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importExport.md

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Import and Export functionality

Supported Types

  • Mesh
  • NurbsCurve
  • Transforms (always exported)
  • Camera (no special flags needed - will auto-detect cameras and export correctly)

Import

You can either use:

  • the command (AL_usdmaya_ImportCommand)
  • the maya "file" interface

Usage is as below (see Example Usages for Export)

Full set of supported import options are described here

Export

Will export a part of your maya scene into the USD format for selected types

You can either use:

  • the command (AL_usdmaya_ExportCommand)
  • the maya "file" interface

If you want the export to happen from a certain point in the hierarchy then select the node in maya and pass the parameter selected=True, otherwise it will export from the root of the scene.

Full set of supported export options are described here

Example Usage

Using File interface (also available from File->Export menu - File type is "al_usdmaya export"

file -force -options "Dynamic_Attributes=1;Meshes=1;Nurbs_Curves=1;Duplicate_Instances=1;Use_Animal_Schema=1;Merge_Transforms=1;Animation=1;Use_Timeline_Range=1;Frame_Min=1;Frame_Max=1;Filter_Sample=0;" -typ "AL usdmaya export" -es "/var/tmp/vv.usdc";

Command Examples If you want to export everything keeping the time sampled data, you can do so by passing:

AL_usdmaya_ExportCommand -f "<path/to/out/file.usd>" -animation

Exporting attributes that are dynamic attributes can be done by:

AL_usdmaya_ExportCommand -f "<path/to/out/file.usd>" -dynamic

Exporting samples over a framerange can be done a few ways:

AL_usdmaya_ExportCommand -f "<path/to/out/file.usd>" -frameRange 0 24
AL_usdmaya_ExportCommand -f "<path/to/out/file.usd>" -ani

Nurbs curves can be exported by passing the corresponding parameters:

AL_usdmaya_ExportCommand -f "<path/to/out/file.usd>" -nc

The exporter can remove samples that contain the same data for adjacent samples

AL_usdmaya_ExportCommand -f "<path/to/out/file.usd>" -fs

Mesh Export

For meshes normally we export:

  1. Topology and Point Positions
  2. Indexed UVs
  3. Colour Sets (by default RGBA and faceVarying)
  4. Subdivision Edge and Vertex Creases
  5. Dynamic Attributes

An option "meshUV" indicates AL_usdmaya_ExportCommand to only export Indexed UVs and leave other part of USD hierarchy as empty "over" prims. Under this mode, "leftHandedUV" is used to adjust UV indices orientation.

Use Animal Schema The "Use_Animal_Schema" option when exporting was originally intended to export mesh data in a way which was easier for our inhouse Renderer to consume. The salient differences are:

  • Export Indexed Subdiv Crease Data
  • Imports/Exports Subdivision-related Flags used by our Renderer as attributes
  • When exporting Colour Sets, names them prefixed with "alusd_colour_" to avoid clashes with Pixar Schemas, and force to RGBA and per Face

Note: This special schema needs to be revisited, possibly removed/made more generic (@todo Reference Github issue) The colour handling in particular is a bit of a mess at the moment, and is hardcoded to support some very specific use cases. Until we fix all this, might be worth using the Pixar Plugin for mesh import/export

Tech Note: Colours on Geometry

USD has an convention enforced in a Schema related to having a Colour Set on your mesh called "primvars:displayColor" see here. This will be used as a fallback by Hydra and other viewers if there is no other shading information (There is also a matching displayOpacity)

To have geometry colours displayed by Hydra your data needs to look something like one of the 3 meshes in this usd file. Colour Data can be exported:

  1. Per Vert
  2. Per Face
  3. Per Vert per Face (FaceVarying) Not 100% sure that this is either working or supported by Hydra

The exporter will export the data as it is stored in Maya - non-indexed

How do we export in AL_USDMaya so we have colours?

  1. Make sure that one of the colour sets in your mesh is called "displayColor" (there is an example maya file here)
  2. If we don't use the "Animal_Schema" when exporting the names of Colour Sets will be preserved (but exported as faceVarying)

A note on Alembic+Maya+USD

If I export to alembic from maya e.g

AbcExport -j "-root |pSphere1 -writeColorSets -f /var/tmp/colouredVerts.abc";

...where -wcs is "Write all color sets on MFnMeshes as color 3 or color 4 indexed geometry parameters with face varying scope".

Then I re-import I see the colours again in maya - great!

Looking at the alembic with abctree:

:--.childBnds
 :--statistics
 `--pSphere1
     | `--.xform
     `--pSphereShape1
         `--.geom
             :--.selfBnds
             :--P
             :--.faceIndices
             :--.faceCounts
             `--.arbGeomParams
                 `--colorSet1
                     :--.vals
                     `--.indices

shows there is an indexed colour set..

In the current version of the USD ABC Plugin(0.7.6) it looks like these indexed UV sets are correctly converted to Face-varying data...and display properly in the viewport (this was not working in older versions of USD)