Clamped to ground billboard rendering issues in top-down view

[jonathan-hepp]

What happened?

The latest changes seem to have affected the way billboards are rendered, particularly when clamped to ground and on a top-down view. Once you start zooming in on the map, the billboard starts to flicker, parts of it get cut out and it can even disappear completely in certain cases.
If the camera is tilted in any angle this issue does not happen. As soon as you set the camera to top-down view again the same rendering problems are back.

I understand that setting the vertical origin property to BOTTOM should prevent the clamped to ground billboard from cutting out at the middle so, to make sure that the issue is not related to that, the example on Sandcastle includes verticalOrigin: Cesium.VerticalOrigin.BOTTOM.

This issue also seems to be more prominent on certain maps, therefore the sandcastle is set to use the Google satellite imagery.

Reproduction steps

1. Add an entity with a billboard whose heightReference property is set to Cesium.HeightReference.CLAMP_TO_GROUND
2. As you zoom in on the map, the billboard will start to show rendering issues, cutting out parts of the image or even disappearing entirely
3. Tilt the camera at any angle and the billboard is rendered again without any issues

Sandcastle example

https://sandcastle.cesium.com/#c=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

Environment

Browser: Chrome
CesiumJS Version: 1.137
Operating System: Ubuntu

[jjspace]

Thanks for the report @jonathan-hepp, I can confirm this happening for me too. The weird part is that it doesn't seem to happen consistently with similar camera angles. I had to spin the camera a bit at different places to trigger it and even then I don't know a reliable angle. It almost feels like this is an issue with the fact the globe is a sphere and billboards are angled slightly for different cameras and only sometimes the billboard gets folded under the terrain.

@mzschwartz5 any ideas here?

[mzschwartz5]

Hmm.. this is a weird one. Something almost seems non-deterministic here. Like, if I find a camera angle that causes the issue, then move the camera very slightly so that the billboard is visible again, and then try to move the camera back to the "bad" angle, the issue no longer presents, often.

It also seems like I'm more likely to trigger the issue if I move the camera quickly to a top-down view. If I move very slowly, it happens less often... so now I'm wondering if this issue has more to do with billboard rendering, or with the camera. Like, maybe the camera matrix is becoming degenerate at a perfectly top-down angle...? And the controller tries to prevent that but sometimes fails if you move too fast?

[mzschwartz5]

Or, it could just be as simple as - at a perfect top-down angle, with the billboard clamped to ground, the billboard is angled to be completely flush with the ground, and thus clips with is / z-fighting. (Though I think we do a "nudge" to the billboard to prevent this...)

[jjspace]

if I find a camera angle that causes the issue, then move the camera very slightly so that the billboard is visible again, and then try to move the camera back to the "bad" angle, the issue no longer presents
It also seems like I'm more likely to trigger the issue if I move the camera quickly to a top-down view

This was my experience too. The fast movement to "flip" the billboard down into the ground especially seems like it could be telling. Something in an update is getting skipped so they're not in the same "order"?

[mzschwartz5]

Some of the ideas in this issue maybe be helpful / relevant here

[jonathan-hepp]

Thank you @mzschwartz5 , @jjspace , after reading more on some linked issues it seems like setting disableDepthTestDistance to a sufficiently large number (or just Number.POSITIVE_INFINITY) completely fixes the clipping issue from a top-down view. I updated the Sandcastle to compare:

Of course it isn't a proper fix as this will keep the billboard visible behind terrain if the camera is tilted, but maybe it offers some insight into the root causes. My understanding is that an undefined disableDepthTestDistance used to default to 50000 but this has recently changed?

[mzschwartz5]

If increasing the disableDepthTestDistance is an appropriate workaround for you, that's great. It is just a workaround, though. And it comes with drawbacks - like the label will appear visible even when on the other side of the globe.

The default behavior of that setting has changed recently, but it's more complex than just changing the default value when undefined. It was bugged before, and ended up basically being effectively set to infinity when the camera was up close to the globe. That sometimes worked in favor of users (like in this case) but was problematic in other ways.