Areas covered by "n % 8 == 0" polygons are not drawn

[outlandnish]

On 0.15.0, we have some weird artifacts when multiple geometries on the same layer are rendered and happen to overlap each other.

You can also view it live at https://app.airmap.io and searching for Anchorage with Recreational - 5 mile radius airports turned on.

Any ideas?

[lucaswoj]

Thanks for the report @NSamala. I can confirm the bug and will investigate as soon as I can.

[ansis]

Any spot covered by 0, 8, 16, 24, etc polygons doesn't correctly. This will be fixed by the switch to earcut #1606 . I don't think we can do anything before that to fix this.

---

We currently draw polygons by creating a stencil using the stencil buffer and then. We only use 3 of the bits of the stencil mask for polygon rendering because we use the rest for tile clipping masks. This means that if the number of polygons overlapping a pixels is a multiple of 8 then it will treat it as zero.

Removing the wrap from here doesn't fix the problem because the clamping is applied before the mask.

[ansis]

It looks like the second image may be of a different bug that we can fix pretty easily: #2089

[outlandnish]

Thanks for the quick response! It does look like that earcut branch will solve both of these problems. For that second picture, would it work to ensure our polygons follow the proper winding order for their linear rings as a short term fix?

[ansis]

@NSamala yes, ensuring all your polygons follow the same winding order should fix it. This should be fixed in the next release: #2285