Tuesday, August 23, 2016

Marble: GSoC 2016 wrap-up

Today is the deadline for submitting the final evaluations for Google Summer of Code 2016, that gives me the opportunity to write a wrap-up post about my project this summer.

About my project

The aim of my GSoC project this summer was to bring fluent rendering to Marble's OSM Vector Tile map theme. The idea for this map theme is to render the vector data taken from the openstreetmap and natural earth databases. These are merged and cut into many many tiles that are stored in .o5m format on a server, and downloaded by Marble. To achieve this in the frst place we needed a tool that will handle this for us. Creating that tool was my main objective.

osm-simplify tool

This little program's purpose is to generate the data used by the OSM Vector Tile map theme. It does many different things regarding map data manipulation, but one of it's most important job is to create the tiles. That means to cut a huge world map to tiny little tiles that can be rendered in Marble.
osm-simplify tool
The tool uses Marble's data handling and parsing which I demonstrated in my previous post. That way you can even use this tool as an example if you want to use Marble for your map data manipulation project, but this tool is far from just an example program.

Tile cutting

More exactly this turned out to be a very resource hungry program. If you look at this table, you can guess why is that:

The hardest part of the tile cutting algorithm was the processing of polygons. This turned out to be a little challenging, but in the end, the solution was to inject another algorithm into Marble's clipping algorithm. The Weiler-Atherton polygon clipping algorithm works on concave polygons too, that solves the borderline issue which I described in my previous post, that comes from the Sutherland-Hodgman polygon clipping algorithm. The tool now uses the more simple Sutherland-Hodgman algorithm for clipping non-polygons, and the Weiler-Atherton kicks in when dealing with polygons.

Here is a nice demonstration of the results of the osm-simplify tool. I loaded 20 separate map files, each is a level 5 tile generated with this tool. I marked the corners of the tiles with red, because they are generated without any gaps between them.

Turning on polygon debugging gives us a nice view of each tile and the underlying cutting process:

To be continued...

I've done a little benchmark to check the performance of the tool, and the results were not that promising. Just for zoom level 9 it needed 6 hours to generate all of the tiles from a single map. The good thing is, there is many ways to improve on the performance. Parallel processing and tile tessellation comes to help. That will be my autumn project, because in the end, Google Summer of Code is about to encouraging students to get involved with open-source projects. 

Final words about GSoC 2016

This year I learned a lot about time management and programming, it was a really great experience. I don't know if I'll have time next year for GSoC, because that will be my last year in university, but I'll just encourage anyone who wants to participate in it, especially for KDE and the Marble team. 

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