KHR_texture_transform shouldn't emulate sampler repeat options

[emackey]

The KHR_texture_transform extension was looking at texture repeat options and trying to factor them into the transformed texture. This is not needed, as WebGL will do the correct thing with sampler repeat options regardless of whether the coordinates have been transformed or not.

Fixes #7916

Alternative solution to #8065

[cesium-concierge]

Thanks for the pull request @emackey!

• ❌ Missing CLA.
  • Please send in a Contributor License Agreement (CLA) and comment back here to let us know to check this!
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• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

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[emackey]

Missing CLA??

[emackey]

Not sure if a unit test is needed. This is strictly a removal of code, there's nothing new to test.

However, I can demonstrate that the clamping and mirroring features work fine even with this code removed:

TexRotateTest_clamp.zip

TexRotateTest_mirror.zip

The original test model shows repeating, which is the 3rd option and the default.

[mramato]

@OmarShehata can you look into why @emackey's CLA is missing?

@lilleyse is this good with you?

[mramato]

Closes #8065

[emackey]

Just for visual reference, this is the original test model from #7916, which doesn't specify a sampler and thus ends up with the default REPEAT behavior. The texture coordinates are axis-aligned, but, the KHR_texture_transform extension introduces a small rotation, producing a slight diagonal edge.

The rotation reveals an area where the texture coordinates begin repeating. revealing block A1 and its neighbors.

With the samplers set to CLAMP_TO_EDGE, the texture rotation reveals an area that appears smeared. This is intentional, from the texture coordinates being clamped along the edges after the rotation has taken place.

Finally, the text model above with samplers set to MIRROR will show some reverse blocks, such as blocks G8 and F8 being shown from mirrored texture coordinates.

The upshot of all this is, Cesium's implementation of the KHR_texture_transform extension was trying to emulate two of these options with shader code, but, the underlying WebGL sampler enums are still in effect from the main model, so it's safe to remove this code without losing the functionality.

[mramato]

This all sounds correct to me, but I'm not qualified to merge. @lilleyse or @likangning93 any thoughts here? Please merge if you are good with this.

[lilleyse]

Yup this looks good to me. I'll merge after the last merge commit passes CI.

[lilleyse]

Thanks @emackey!