USD Preview Surface material export.

Add `USD Preview Surface From Nodes` export option, to convert a
Principled BSDF material node network to an approximate USD Preview
Surface shader representation. If this option is disabled, the original
material export behavior is maintained, where viewport setting are saved
to the Preview Surface shader.

Also added the following options for texture export.

  - `Export Textures`: If converting Preview Surface, export textures
    referenced by shader nodes to a 'textures' directory which is a
    sibling of the USD file.
  - `Overwrite Textures`: Allow overwriting existing texture files when
    exporting textures (this option is off by default).
  - `Relative Texture Paths`:  Make texture asset paths relative to the
    USD.

The entry point for the new functionality is
`create_usd_preview_surface_material()`, called from
`USDAbstractWriter::ensure_usd_material()`.  The material conversion
currently handles a small subset of Blender shading nodes,
`BSDF_DIFFUSE`, `BSDF_PRINCIPLED`, `TEX_IMAGE` and `UVMAP`.

Texture export is handled by copying texture files from their original
location to a `textures` folder in the same directory as the USD.
In-memory and packed textures are saved directly to the textures folder.

This patch is based, in part, on code in Tangent Animation's USD
exporter branch.

Reviewed By: sybren, HooglyBoogly

Differential Revision: https://developer.blender.org/D13647

1 files changed, 6 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_customdata.h b/source/blender/blenkernel/BKE_customdata.h
index 00eae2e8e6e..76389b0c66f 100644
--- a/source/blender/blenkernel/BKE_customdata.h
+++ b/source/blender/blenkernel/BKE_customdata.h
@@ -437,6 +437,12 @@ int CustomData_get_clone_layer(const struct CustomData *data, int type);
 int CustomData_get_stencil_layer(const struct CustomData *data, int type);
 
 /**
+ * Returns name of the active layer of the given type or NULL
+ * if no such active layer is defined.
+ */
+const char *CustomData_get_active_layer_name(const struct CustomData *data, int type);
+
+/**
  * Copies the data from source to the data element at index in the first layer of type
  * no effect if there is no layer of type.
  */