Change units of line-dasharray to pixels

[mollymerp]

After discussing with @ansis and @jfirebaugh it seems that the most straight-forward way to make data-driven line-width and line-dasharray possible, would be to change the units of line-dasharray to absolute pixels (from relative units of line-width) because

Dashed lines rely on scaling the dash length by the line width at the previous round zoom level.

This would introduce some complexity in stying lines with line-dasharray because they would need additional zoom functions to stay in sync with line-width to make sure the line-dasharray appears consistent as the map zooms. This seems like a reasonable tradeoff to keeping property function evaluation consistent across all paint properties (we'd need to introduce many exceptions to the current code path if line-dasharray stays a relative value).

Interested to hear thoughts on this! cc @lucaswoj @nickidlugash

Previous discussion on the possible approaches to solving this problem here: https://github.com/mapbox/mapbox-gl-js/issues/3682

[1ec5]

This would also be good for parity with MapKit in the iOS and macOS SDKs. (MapKit’s MKOverlayPathRenderer.lineDashPattern property is measured in screen points.)

[nickidlugash]

@mollymerp sounds like this is the best option.

If we do this, is it possible to also refactor the function behavior of line-dasharray to interpolate at integer zoom levels (similar to the behavior of layout number properties), as opposed to being piecewise? I believe that the reason this wasn't done in the first place is that line dasharrays are rendered like line patterns, so we matched the function behavior of line patterns. However, since we are generating the line-dasharray images in the renderer, would it not be possible to generate a new line-dasharray image at every integer zoom level, based on the interpolated value?

If it is possible to implement this ^, I think it would greatly reduce the burden on designers for matching line-dasharray to line-width, because it would then be possible to match the stops and base values. I think this would also more closely match user expectations for the line-dasharray values (I think it's currently the only number property that is piecewise).

FWIW though, I do personally already write line-dasharray values as functions about 50% of the time, because it is often desirable for the proportions of lines and gaps to change across zoom levels (e.g. it's often desirable for gaps to tighten up as you increase zoom level).

[lucaswoj]

This issue was moved to mapbox/mapbox-gl-js#4158