Round trip between Maya and Houdini

[Steven Shapiro]

Is there any design/workflow documentation on the current
implementation of the export of geometry from Maya into Houdini and
back? Specifically, I'm wondering about how hierarchy and grouping is
handled. Houdini flattens things out, currently, upon import.
However, there is some initial attempt at preserving the full dag
path. If this were more fleshed out, it would be possible to import
geo from Maya into Houdini, export from Houdini, and import back into
Maya with the hierarchy preserved (though with the transform composed
to the end node).

Since the writing out of Houdini is just starting, it seems like a
good time to organize that round trip workflow. We're planning on
changing things a bit from the current reference implementations, but
it would be better to do something consistent with the format and
vendors' future plans for native support.

[Steve LaVietes]

The current Houdini import tools give you the choice of flattening out or retaining the transformation hierarchy.

Even when flattening out the hierarchy (by loading into a single SOP), it creates primitive groups to retain the hierarchical location -- but not the individual transformations (which are baked into the points).

It seems reasonable to expect the export tools to be able to preserve whatever hierarchy information is present in both cases.

-stevel

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[Ashraf Ghoniem]

So I am trying to figure out how to make a roundtrip with maya to houdini and back again. Has anybody tried this with ogawa and reading the alembic file in sops or "flat" and some how preserving the maya hierarchy all the way through. 

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[Ashraf Ghoniem]

just for clarification sake my alembic file retains hierarchy within maya but once i read into houdini within sops I am unable to export that hierarchy again. 

[Lucas Miller]

It sounds like the ROP needs a mode where given the full path from the original archive that full path can be reconstructed on export.

Without extra data, or a separate utility, you would lose any transformation (and additional arbitrary attributes) along the way because the exported points would be in world-space.

Lucas

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[Ashraf Ghoniem]

yeah talking to sesi they seem to be indicating that the current way it's setup it cannot preserve hierarchy unless you import via the file menu or unless you use an alembic archive object level node which essentially creates the hierarchy in obj level. Which let's be honest isn't the ideal way to interact within houdini. Especially if you want to do an effect on a character are you going to work on each individual object node? Or do you object merge / use the sop import than write some sort of python tool to create the network in object level before export both seem rather clunky. I can admit that I am possibly completely missing something here. 

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[Alex Suter]

You could hold onto the hierarchy as string attributes, then modify the exporter to create the OXform objects on write. 

You end up with a big mess of strings, but it maintains the hierarchy. We did something like this with our exporter. Not sure if the official one does the same.

Definitely don't want to create a Houdini object per Alembic object. 

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[Steven]

Another possibility is that the Alembic ROP could take an optional Alembic hierarchy cache and re-create the hierarchy in the export based on any shapes that map. That could be the input geo used or a sidecar-style file that's just used to determine the hierarchy.  Hopefully that's something that SESI is considering...

None of that would help with decomposing the transforms, unless you wanted to include some additional transform data and reverse that out.  Though that doesn't make a lot of sense to me in most cases.

-Steve

[Matt Ebb]

When I was at Animal Logic on Lego last year there was a similar issue with their internal format named bobject, which is similar to Alembic in the sense that it contains an internal transformation hierarchy as well as geometry. We in the fx dept were having problems with being able to manipulate those files, and the structure inside, procedurally inside Houdini.

After a few prototypes based loosely on the Alembic reader that didn't work out so conveniently, and then some further discussion, one of the rnd/fx guys developed a 'bobject skeleton' SOP that would read the internal transformation hierarchy of the file and represent it in Houdini as a point cloud, with position, scale and orient attributes, and a bunch of meta-data as well. Then with a new option on the bobject SOP we could read in all the geo in the file (eg the leaf nodes of the hierarchy) in their own local space and use some additional tools to instance them on the point cloud for visualisation. An additional ROP would then interpret this combination of 'skeleton point cloud' and geo, and write it back into the file in the correct manner.

This allowed us to easily animate transforms and visibility within the bobject file, in a very lightweight fashion too, since a lot of the time we just needed the points and not the geo. And of course it was convenient since we were just dealing with SOP geometry, not big object hierarchy structures, which aren't so nice in Houdini.

Just some alternative ideas :)

cheers