Generate tangents

[david-lively]

Description

When tangents are missing but required (specifically, when the tileset is lit, or has a normal map but no tangents), they will be generated on the CPU when the vertex buffer is prepared. An option alwaysIncludeTangents is added to Cesium3DTileset to force tangent generation.

Note: requires changes from this cesium-native branch: https://github.com/CesiumGS/cesium-native/tree/generate-tangents

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

Remaining Tasks

Testing plan

[j9liu]

Just to clarify @david-lively, is this meant to build on #596 or replace it? Or is it entirely separate?

[david-lively]

@j9liu This replaces 596.

[j9liu]

Thanks @david-lively, can you close #596 to avoid confusion?

[j9liu]

Thanks @david-lively ! I still have to test this out locally, but I have some suggestions after taking a pass over the code.

[j9liu]

I tried loading the NormalTangentTest sample model on this branch, but it looks off:

Here's the simple tileset I made to wrap around it; you can use a georeference with True Origin to more easily visualize it.

NormalTangentTest.zip

[j9liu]

Let me do a clean rebuild and test again with the above. It's possible that the native part got messed up somehow while jumping branches

[j9liu]

Sadly still seeing the same behavior after a clean build. I also tried this with the NormalTangentMirrorTest model. It's kind of better, but still a bit wonky...

[kring]

Moving this out of this month's release because it's not quite ready yet.