GroundPrimitive materials and Polylines on Terrain not available when classifying 3D Tilesets

[likangning93]

GroundPrimitive materials aren't available for classifying 3D Tilesets because the depth information for that isn't available in the GroundPrimitive pass. There's also the possibility that shadow volumes might not be tall enough, but in practice I think that's a non-issue at the moment because our shadow volumes are taller than they need to be.

I think I have support hacked up here: https://github.com/likangning93/cesium/tree/groundPrimitiveMaterials3DTiles

But it needs consideration by someone who knows more about the overall renderer.

[mramato]

Do we probably document this lack of functionality in CHANGES (where it was introduced) and in any related API dealing with it?

[likangning93]

@mramato it's documented

[likangning93]

Polylines on terrain on 3D Tilesets

The polylines on terrain algorithm currently in #6615 depends on the depth buffer, so that's a similar implementation hurdle for getting Polylines on Terrain to work on 3D Tiles.

Bigger problem though is that the geometry generation for Polylines on Terrain makes a lot of 2.5D assumptions, and won't behave as-expected for, say, drawing lines on the sides of buildings or the undersides of overpasses. There's a similar problem for GroundPrimitives except that you can use ClassificationPrimitives without materials to do this sort of thing for most use cases.

[ggetz]

Brought up on the forum: https://groups.google.com/forum/#!topic/cesium-dev/2h_uVa6JHAM

[OmarShehata]

Just came up again on the forum as a way to label parts of 3D Tiles.

[cesium-concierge]

Congratulations on closing the issue! I found these Cesium forum links in the comments above:

https://groups.google.com/forum/#!topic/cesium-dev/2h_uVa6JHAM
https://groups.google.com/d/msg/cesium-dev/HsC4mRtcm8E/O717hmi1BgAJ

If this issue affects any of these threads, please post a comment like the following:

The issue at #6636 has just been closed and may resolve your issue. Look for the change in the next stable release of Cesium or get it now in the master branch on GitHub https://github.com/AnalyticalGraphicsInc/cesium.

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