Hello everyone,
I’m running into a persistent issue where I’m unable to read custom user attributes in V-Ray for Maya from an Alembic file exported from Houdini. I’ve followed all the standard procedures without success and would appreciate some help in determining if this is a known bug or a step I might be missing.
Software Versions:
- Houdini: 20.0.547
- Maya: 2026
- V-Ray: v7.10.00
Goal
My goal is to use an integer ID attribute (select_id) created in Houdini to drive a VRaySwitchMtl in Maya.
Workflow
I’ve set up a very simple test scene to isolate the problem:
1. In Houdini:
- I created two separate spheres.
- For each sphere, I created two primitive attributes:
- An integer attribute select_id with a unique value for each sphere ( i@select_id = 1; and i@select_id = 2; ).
- A string attribute path to build the scene hierarchy ( s@path = “/geo/sphere_A/sphere_A_Shape”; and s@path = “/geo/sphere_B/sphere_B_Shape”; ).
- I’m using a ROP Alembic Output node with the following settings:
- In the Hierarchy tab, Build Hierarchy From Attribute is enabled and set to use the path attribute.
- In the Geometry tab, select_id is added to the Primitive Attributes export list.
2. In Maya:
- I import the Alembic file. The hierarchy is correctly created, with two separate objects: sphere_A and sphere_B.
- By selecting each object’s Shape node, I can see the Select Id attribute listed in the Extra Attributes section of the Attribute Editor. This confirms the attribute is present in the file.
3. V-Ray Shader Setup:
- I create a VRaySwitchMtl with two different materials.
- I create a VRayUserInteger node and connect its output to the “Material Switch” input of the SwitchMtl.
- In the VRayUserInteger node, I set the User attribute name to the exact, case-sensitive name: select_id.
The Problem
At render time, both spheres render with the material connected to the default value (0) of the VRayUserInteger. This indicates that V-Ray is failing to read the select_id attribute value from the geometry.
Troubleshooting Steps Already Taken
I have already tried the following, without success:
- Attribute Promotion: I promoted the select_id attribute from Primitive to Vertex in Houdini before exporting. The issue remains the same.
- Testing with VRayUserColor: I replaced select_id with the standard Cd (color) attribute in Houdini and tried to read it with a VRayUserColor node plugged into the diffuse color. The result was the same; the attribute was not read.
- Manual V-Ray Attributes: I manually added String attributes to the transform nodes in Maya, named vrayUserAttr_select_id with values “1” and “2”. Even after updating the VRayUserInteger to use this new attribute name, the render still failed.
It appears my V-Ray setup is fundamentally unable to read any user attribute data from my Alembic files. Is this a known issue, or am I missing a specific V-Ray setting required for this workflow?
I can provide the simplified test scene if needed.
Thank you in advance for your help.
Thomas.
