This adds the processing required to import and export, simple, material
graphs utilizing the UsdUVTexture Scale and Bias inputs.
Since Blender does not have equivalent inputs on its Image node, a
Multiply-Add node is used instead. This matches the calculation as per
the UsdPreviewSurface spec[1]
A complicating factor here is when these two inputs are used for normal
map textures. The Scale and Bias inputs are authored in such a way to
take the [0, 1] image data and expand into the [-1, 1] tangent space
range as per the spec. However, the Blender Normal Map node expects to
do this transformation itself. The processing in this patch needs to
account for this. For the case of normal maps it will:
- Apply the Scale-Bias calculation directly as authored
- Apply an additional transform to move from [-1, 1] back into [0, 1]
- Feeds this into the Normal Map node
This processing extends to Export as well. During material graph
traversal we need to "skip" the middle adjustment transform and only
export the "real" Scale-Bias data. Traversing the graph like this can be
error prone but is probably the best we can do without having a native
Scale-Bias concept on Blender's Image node.
[1] https://openusd.org/release/spec_usdpreviewsurface.html#texture-reader
Pull Request: https://projects.blender.org/blender/blender/pulls/115224
f355b20262