Mapnik v3.1.0 extend

[dch0ph]

A draft attempt to document PR #3512.

Queries:

• expressions: true needs checking. I didn't know how to determine from the code whether expressions would be supported.
• From what I could tell, the extend key is parsed in parse_symbolizer_base, which is used by multiple symbolizers rather than just Line. Is this correct? But presumably it only makes sense to document extend for the line symbolizer?

I would like to have a way to double-check that a given property is present in 3.1.0. I would have expected "extend" to be found when using strings libmapnik.so.3.1.0, but it drew a blank. I tried scripts/validate_mapnik_instance.py, but I don't think this will work unless the python bindings match 3.1.0 (which they didn't seem to).

I do think it would be helpful to retrospectively create an approximate entry in the mapnik CHANGELOG.md to separate out then changes up to 3.1. It looks like everything after 3.0.20 has been lumped under the 4.0.0 entry?

[dch0ph]

@artemp It would useful to get some feedback on the queries above (particularly the query about expressions). I've marked the PR as "draft" while this is unresolved otherwise the added documentation risks being incorrect.

[artemp]

@dch0ph - apologies for taking long to reply. I'll reply properly first thing tomorrow, thanks for your effort!

[artemp]

@dch0ph - Just looking at extend symbolizer property - it was introduced in mapnik/mapnik@6e489a9 (post https://github.com/mapnik/mapnik/tree/v3.1.0)

Implementation in https://github.com/mapnik/mapnik/blob/master/include/mapnik/extend_converter.hpp suggests it can be applied to any geometry not just a line.

@talaj - are you around? could you verify the above statement ^ Also, cool FSM 👍

[artemp]

expressions: true needs checking. I didn't know how to determine from the code whether expressions would be supported.

As far as I can see all Symbolizer properties which are key/value type can be expressions in >= v3.1.0.
https://github.com/mapnik/mapnik/blob/8b70a205c8f751bd880d890148f17bc1cdab8075/include/mapnik/symbolizer_base.hpp#L84-L98

TextSymbolizer is somewhat different. It has inner <Layout>, <Format> elements and it is difficult to follow implementation (https://github.com/mapnik/mapnik/blob/v3.1.0/src/text/text_properties.cpp)

Here's some pointers
https://github.com/mapnik/mapnik/blob/8b70a205c8f751bd880d890148f17bc1cdab8075/src/text/text_properties.cpp#L197-L213

https://github.com/mapnik/mapnik/blob/8b70a205c8f751bd880d890148f17bc1cdab8075/src/text/text_properties.cpp#L260-L274

Looks like most if not all can be expressions also..
Agree we need a reliable way to verify all this .. need to think about it.. 🤔

[artemp]

Looks like this shouldn't be in v3.1.0