Enable a gltf_gltf2_hook

[jjcasmar]

This patch allows the user to create a gltf2_hook that returns a new list of scenes and animations. The purpose of this is to allow a second pass that can do more things than what currently the exporter does.

As an example, the exporter checks if an object in bpy.data.objects is exported, and in that case, it checks if it has animations. If it has animations, it exports the animation.

If you want to use glTF2 as an animation library, its not possible, because the animations wont be exported. EXT_property_animation (although not ratified yet), can't be implemented neither as you cant export material animations.

With a second pass, we allow this use cases and more.

@scurest @emackey @julienduroure

[scurest]

I think you could simplify and generalize it by just adding this to __gather_gltf

 def __gather_gltf(exporter, export_settings):
     active_scene_idx, scenes, animations = gltf2_blender_gather.gather_gltf2(export_settings)
 
+    plan = {'active_scene_idx': active_scene_idx, 'scenes': scenes, 'animations': animations }
+    export_user_extensions('gather_gltf_hook', export_settings, plan)
+    active_scene_idx, scenes, animations = plan['active_scene_idx'], plan['scenes'], plan['animations']
+
     if export_settings['gltf_draco_mesh_compression']:
         gltf2_io_draco_compression_extension.compress_scene_primitives(scenes, export_settings)
         exporter.add_draco_extension()

ie. we just give extensions a chance to modify those three variables.

You'd also have to take my suggestion from here about making export_user_extensions more flexible.

[jjcasmar]

The code you have posted would work perfectly without modifying the export_user_extensions, no?

Edit: it doesnt work, as we expect the gltf2_object to have an extensions attribute.

So if we allow not to pass a gltf2_object, how can we do the extensions is None check to generate a dict?

[scurest]

Easier to do

if args and hasattr(args[0], 'extensions'):
    if args[0].extensions is None:
        args[0].extensions = {}

Like I said before, I think this check should just go away though.

[jjcasmar]

Then you need to do it in all the extensions... By default, the extensions is None, and you dont know which extension is going to be called first

[jjcasmar]

Ive modify it as you said

[jjcasmar]

Any news on this?

[emackey]

Should the exception get logged to the console? Might be hard to debug without that.

[emackey]

@jjcasmar Just curious, would this also enable #231, material variants?

@scurest Are your concerns resolved here? Do you think this is a good idea?

[jjcasmar]

@emackey It would be possible, but not nice. The problem is that the exporter creates a glTF.Scene, which contains glTF.Nodes. Each node may contain a mesh, which may contain a material. There is no other way to specify a material in the glTF.Scene. So with this patch you would be able to add a dummy node, with a dummy mesh, with a material, but there is no way to add directly a material.

This is a general problem created by how the exporter works. The exporter creates an intermediate representation which is not 1:1 a glTF representation (for example, there is no textures root store).

[julienduroure]

So ... any problem remaining on this, or is that ready to merge?

[julienduroure]

@scurest @emackey Any concerns here, or could I merge it?

[scurest]

I don't really have any problems, but I'm not familiar with this code so I'm not really good to ask (my comments were just ways to make stuff shorter).

[jjcasmar]

Is there any other review?

[emackey]

Seems fine. Thanks @jjcasmar.