Extend mix node to allow per channel mixing

[friedererdmann]

Extending mix node to allow per channel mixing

The mix node's mix input should allow for all data formats as input and do a per element linear interpolation, not just float. Example given:

in1 = Vector2(1,1)
in2 = Vector2(0,0)
mix = Vector2(1,0)
out = Vector2(1,0)

For backwards compatibility float mixing remains supported.

Adding node definitions with the extra suffix of the mix data type like ND_mix_color3_color3.

Note: So far I've only tested in MaterialX Viewer, I'm still working on some tests and will try to see results in other renderers.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: friedererdmann / name: Frieder Erdmann (c0cd1a6)

[jstone-lucasfilm]

This proposal looks good to me, thanks @friedererdmann. I'm CC'ing @dbsmythe and @niklasharrysson for their thoughts.

[niklasharrysson]

Looks good for GLSL and OSL. See attached renders from the testsuite. For MDL there is a small change needed, see above.

mix_nodes.pdf

[niklasharrysson]

Looks good! Thanks @friedererdmann

[dbsmythe]

I've updated the v1.39 Spec document to mention support for per-channel operations:
The Mix node takes two 1-4 channel inputs fg and bg plus a separate 1-channel (float) or N-channel (same type and number of channels as fg and bg) mix input and mixes the fg and bg according to the mix value, either uniformly for a "float" mix type, or per-channel for non-float mix types.

[friedererdmann]

Looks good! Thanks @friedererdmann

Thanks Niklas for your review and feedback! I don't seem to be able to merge myself, will you merge the PR?