Label collision detection preventing labels not shown on the map but according to clamped min-zoom

[dieterdreist]

There's maybe a bug in mapbox-gl-js/v0.42.2 (and maybe earlier versions), where the collission detection for labels prevents the rendering of labels in a lower layer where there is a different layer with the same features above, but the features are not rendered (excluded by float Min-Zoom of 8.8)

Description of the situation:
There is a layer B which selects a subset of the features in Layer A.
Layer A is above Layer B in MB Studio.
Layer A has its zoom limited from z8.8 to z14.
Layer B has its zoom limited from z6.5 to z15.

I've triple checked the zoom-dependent styling, and it has nothing to do with it.

Features from Layer B display from z6.5 to z.8.
Then they disappear and reappear when zoom 8.8 is reached (because now it is Layer A which displays them). I can see that disappearing of features in Layer B is caused by Layer A because the features re-appear in Layer B (in Zoom 8-8.8 and above) when I disable Layer A or disable collision prevention in Layer B. Unfortunately, disabling collision prevention is not an option for me. For now I workaround by setting the min-zoom for the source to 9 rather than 8.8

[dieterdreist]

Looking at it, the problem could be more concisely rephrased like this:
a layer minzoom float is treated as a clamped integer in collision detection when interacting with other layers, but working fine when it comes to visibility in rendering.

The specs state:

minzoom
Optional number between 0 and 24 inclusive.
The minimum zoom level on which the layer gets parsed and appears on.

A number is "A number value, often an integer or floating point (decimal number).", so this is likely a bug.

[eulo]

map.setLayoutProperty('text-layer', 'text-ignore-placement', true);
map.setLayoutProperty('text-layer', 'text-allow-overlap', true);

Fixed this issue for me

[ChrisLoer]

Thanks for the detailed report @dieterdreist -- I'm not sure if this is new, but it's definitely something that would be affected by the recent #5150 PR. Sorry it took me so long to see this issue. Would you be able to put together a JSFiddle or a Codepen that shows the behavior?

I'll try to investigate next week.

/cc @ansis

[ChrisLoer]

I can reproduce this using just two layers with fractional minzooms that draw a single point from a GeoJSON source (so no need to worry about the interaction with Studio, I think).

It looks like the current behavior was introduced with the fix to issue #2929 in v0.40. Before that there was a different problem, which would cause Layer B in the example above to display over all of the zoom range [8,9), and then Layer A would take over at zoom 9. If Layer A and Layer B were styled the same way, you wouldn't notice that the shift from A to B happened at the wrong zoom.

After @asheemmamoowala's fix to support fractional zoom levels, what happens at zoom 8.2 is that the minzoom filter prevents Layer A from drawing, but it doesn't prevent Layer A from being added to the collision index. Layer B is allowed to draw, but it gets collided out by the "phantom" layer A label.

I think this should be a pretty straightforward fix -- just a matter of testing for fractional min/maxzooms at the time we do collision detection.

[ChrisLoer]

We should double check once we have a regression test for this, but it looks like gl-native mostly isn't affected by this problem because the layer->needsRendering() check will filter the layer out of the set of layers that placement is run against.

The one caveat is that there will be a small (<300ms) delay between one layer disappearing and the other starting to appear, and there will be cross-fading between the two layers, which may look like "flicker".

[ChrisLoer]

Fixed in #5974. Thanks again for the report, @dieterdreist! Let us know if there's still a case that causes trouble.