River label renders on top of bridge

[packi]

Example:

Bridge: http://www.openstreetmap.org/way/8147286
Waterway: http://www.openstreetmap.org/way/27600092

[matthijsmelissen]

How should we solve this? What would be the desired behaviour?

[packi]

Not sure about the how, but I don't think I've ever seen it before (or rather noticed it). In this special case it's a bit awkward because the bridge is very wide (more like a place).

[HolgerJeromin]

Should the waterway be tagged with tunnel? Then it will be same as all other waterways under features.

[matthijsmelissen]

I'm still not sure what the desired behaviour would be, and if there is actually a problem.

[matkoniecz]

River labels should be displaced to be rendered only over river, not over objects on river.

[matthijsmelissen]

It is not easy to specify that an object cannot be rendered over another object, as far as I know. The main tool to accomplish that is render order, but I'm not sure how that can help us here. We also don't want half labels if a river is partially covered.

[Rovastar]

This happens in my local area at certain zoom levels
http://www.openstreetmap.org/#map=16/52.9267/-1.4750
but I cannot think of anything sensible to stop this. I think it would be too complex to implement so I would say close this for now.

[daganzdaanda]

In the case of the screenshot, it might make sense to add a tunnel=yes to the river because it's a big area (would a covered=yes do anything?)
But many bridges get crossed by a river's name, if you look at any city. That's not very elegant, and you can't really add a tunnel in that case.
Up to z14 or 15, I think it's good if a river gets labelled over roads. But at 16+, you see so much detail in a city, and then it looks awkward when a bridge is obscured.

How does displacement work?
It looks as if the river is rendered first, then a bridge on top, then maybe a name of the road, and then the name of the river? I've not seen names of a road and the river colliding -- is there a collision detection going on, or was that just by chance?
ETA: Or is this really a mapnik issue?

[gravitystorm]

The way to fix this is to create "labelling features" by taking the water lines/polygons, and 'subtracting' areas that overlap them (and subtracting areas created by adding buffers around linear features like roads). The same approach can be taken to solve e.g. street labels where lots of railway lines passing over the top, e.g. http://www.openstreetmap.org/#map=18/51.49871/-0.11458

But this is all far too complex a solution, given the size of the problem. Perhaps there is an easier way?

[vincentdephily]

FYI, it happens even for single-way bridges. Just need to be unlucky enough. And it's noticed by users : http://www.openstreetmap.org/note/191799

[matthijsmelissen]

As we currently have no simple way to solve this, I will close this issue.

[vincentdephily]

One easy fix would be to render river labels after rivers but before bridges. It's not great as we'd be obscuring the label (maybe even partially), but it's arguably better than the current situation.

There was an inspiring talk about intelligent label placement at SOTM-EU 2014. Can any of it be applied to osm-carto ?

[alfmarcua]

A partial humble solution to this issue could be to have a phantom label for bridges that displaced river labels. In particular, it could be achieved by giving a label style with opacity 0 to the bridges whenever their own label it is not displayed.