Primvars are not using the Channel Name if is defined

[SttevanCJoga]

Describe the Bug

During the export of an object, the Channel Name Map does not correctly use the custom channel names defined in 3ds Max.

In the example below, channels -2: Alpha and 0: vc are exported correctly and preserve their assigned names. However, the other channels ignore the configured Channel Name Map and are exported using the default USD naming convention (st*).

For example:

• Channel -1: Illum was renamed to LayerWeights
• Channel 3: map was renamed to Apple

After exporting to USD, both custom names are ignored and replaced with the default name or st* names instead of the user-defined channel names.

Expected Behavior

The USD export should preserve and use all channel names defined in the Channel Name Map for every exported channel.

Actual Behavior

Only some channels (such as -2: Alpha and 0: vc) correctly use the assigned names. Other channels ignore the mapping and are exported with default st* names.

Additional Information

Using the MaxUsd Python API, it is possible to manually define replacement names for exported channels. However, this is not a practical workaround in production scenarios where multiple objects in the same scene may use different channel mappings. The exporter should automatically respect the Channel Name Map configured on each object during export if is defined.

# Those are okay, each one respect the channel in the 3dsMax

    export_options.SetChannelPrimvarMapping(-2, "displayOpacity")  # Vertex Alpha
    export_options.SetChannelPrimvarMapping(-1, "LayerWeights")  # Vertex Illum
    export_options.SetChannelPrimvarMapping(0, "displayColor")  # Vertex Color

# Those should be following the Channel Name if is define instead here

    export_options.SetChannelPrimvarMapping(1, "st")  # UV's
    export_options.SetChannelPrimvarMapping(2, "st1")
    export_options.SetChannelPrimvarMapping(3, "st2")
    export_options.SetChannelPrimvarMapping(4, "st3")
    export_options.SetChannelPrimvarMapping(5, "st4")

    export_options.SetChannelPrimvarMapping(6, "Different_Data")
    export_options.SetChannelPrimvarMapping(7, "Different_Data2")

Attachments

Scene 3dsMax :

Name of Primvars are not following the name defined :

Specs (if applicable):

• Windows 11
• 3ds Max 2024 / 2025 / 2026 / 2027
• Max-Usd 0.16.2.31

[deboisj]

Thanks for the report @SttevanCJoga, we did discuss this in the past.

Unless I am mistaken, a problem with doing this, is that USD materials need to specify the primvar to use for UVs.

If we did this and had different objects export different primvars for the same 3dsmax channels, and then those max objects share the same max materials.. we cannot just assign the same exported USD material on the USD side - The USD PrimvarReader UsdShade node would need to point to different UVs depending on the mesh prim it is bound to.

Any thoughts on this problem?

[SttevanCJoga]

Hi @deboisj ,

Maybe I wasn't clear in my original report. My concern is not about UV channels used by materials, but about vertex color / map channel naming during export.

I agree that for UV channels, keeping the st* convention makes sense, especially because USD materials reference specific primvars through PrimvarReader nodes. As you mentioned, if different meshes exported different primvar names for the same Max channel while sharing the same material, that could create binding issues on the USD side.

What I'm referring to is the exporter not respecting the Channel Name Map for channels that are exported as primvars (for example vertex color data). Some channels already preserve their custom names correctly (Alpha, vc), while others ignore the mapping and fall back to the default naming.

So unless I'm misunderstanding how those channels are currently handled, I would expect the exporter to honor the user-defined Channel Name Map for those exported primvars. Is there something I'm missing about how vertex color and map channels are treated differently during USD export?

[deboisj]

In principle none of them would preserve their custom names as primvar names, are you sure you saw this? I see no code reading channel info on export, for any map channel -2 to 99, so including special channels. We do however read channel info in import. but that's a different thing.

But regardless the issue with UVs would be the same with other channels, all map channels are exported as USD primvars, and there are at least some maps i believe that let you choose non-uv channels as source (I.e map channels <1), like the OSL Uber bitmap lets you pick channel 0 for example. Similarly might impact materialX exports.