Color sets from Houdini Alembic files using instancing.

[galakgorr]

Here's a tricky problem.

I'm exporting an Alembic from Houdini. In order to take advantage of instancing, the objects I'm exporting are packed primitives. These packed primitives have a Cd point attribute. I'd like to be able to use the VRayMesh node in Maya to load in the Alembic file as a proxy, but it's not detecting Cd as a color set. Is there a way to get this attribute across without unpacking and losing all the benefits of instancing?

[yolov]

I'm guessing that you would need the attribute to drive a texture, in which case you can add extra vray attributes to the placement node of the texture. This will add a text field to the bottom of the attribute editor, where you can just type in the name of the color set.

We have it documented for V-Ray Next for Maya, but the docs are still closed, as they're still in the making. Also, we have houdini to max and maya workflow tutorials in the making, both for alembics and simulations. Hopefully, they'll be useful to more people.

What I'm suggesting is based on what I know for reading the rest attribute, so apologies in advance if I'm not on point. I'm not a houdini expert, but I can get one of our guys to help you out in case the above doesn't work.

[yolov]

Actually, that particular page of the docs is done so I just transferred it to 3.6. Here's a link: https://docs.chaosgroup.com/display/...ent+Attributes

[galakgorr]

Thanks for the fast response!

The instancing part is what's making all of this difficult. I have successfully gotten color sets and UV sets to work from Houdini -> Maya VRay, but only when the Alembic file doesn't involve instances. Whatever Houdini is writing, it seems like attributes added to the packed primitive objects themselves (not the meshes they represent) are ignored by the exporter. Maybe one of your Houdini guys knows a workaround?

Thanks again!

[yolov]

Sorry, I missed the instancing part. I'll have one of the guys look into this tomorrow, as he's way more familiar with houdini than I.
Btw, if you could get us a scene, that would be really helpful.

[galakgorr]

Sure, here's the .HIP file. There's two color-type attributes being added to the packed primitives as point attributes. The attributes don't appear to exist when reading the Alembic back in.

[Gosho.Genchev]

Hey there,

the way I see it, you've got 2 options:

1. Break the instancing by unpacking and transferring the attribute to the geometry, then pack again or
2. Use the template points or copy-stamping to set the attribute value and transfer it to the geometry before packing

I tried finding an alternative and stumbled upon abc_userProperties but a) could not figure out a way to set one in Houdin and b) the dev team told me it's unlikely you'll be able to use them for rendering anyway

If you don't mind me asking - is it possible to tell us what your end goal is so we could try to figure out a solution? If you don't feel comfortable posting here, send a ticket to the support team at support@chaosgroup.com

Cheers !

[galakgorr]

The goal was to be able to transfer potentially millions of instanced objects from Houdini to Maya via Alembic. "Instancing" in Alembic keeps the file size to a more manageable level, though after spending more time looking at the Alembic spec it seems that it's data de-duplication and not actual instancing. My hope was that I could write point attributes like @Cd to the packed primitives and have that transfer along with the Alembic, but it seems even Houdini can't read these attributes back in, so it's likely just out of scope for what Alembic can handle. If there's some other way to vary attributes per-copy without unpacking / deinstancing, I'd love to know about it.