary objects only. In which case, it would be almost equivalent to building the new scene from scratch.

[njroussel]

Indeed, you are correct. I hadn't really considered nested plugins, where the enclosing plugin would need to be re-created.

[georgy95]

Thanks! @njroussel

[gioarma]

I think that having the ability to add and delete objects would be nice. In my case, I use the Blender plugin to create a base .xml scene. Then, I would like to programmatically add other elements to the scene.
A workaround would be to convert the loaded xml file to a dictionary, modify the dictionary adding the objects, and then using mi.load_dict. However, looking at the documentation I see that there is no function to do that, I only see mi.xml.dict_to_xml, which does the opposite.

[njroussel]

A "better" workaround is to do what you suggest the other way around: keep the original scene description in XML, convert any additional elements you might want to add to your scene into XML (using mi.xml.dict_to_xml for example), and finally insert these new XML elements into the main scene XML which you finally load with mi.load_file() or mi.load_string().

Of course, directly modifying a partially-loaded scene would still be better that re-loading new ones just to add or remove a single object. But that isn't available yet.

[gioarma]

Hi @njroussel, thanks for your answer. By "insert these new XML elements into the main scene" you mean programmatically opening the main XML and appending the new XML content, or is there another way of doing it that I'm not aware of?

[njroussel]

you mean programmatically opening the main XML and appending the new XML content

Yes 😄 . Either just loading the file as a string and editing it or using whatever XML parsing/editor library you prefer.

[georgy95]

Maybe what I did will help you @gioarma

Create a class to manage all of the scene assets and use the observer pattern to propagate the child component updates all the way up-to the parent.

So when Texture changes, you update the Material, which then updates the Scene.

Scene rebuilds took around 0.5s for me.