Unpaved rendering for turning circles

[sommerluk]

Changes proposed in this pull request:

• Currently, unpaved turning circles are rendered as if they were paved. This PR renders them actually as unpaved.

Test rendering with links to the example places:
https://www.openstreetmap.org/#map=19/6.53649/2.40256

Before

After

[pnorman]

I don't like generating fake lines. Why can't we use a point and crop out an appropriately sized circle from the unpaved pattern?

[imagico]

That would mean cutting about 30 different circular SVGs from the patterns. And you would still get a discontinuity of the pattern rendering at the transit from the turning circle to the linear road.

The other alternative would be using ST_Buffer() + polygon pattern, but that would require the road width progression to be available in SQL. On the other hand, that would allow merging the turning circle and highway area layers and this way could reduce size of roads.mss quite a bit...

[sommerluk]

As @imago explained, using a point geometry

would mean cutting about 30 different circular SVGs from the patterns

Therefore, I think what is proposed in this PR is easier in code and maintenance.

[dch0ph]

It might seem in slightly poor taste to highlight this PR following the recent changes in maintainers, but ...

I don't see a problem in creating a fake line to address a rendering issue - it's not as though the underlying data(base) is being manipulating.

The other alternative would be using ST_Buffer() + polygon pattern, but that would require the road width progression to be available in SQL.

Or is this a more promising long-term direction? I was wondering about how you might render passing places, but this would involve a lot of MSS code duplicating the road width progression, which would be avoided if the road width were evaluated in SQL.

[imagico]

I don't see a problem in creating a fake line to address a rendering issue - it's not as though the underlying data(base) is being manipulating.

I don't think it is a particularly elegant solution but i also don't have principal objections against that.

Background is that rendering line features with a polygon-like 2d fill pattern was a late addition to Mapnik - and it was done selectively for line features, not as a generic feature to be able to use a 2d pattern for anything that would otherwise be shown in a plain color. The need to also allow this for other features (in particular marker symbolizers and SVG symbols) was discussed in mapnik/mapnik#3977 - but never implemented. The choice by @sommerluk for using fake lines is likely based on the realization that this would not happen and we need to use a workaround of some kind.

The three principal solutions for that have been mentioned already:

1. using a very short artificially generated line string and rendering that like the road lines with a 2d pattern and round line caps - somewhat awkward on the code level but still fairly strait away, visual results without issues.
2. script generating circular cutouts from the patterns as SVGs and using those in rendering. This is much more complex to implement and maintain and will be visually inferior because the patterns of road lines and turning circle will not match.
3. using ST_Buffer() + polygon pattern. This is somewhat complex to implement because it requires the road width progressions to be available in SQL. It could still be beneficial as part of a general re-design of the road layers - which i, however, don't see happening as a volunteer project (this would be a multiple months effort requiring in-depth knowledge and it would be unclear if that would be ultimately of benefit under our goals).

Among these option i see the first clearly as the most viable one. This would not rule out moving to the third in case a larger road rendering re-design should be pursued by someone in the future.