Re-support glTF 1.0 premultipliedAlpha

[OmarShehata]

Cesium used to support the glTF 1.0 premultipliedAlpha property. This was removed in 1.50 when we we updated the glTF pipeline:

#6805 (comment)

This would only work when OIT is disabled. See forum thread:

https://groups.google.com/d/msg/cesium-dev/6dCttPKnJWw/vxCNm2jEBAAJ

This breaking change was not mentioned in CHANGES.md, which made this much harder to find. It might be worth adding a note about it there too.

[lilleyse]

This affects some of our glTF 1.0 sample models. The propellers should be the lighter version, which I got only after editing the shader to not premultiply.

[xtassin]

Using Cesium > 1.50 on iOS 12.x appears to always use premultipliedAlpha, no matter what value is specified in the GLTF model. On the other hand, Chrome on Windows 10 never uses premultipliedAlpha. This is a problem as it is impossible to use the same models and shaders on all platforms.

[xtassin]

This is still an issue as of June 2019. Any chance of a fix coming soon or perhaps someone could give me a pointer on where to look so I could attempt to fix it myself?

[OmarShehata]

@xtassin there was a recent fix where Chrome on Windows was incorrectly setting premultiplyAlpha to true, this was turned off in this PR: #7804

I imagine this is what's causing:

On the other hand, Chrome on Windows 10 never uses premultipliedAlpha.

Based on the discussion here: #7795 (comment) it sounds like always set premultiply to false was the intended outcome, so the fact that iOS is using premultiply is the problem here?

[xtassin]

@OmarShehata yes, it appears iOS always requires premultiplied alpha. I am testing with an (old) iPad Air 2 with iOS 12.1.1

[xtassin]

I would like to give an update: I have received screenshot from users on desktop where this problem is visible (wrong handling of pre-multiplied alpha). This would indicate that the issue may also be GPU/OS dependent. I will try to gather more details.

[xtassin]

@OmarShehata, disabling orderIndependentTranslucency forces Cesium to use pre-multiplied alpha correctly on all platform. However, it creates other rendering issues that make it un-usable as a workaround in production.

[j9liu]

I'm closing this issue, since as of 1.97 we're only providing limited support for glTF 1.0.