Gaps in terrain with fog and small horizon-blend

[rreusser]

mapbox-gl-js version: 2.3.0-beta.1

browser: Chrome

Steps to Trigger Behavior

1. View a location at elevation such that the terrain is slightly above the horizon : http://localhost:9966/debug/fog.html#10.27/29.5925/81.5995/-38/83
2. Set fog range to [-1.3, 4]
3. Set horizon-blend to 0
4. Small gaps may appear in terrain as you pan

Link to Demonstration

Codepen: https://codepen.io/rsreusser/pen/QWpymwo?editors=1010
Debug page: http://localhost:9966/debug/fog.html#10.27/29.5925/81.5995/-38/83

Expected Behavior

Gaps in tile coverage are not visible

Actual Behavior

Gaps in tile coverage are visible

Possibile remedies

• Disable tile culling when horizon-blend is below ~0.04 or so. That represents a probably-undesireably-harsh transition from fog to sky, so this would probably be an acceptable threshold at which to stop culling to avoid these artifacts.
• Disable tile culling if no sky layer is present. Even with a threshold, the above fix won't address the case where there's simply no sky present to catch the fog color. Gaps will be unavoidable unless there's something there to catch the color.

[rreusser]

Edit: I forgot that we render fully opaque fog on a blank background, so the no-sky case is not a problem. Then I think the best solution is to disable culling at some minimum reasonable threshold.