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vectorencodinggraphics

What is this vector overlay format?

I'm trying to reverse-engineer the weather maps of the Swiss Weather service because I want to extract information for display on some e-paper displays. You can see such a map here: https://www.meteosuisse.admin.ch/services-et-publications/applications/precipitations.html

The overlay I'm interested in is contained in JSON files with names following the pattern rate_YYYYMMDD_hhmm.json. It contains an array of "areas", each given by an RGB colour and an array of "shapes". So far, so good. However I don't understand the encoding used for those shapes. Here is an example:

{"i":621,"j":120,"d":"NLNLNLNNMOLNKM","o":"01257867","l":0}

I'm guessing i and j are the location of the shape and d and o somehow describe its boundary? I noticed that d.length is always equal to (o.length - 1) * 2, and l seems to be equal to the index of the shape in the area's shape array.

d seems to only use N, M, K, L and O. o seems to use all digits between 0 and 9.

Does anyone recognise this? Is it standard, or at least some format used by some library? The JavaScript is obfuscated so it's hard to figure out where this is being parsed.

Solution

  • Ok it doesn't look like this is a standard encoding. The decoding function is called f in the obfuscated Javascript of the page and works roughly as follows:

    • i and j are coordinates of a one vertex of the polygon
    • each consecutive pair of letters in d indicate in which direction to go for the next vertex. Letters from K to O are mapped to offsets from -2 to +2. For instance the initial NL moves from (621; 120) to (621+1; 120-1)
    • the numbers in o provide additional precision by specifying an offset between 0.05 and 0.95. For some reason the encoding only allows adding such an offset on either the horizontal coordinate (if it is an odd number) or the vertical coordinate (if the horizontal coordinate is odd).
    • finally, a linear transformation is applied to those coordinates, which is then passed to a function called CHtoWGS in the same file, presumably to convert to one of the WGS coordinate systems.