water label placement offset

[sommerluk]

The label for water features seems to be sometimes placed at wrong positions.

Consider the “Lagune Ébrié” at http://www.openstreetmap.org/relation/1982728#map=10/5.2168/-4.2078 which has a label that is displayed far away from the lagoon itself:

I’m quite sure that this paricular lagoon had a well-placed label some monthes ago.

[pnorman]

Could that label be coming from a different object? Not only is it not the centroid of that relation, it does not lie within the convex hull or bounding box of it.

[sommerluk]

I’m not sure.

Just a few observations:

→ Some monthes ago this particular lagoon had a well-placed label (within the water area)
→ This label disappeared some monthes ago.
→ I’ve seen the label in the ocean only a few days ago.
→ I’ve searched (www.openstreetmap.org/search?query=Lagune Ébrié) for the text, and there is only one OSM element in the results
→ I’ve also loaded with JOSM all the area of the ocean around maybe 50 km and didn’t found anything in this sense.
→ The label in the ocean is only visible from z10 to z13. Starting with z14 it disappears.
→ I’ve tried to reproduce this locally with TileMill, the current style and a current data extract of Ivory Coast from Geofabrik. There, I see the label in the ocean from z10 to z11.
→ Within the water area of the lagoon, I didn’t find a label neither at openstreetmap.org nor in my local TileMill rendering.

[matthijsmelissen]

Very strange.

→ I’ve tried to reproduce this locally with TileMill, the current style and a current data extract of Ivory Coast from Geofabrik. There, I see the label in the ocean from z10 to z11.

Especially this is interesting, means it's not an issue with the posgis database at the production server.

[sommerluk]

@pnorman

Could that label be coming from a different object?

No, it doesn’t. I’m now sure. I’ve loaded the hole region (step by step) in JOSM. Then, I’ve selected the lagoon relation and copied it to a new layer. This layer I’ve saved and loaded it in the database. So there was only the lagoon relation and its members in the database; no other elements. Here is the result in TileMill:

[matthijsmelissen]

Weird. Thank you for in-depth testing. Would you also be able to create a minimal code-example that causes this behaviour?

[sommerluk]

A minimal code example is available at https://github.com/sommerluk/openstreetmap-carto/tree/minimcode01 and contains also example data.

I could reduce the code a lot. However, concerning the example data, I could only remove unnecessary tags. When I start to remove “inner” elements of the polygon or when I start to simplify the outer of the polygon, the label placement gets directly better (but still not always within the water area).

[sommerluk]

By the way: Locally still the same behaviour: Starting from z12 I don’t see the label anymore.

[matthijsmelissen]

Great, thank you. Seems to be a Mapnik issue. We'll leave the issue open here, but in the end it needs to be fixed on the side of Mapnik. Their repository is https://github.com/mapnik/mapnik. Could you report it there, or shall I do that?

[sommerluk]

As I don’t much about how Mapnik works (and how it works together with CartoCSS), I would appreciate if you could make the bug report at Mapnik. I think this will lead to a clearer description. I’ll subscribe the bug report anyway …

[sommerluk]

Upstream bug report at mapnik/mapnik#2762

[matthijsmelissen]

Thanks! Sorry, I did not have time to report the bug myself earlier.

[pnorman]

Labeling seems fine under Mapnik 3.

[matthijsmelissen]

Mapnik 3 now has been rolled out. If there are still problems, I'd be happy to hear about them.

[sommerluk]

This issue is not solved with mapnik3. The label of the lagoon is still not within the lagoon. Instead it is within a nearby lake that has no connection to the lagoon:

The screenshot is from openstreetmap.org but I’ve tried it out locally (openSUSE/kosmtik/mapnik3) and the result is the same.

Label placement is “interior”, isn’t it? So this should be within the actual water area (and never on land) even on multipolygons with islands, shouldn’t it?

[matthijsmelissen]

Just to be sure, is this a fresh tile?

[matthijsmelissen]

Re-opening for further investigation.

[sommerluk]

Current rendering. The label “Lagune Ébrié” is displayed over a lake that is not connected at all to the lagoon. The lagoon that should get the label is marked with the red cross.

[Skippern]

Case mentioned in #2613 is not a complex polygon.

[kocio-pl]

Please find ticket discussing this (may be not present and require creating it), rather than post in a related issue.

Just a note: Mapnik is the rendering software. This style is called osm-carto.

👍

In all cases, the nodes with "label" role placed by humans will be always smarter

I don't agree - they could only be fine tuned for a given map. You just can't predict which labels/icons/features will be needed on every map and if this placement will not be in conflict with other desired features. Hence automatic collision solving by Mapnik (I mean rendering library) is a way to go - and we just wait for a proper code to be ported upstream.

Labels and icons on the opposite should be rendered as vector MBTiles by the client with additional benefits according to user preferences: support for multiple languages, support for more accessible font sizes, support for more accessible font colors/contrasts.

I agree with you that vector layers are nice and it can be a final solution, but it's a complex technical and testing challenge: different vector formats and servers (MBTiles is just one of them - see http://paulnorman.ca/blog/2016/11/serving-vector-tiles/ ), a machine for testing it before we can start thinking of switching...

In my opinion it's best to start with language versions layer, because osm-carto can not deal with it anyway (there can be hundreds of name:xx labels) and it would be a big usability enhancement. Could you help with it somehow?

[Klumbumbus]

If I didn't misunderstood the problem was fixed with #1465 (comment). I still have an example of label placement out of the polygon: The label of the forest "Zeisigwald" https://www.openstreetmap.org/relation/229790 is placed at this scrub: https://c.tile.openstreetmap.org/16/35124/21984.png in the east which is not part of the forest multipolygon.

[matthijsmelissen]

Is the new Mapnik already running on the OSMF servers?

[verdy-p]

@kocio-pl the labels are NOT intended to be tagging for rendering, they are suggested optimal placements, and should be the first position to try in renderers, but it does not mean that a renderer can move it around to avoid collisions with other labels. This "optimal" placement is only correct if the label is alone on the map and so there's no collision.

How a renderer will then adjust these positions will be renderer specific. But these labels should be correctly centered in the zone (and not necessarily on the largest subzone if the zone has separate islands, because its optimal placement is sometimes on a small island and not a large dependency): as a renderer CANNOT guess which subarea to label, it MUST be hinted about where to look for a placement. It still has the freedom to move this placement by first looking on other possible nearest places in the same subzone (avoiding if possible to cover inner "holes") then it can allow the label to extend outside the zone (while remaining attached to the initial zone; if the label cannot still be placed, it can place it elsewhere (outside the zone, but not too far away), by drawing a connection line/arrow pointing to the zone.

Labels offer that level of flexibility: they are hints which can be used to improve the placement to be near optimal. Automatic placement from the geometry will frequently be inaccurate because arbitrary choices are made (e.g. using as start position only the central point of a global bounding box, or the central point of the bounding box for the largest subarea, and then it will be randomly be using start positions that could be in a hole or still outside the area.

Hinting the placement with labels avoids most of these arbitrary choices that lead to frequent inaccurate or false placements made by automatic guessers. (Note: labels do not need any tag, they are just used for their geographic coordinates and nothing else, so they don't even need any name, the name to render should always be taken from the relation).

So yes, this talk about "labels" is clearly "on-topic" for this precise subject. it has NEVER been designed to be "tagging for rendering" (not even specific to any one in a specific style like carto-css): your assumption or frequent use of this "tagging for rendering" argument is false here, you clearly don't understand the issue when location of labels are accurate data fully independant of the rendering, and because this location is not specified at all by the outline geometry.

It is a generic instruction/hinting intended for ALL renderers so they can ALL optimize their placement instead of using very fuzzy (and frequently very inaccurate) heuristics based only on the geometry of area outlines. The rule about labels is that its node should be positioned as precisely as possible to the location that gives the name to the whole area, but there's absolutely no need to tag this location, it may be a single node without any tag (only referenced by the relation, possibly several relations, but usually the label should be referenced only once as embedded subareas normally have a name distinguished from the container area; for this reason, labels should not be attached to any other object geometry: not part of any way and not members of other relations).

If you don't want label nodes in the database (or you want to deprecate them), they can be safely replaced by tagging the relation itself with a "label=*" tag specifying coordinates of a node, but this is less easy to edit correctly. In that case there would be no longer any node member with "label" role in relations.

[matkoniecz]

the labels are NOT intended to be tagging for rendering, they are suggested optimal placements

This is contradictory. It is not the worst case of tagging of rendering (deliberately adding wrong data), but adding/changing something solely to change rendering, without adding any new information is also bad and should be avoided.

Further text: tl;dr

[verdy-p]

You've not demonstrated any contradiction. NO. this is not for rendering but for tagging the effective best position of the feature (if it was alone to render, it would be that one). This is really actual data, independant then of how the renderer will finally place it (but still very useful for renderer as it is an important hint against their completely arbitrary and often wrong choice of possible placements)

The label hint also allows a renderer to easily avoid placing dual labels (one for the area, another one for the label node representing it within the area): a frequent anomaly in cartocss renderer: it can eliminate the label drawn for the node and only place the label for the area (at the same suggested position, but possibly slighly moved at minimal distance from that position but still if possible within the area)

Further text: ts;dr (i.e. no argument at all in your comment)

[kocio-pl]

Is the new Mapnik already running on the OSMF servers?

Probably not. Since v3.0.18 has the fix for this problem, but introduces another one (labels can move location when changing zoom level), I would not advise OSMF sysadmins to use this package and wait for v3.0.19 to be released and packaged for Ubuntu, because it would have the fix also for this newer problem.

The good news is that v3.0.19 would also contain grid algorithm for searching best available place for a name label automatically, so we could be smarter than placing "optimal" label points by hand - manual placements can be good just in some cases, but not in general. It's the renderer job to find such place individually in each case. We could try this new algorithm for areas then (I have compilation problems with the pure repo code, so I'm not able to try it before packaging code is ready).

[kocio-pl]

@Klumbumbus The problem is completely external for us, however it's fixed in the Mapnik code and anybody could use this code on their own server, but for the reasons mentioned above it's better to hold on for a moment and not deploy anything less than v3.0.19 on OSMF servers (which are also external from the point of view of osm-carto). This is just unfortunate chain of problems with Mapnik bugs, otherwise it would be solved on OSM.org months ago.

@verdy-p I also think this is mild version of tagging for rendering, but if we could just avoid it, let's not go for half-solutions.

It is a generic instruction/hinting intended for ALL renderers so they can ALL optimize their placement instead of using very fuzzy (and frequently very inaccurate) heuristics based only on the geometry of area outlines.

What do you think might be wrong with this simple algorithm used by grid (see the section "Finding single placement inside a polygon")?

Labels offer that level of flexibility: they are hints which can be used to improve the placement to be near optimal.

Only in some cases, but with a reasonable algorithm they are just not needed, because the optimal placement will be found for any given case. I would also not name a predefined point a flexible solution - it's fixed, but dynamic place searching is flexible.

Automatic placement from the geometry will frequently be inaccurate because arbitrary choices are made (e.g. using as start position only the central point of a global bounding box, or the central point of the bounding box for the largest subarea, and then it will be randomly be using start positions that could be in a hole or still outside the area.

If the grid would use the area outside the polygon or in the hole, I would simply report a bug in the code to be fixed.

[Klumbumbus]

it's better to hold on for a moment and not deploy anything less than v3.0.19 on OSMF servers

OK, thanks.

[verdy-p]

The algorithm by grid is also an heuristic, and even if it is "perfect" geometrically, it is still not the best placement as it will be too far from the designated place, even if it's inside the zone.

You seem to also forget the case of places with exclaves: the largest exclave which is most likely to be found by a "perfect" algorithm (in fact only an heuristic, not a decisive "algorithm") is sometimes not the appropriate one: the smallest exclave is the main place, the largerst one is only a dependency (which has its own local name !

Once again:

• the label position is the hint that allows the "perfect" algorithm to choose the appropriate exclave.
• even if there's no exclave, the "central" position is not always the best position when the label is in fact the name of a place near the border of the area ! The label position suggests the ideal position where the label should be placed, then the algorithm can look for other positions and if there are several candidates it should use the candidate which is the nearest from the ideal.
• if the label can't be placed in the area and is moved slightly so that it now appears outside the area, the renderer will want to trace a ray/arrao connecting the area where the label is drawn with the actual expected location (which is not the center of the area)

Now reread what I said: this is not a position intended for rendering, it is the ideal position where the place should be indicated and the renderer of course can move it elsewhere (possible for text labels, but NOT for placing icons when the area itself cannot be represented because it is too small!).

Label nodes are REAL data. IT does not mean that text labels must be placed there, but that they should not deviate too much away from that ideal position.

[matkoniecz]

To repeat #1465 (comment)

Proposing using label role from relations is offtopic in this closed and already fixed issue.

Please find ticket discussing this (may be not present and creating it may be required), rather than post in a related issue like this one. If it was proposed and already rejected or what you propose was already stated - please do not repeat it or create duplicated issues.

In case of continued offtopic it will be necessary to lock conversation in this issue.

[verdy-p]

Now consider the case of most towns/villages that are municipalities: the ideal position indicated by the "label" node position is generally in the middle of the centre of activity of that municipality but nowhere in the geometric center of the boundaries of the area which is adminsitered: most of these towns/villages have very irregular shapes around for their boundaries. and the label should remain within the central area of where activities in the village/town occurs (this is generally near the location of the townhall, not the building itself but the major crossing or square connecting it to all places in the town village and around which there are most commercial activities and services).

Here again the label position gives the ideal position. Yes it is manually tuned, but this manual tuning should be considered as an important hint by renderers about where they will place the labels and should be even more strongly considered if placing icons, or if the full area cannot be represented with all its details (the text label will then be rendered largely outside the area, but should still cover the ideal location indicated by the label node member, using a best effort heuristic, and if not it should draw a ray to connect the text label to the ideal position).

Note that rays may also be drawn to conenct the same text label with other exclaves (and sometimes it is preferable to rendering the same text label multiple times, because these rays can be partly covered in the middle by other text labels, and because the other terminal position of these rays on exclaves will not need the placement of any additional text label that will collide with other unrelated labels and could fordid them to be rendered separately).

I maintain: label positions are real data, not strict rendering orders, but are important hints for renderers.

Also it is completely ON TOPIC, because for now text labels are far from being ideal and are rendered in wrong positions in frequent cases. With what I indicated, the initial bug shown above (in the initial report as well as all further reports below it) would have NEVER occured !

In addition, these labels allow your renderer to have a measurable quantity to evaluate its placement: any large difference (relative to the current zoom level and the physical area covered by the rendered label) can be reported: it allows your heuristic to be better tuned and improved to better respect the placement hints (sometimes the report will be that the suggested manual label was completely incorrectly placed).

The label finally allows the database to be used by application that only want coordinates of a single point and will zoom on it at a default zoom level where the place should normally be found with its visible label on the rendered map, even if they won't display the full area. Label positions are real data giving the position that people expect to find, independantly of the area they really cover. Such applications include GPS driver's navigators: enter the name of a location and go there: people don't expect to go in the middle of nowhere in the surrounding rural area when the place has an evident "center" (which is not the geometric center), also represented by the ideal "label" node member.

[matkoniecz]

To repeat #1465 (comment) and #1465 (comment)

Proposing using label role from relations is offtopic in this closed and already fixed issue.

Please find ticket discussing this (may be not present and creating it may be required), rather than post in a related issue like this one. If it was proposed and already rejected or what you propose was already stated - please do not repeat it or create duplicated issues.

In case of continued offtopic it will be necessary to lock conversation in this issue. I am not locking now as it is likely that last comment was posted before warning was visible.

[kocio-pl]

I think a proper place to discuss labels placement might be #1391. It's also about particular issue, but the real problem is general and includes discussing dynamic placement versus manual hints.

[dieterdreist]

2018-03-04 23:07 GMT+01:00 Philippe Verdy <[email protected]>:

Now consider the case of most towns/villages that are municipalities: the ideal position indicated by the "label" node position is generally in the middle of the centre of activity of that municipality but nowhere in the geometric center of the boundaries of the area which is adminsitered

The role name "label" is somehow misleading here. There is actually something like a "centre" in towns or villages (usually), and this is what the node role name should refer to (e.g. "centre", "central_point" or similar). By calling it "label" you suggest it is about rendering a label, not about geographic information describing the place. Btw., no need to mention "municipalities", as this is about administration, when discussing villages and towns we are speaking about settlements here.

[kocio-pl]

Another example of a similar problem - it works with 3.0.15 though and OSMF servers still use 3.0.9, which probably uses the same algorithm, so it might be some other reason (metatiles maybe?):

OSMF server:

Kosmtik rendering:

[kocio-pl]

We are waiting now for the deployment of packages on OSMF servers, which is probably the last step - see openstreetmap/chef#155. The package set for Ubuntu Xenial (16.04 LTS) with a fixed Mapnik version is available here:

https://launchpad.net/%7Etalaj/+archive/ubuntu/osm-mapnik/+packages