postgis question/project: create tiles for the complement(!) of polygons

[Brendan Farrell]

Dear CUGOS,

I've had an interesting technical challenge come up: I need to create vector tiles from postgresql that show both the polygons of interest and their complement. The end result would be a web map that shows the polygons with their data available for display, and then there also be a display for their complement, i.e. the area for which there are no polygons. In that area we will assign a field that essentially says there is no data available there. I need to do this through postgis rather than precomputing tiles.

Does anybody want to help me figure this one out? I think it might make a nice blog post if we do. I've pretty much got to figure it out this week, so please let me know quickly if you're interested or have ideas. 

Brendan

[tony.ca...@gmail.com]

Brendan - You can accomplish this relatively simply via a combination of `ST_TileEnvelope`, `ST_Difference`, `ST_AsMVTGeom` (convert geometries to tile coordinates), and `ST_AsMVT` (to produce MVT-encoded data). 

Best of luck - 
tony

[Brendan Farrell]

Tony,

I don't think it's so simple. The general idea would be to use those three functions, but the details are a bit tricky. For example, for tiles that do not intersect any polygons, one wants to show the whole tile. But I usually include a where clause in my queries for where the data intersects the tiles and find that that is faster than only having the cut out function in ST_AsMVTGeom. 

Would you like to work on this together? It's not a huge project, but I think it needs a little careful work and I at least think it's a little bit interesting.

Brendan

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