Add allow-overlap and ignore-placement icon/text options

[mourner]

Closes #477

[yhahn]

@mourner want to make a PR for https://github.com/mapbox/mapbox-gl-style-spec as well?

[mourner]

@yhahn already did mapbox/mapbox-gl-style-spec#64

[yhahn]

o

[ansis]

I'm wondering if we could make this one option instead of two. Is there any case where you'd want to allow overlap with the current/previous layers, but not allow overlap with later layers?

[mourner]

Maybe, — that's a question to style designers. cc @nickidlugash
Also, who did introduce those props in carto? He should know cc @tmcw

[tmcw]

Also, who did introduce those props in carto?

Carto's support for this is just a pass-through to Mapnik XML, no real magic or complexity there.

[mourner]

OK, maybe @artemp @springmeyer can weigh in here :)

[ansis]

@ajashton do you have any thoughts on whether allow-overlap and ignore-placement could be combined, or if they are useful to have separate?

[ajashton]

There are use cases where you don't want things within a single layer overlapping, but you want to be able to draw things in subsequent layers on top. One example is big semitransparent labels like you see in Bing or some print maps:

I can also see it being useful for icons-as-texture, like what Saman was experimenting with a while back (tho ignore-placement is not used in this particular style):

[ansis]

awesome, thanks!

[springmeyer]

tl;dr - I agree with @ansis - seems wise to start with just allow-overlap and hide the complexity of ignore-placement until absolutely needed. Although @AJ or @nickidlugash overrule here.

The longer winded version is: Having them as separate options is important only in the rare case where you want icons to overlap themselves but you do not want text or icons from other layers (rendered after) to overlap. E.g. marker-allow-overlap:true; marker-ignore-placement:false; This was added to Mapnik at the request of the OSM stylesheet designers (mapnik/mapnik#564).

The sad part of this design (having two options) in TileMill has been that users often set allow-overlap:true but do not know about the obscure ignore-placement and hit performance problems rendering millions of markers. This is because the ignore-placement value does not change automatically (if unset) depending on your allow-overlap setting and Mapnik then ends up collecting bbox's for collision even if the style does not need to care about collision with features rendered later. To lessen this problem we basically default to recommending ignore-placement:true in TileMill latest: https://github.com/mapbox/tilemill/commit/d612c073d8dbbd23ff77924bb9fde5990887284e, which just feels duplicative.

Also, I just did a quick grep of tm-custom-styles and see a ton of cases where the intention is for them both to be true, so keeping as one option seems wise:

~/projects/tm2-custom-styles[master]$ grep -r 'marker-allow-overlap: true; marker-ignore-placement: true' ./ | wc -l
     194

[springmeyer]

ha. @AJ's comment just appeared once I posted (had this open for a while stewing).