Map projections

[bryevdv]

Should add various map projections. The projections should be specified per-glyph I think, or perhaps plot objects can take a default that could be overridden by glyphs.

[ThomasLecocq]

I think this should be left to external modules. I'd suggest creating tight links with cartopy (https://github.com/SciTools/cartopy), as @pelson and the team have made a massive work on that subject.

[bryevdv]

Thomas, absolutely the plan is to leverage a robust and mature projections library and not roll our own. Sorry the ticket was not more clear, but I meant integration, not wheel re-inventing :) We have not done a survey of available modules yet, though, so thanks for the pointer to cartopy. That said we will likely need some support on the js side also.

[ThomasLecocq]

I remember talking to cartopy guys and expressing the need to allow outputs to js-based cartography, probably towards some kind of leafletjs ... I see a nice convergence there !

[pelson]

Thanks for the heads up. FWIW almost everything you see in the cartopy documentation makes use of matplotlib, but in truth it is only an optional dependency on cartopy, and with the right hooks in bokeh would be relatively easy to add map projection capabilities. If it isn't clear from the cartopy docs, the USP is that both raster and vector data are transformed from coordinate reference system to projection, often resulting in lines/polygons gaining cut-points where things like the dateline are involved.

Almost all the other tools I've seen/used to date deals with line transformation by projecting the coordinates in isolation and then constructing a single line from the new coordinates. Inevitably this results in inconsistencies, for example, lines which cross the dateline will result in segments which span the entire width of the map:

Anyway, if you'd like some more info on cartopy, feel free to get in touch and we can discuss in some more detail what is already there and what isn't.

HTH

[bryevdv]

Thanks for chiming in @pelson, cartopy looks pretty fantastic. Transformation of rasterized lines is a great feature. Just to to give you some idea of our roadmap, I am hoping we will get to adding some support for projections around 0.5 which will be early in 2014. I guess I have to ask the question: how would you feel about porting some or all of the code to javascript? :) We are ultimately interested in developing bindings for Bokeh in as many languages as possible, not just python. To that end, it makes sense to push as much into the bokehjs core as possible.

[pelson]

Transformation of rasterized lines is a great feature

Transformation of lines is the key. No rasterization need take place.

how would you feel about porting some or all of the code to javascript?

Nicely popped 😄 - one of the biggest problems with cartopy is performance. I'm hoping to put a considerable amount of effort into optimising and parallelising the line transformation algorithm in the next 4-6 months - even so, we have this implemented (in a non optimal algorithm) in C++ currently as:

• Python was too slow in the interpolation phase
• the underlying geometry library, GEOS, is a C++ library
  In principle, implementing this is JS with performance would be a hard thing to do, but it might be possible. Ultimately, the solution needs to be callable from Python for the matplotlib interface, but the V8 engine could be used for that.

In truth though, I think if you are really looking for a JS engine, it may be that OpenLayers is the easiest path - obviously you wouldn't get the line transformation glory, but there are plenty of excellent and fully fleshed out features which you would get for free.

Keep me up-to-date on your thoughts and progress on this - it is something I'm really interested in seeing.

Cheers,

[bryevdv]

Definitely will keep you up to date! It is also possible to take a layered approach -- expose a basic functionality from js that is alway there whether using python or not, but make use of a more sophisticated system like cartopy when it is available.

As for performance in python, have you had a look at Numba for jitting python into machine code?

[davidgardenier]

Astronomer here! I was curious if there's any current work being done on map projections in bokeh? :) For sky maps, we often need to use projections such as a Mollweide projection which would be a great functionality to see in bokeh, as we are currently forced to use tools such as matplotlib.

[andreas-h]

@bryevdv Any news on the issue of map projections?

[bryevdv]

Closing this, Bokeh is currently being pared down to an essential stable core, so that other tools can build on top of it reliably. The appropriate place for more sophisticated GIS work is GeoViews and other projects on top of a stable Bokeh core.

cc @jbednar @philippjfr @jlstevens