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Re-support glTF 1.0 premultipliedAlpha #7419

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OmarShehata opened this issue Dec 17, 2018 · 8 comments
Closed

Re-support glTF 1.0 premultipliedAlpha #7419

OmarShehata opened this issue Dec 17, 2018 · 8 comments
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@OmarShehata
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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
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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.

air-premultiplied

air-unpremultiplied

@xtassin
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xtassin commented Feb 12, 2019

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
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xtassin commented Jun 18, 2019

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
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@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
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xtassin commented Jun 20, 2019

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

@xtassin
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xtassin commented Aug 7, 2019

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
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xtassin commented Aug 23, 2019

@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
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j9liu commented Aug 19, 2022

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

@j9liu j9liu closed this as completed Aug 19, 2022
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