Fix #996 by adding intermittent water bodies support

[Penegal]

This adds support for rendering intermittent water bodies, hence it fixes #996.

Rendering examples:
https://www.openstreetmap.org/way/391188151

https://www.openstreetmap.org/way/394053640

https://www.openstreetmap.org/way/466317471

https://www.openstreetmap.org/way/507366289

[kocio-pl]

Here are the keywords for automatically closing tickets:

https://help.github.com/articles/closing-issues-using-keywords/

Could you post some images with the rendering of the actual code from this PR?

[Penegal]

@kocio-pl: Edited.

[kocio-pl]

Thanks! The links to rendered locations are useful for testing - could you post them too?

It looks great and intuitive for me, some more examples:

Intermittent pond in the forest near the river

Intermittent water in the small park

Seasonal pond with a name in the park

[Penegal]

@kocio-pl: Edited.

[kocio-pl]

South Australia example on z6 - I don't know if the tagging is complete, but at least some of the biggest lakes there are clearly intermittent:

Whole continent view on z5 (click to see the full size):

[sommerluk]

Given that the pattern is quite simple and maybe pixel-aligned, is there a chance to have this as SVG pattern avoiding #2750 problems?

[kocio-pl]

Currently we're not sure why such problems exist at all and how to fix/avoid them with vector patterns (looks to me like a Mapnik issue), so raster version is the only safe bet for now.

[Penegal]

What does miss for this to be merged? Approval? Testing?

[kocio-pl]

I just wait one day more to give a chance for anybody to post additional comments/improvements, because it's now more visible than just an old issue, but that's all.

[Penegal]

OK, just wanted to be sure.

[kocio-pl]

Thanks for the work! I like the result, especially because it tunes a look on low zoom (probably not only in Australia).

[turnsole80]

I've updated the tagging on some of the lakes in Australia, so it should look even better now :)

[kocio-pl]

Thanks a lot! I've been thinking lately about asking the Aussie community about it.

There can be similar examples across the globe, if anybody knows one, please tag them too.

[Gazer75]

There is a small issue with this. It will not display over a dam polygon.
https://www.openstreetmap.org/relation/8248594

[kocio-pl]

I think the water just should not be rendered over dam, no matter if intermittent or not.

[Gazer75]

Why not? When the doors are open the water flows over in that area.

[kocio-pl]

The main reason is that there are so much combinations with layers and they are expensive (in terms of performance and maintenance), that we must have a strong reasons to add/split more of them. And it would mean that intermittent water should have its own layer. And that would be always above the whole class of things, not only dams...

We would need some generic solution like global render layers - that would mean that layer=1 is always above layer=-1. Yet we have only per-layer rendering priority (road with layer=1 over road with layer=-1) not the global one (building with layer=1 over road with layer=-1). I'm not even sure if this is doable with our framework, but even if it is, it would be huge code rewrite probably.

For a notable example of layering problem without global rendering layers see #688 (comment).