Parallel LineSymbolizer

[radomir.cernoch]

In short

Imagine following situation: A line is assigned 2 line-symbolizers and I need mapnik to paint them not one over another, but one next to the other. For example if a road, which normally looks like

---------------------

is assigned colours 'Yellow' and 'Blue'. With current mapnik capabilities, there is no problem to achieve a result like

---YYYBBB---YYYBBB---

However the desired result is more similar to

YYYYYYYYYYYYYYYYYYYYY
---------------------
BBBBBBBBBBBBBBBBBBBBB

Motivation

In the Czech Repubic we have a large number of tourist trails, which are assigned one of 4 colours (red, blue, green, yellow). In some places, there are two trails going through the same road. The traditional way of painting them is exactly the way described above.

A typical result should look similar to the overlay in: http://www.mapy.cz/#x=134865920@y=138622720@z=12@mm=TTtP

Proposed solution

To add 'parallel_group=X' option to the line symbolizer. If there are at least two symbolizers within the same group X assigned to one line, algorithm for "shifting" symbolizers gets activated and automatically calculates the offset to each symbolizer on that road.

I can help to write the algorithm for calculating offsets, but I know nothing from the internals of mapnik.

Solutions which (probably) will not work

Preprocessing

One way of dealing with such problem might be pre-computing the desired look of the map by shifting the trails a few meters left/right from the physical road.

Unfortunately we need to keep the distance between Red and Blue independent of zoom level and therefore such solution will not work.

Line symbolizer with offset

There is currently a similar ticket, which proposes to add a "shift" value in pixels to a line symbolizer. http://trac.mapnik.org/ticket/180

Nevertheless I think that such solution will bring further problems when applied to the situation described here.

In case of Czech tourist trails, there are 4 colors and the map must consider every possible combination of trails on a road: {R}, {B}, {G}, {Y}, {RB}, {RG}, {RY}, ..., {RBG}, {RBY}, ..., {RBGY}. This gives total of 15 combinations (size of super-set -1), which is still manageable.

However such solution is hardly optimal. Imagine a "T" crossing, where Yellow goes from left to right, but Blue goes from left to down. The nicely looking solution is:

YYYYYYYYYYYYYYY
---------------
BBBBBBB|
      B|
      B|
      B|

However with fixed "shift"s of each color in each combination, the Red and Blue might unnecessarily cross each other:

BBBBBBB
---------------
YYYYYYY|YYYYYYY
      B|
      B|
      B|

There is an ugly solution, which consideres every permutation of every combinations in the superset: {R}, {B}, {G}, {Y}, {RB}, {BR}, {RG}, {GR, ..., {RBGY}, {RBYG}, {RYBG}, ... Only for 4 colours this gives 64 different line symbolizers.

[springmeyer]

Hello Radomir,

Thanks for posting this thoughtful ticket. I agree that your 'proposed solution' has desirable elements and is a good step toward thinking through the pitfalls of handling lines features independently. However allowing for the grouped handling of lines needed for auto-offsetting and smart handling of intersections will require quite a bit of thought.

In the meantime I think that implementing a shortcut like #180 will still be useful, if not to solve this specific usecase, but at least to set the stage for understanding further potential challenges.

I'm going to be working on #180, so please comment on that ticket if you have further thoughts.