Patrick O'Brian Mapping Project Screensaver

on May 17, 2020, 6:55 am

I have completed a 1.0 version of my Patrick O'Brian screensaver. This software should work (no guarantees) on MacOS 10.12 (Sierra) or later. To install, unzip and then double click Voyages.saver. You will be asked to install the screensaver for "this user" or "all users". Please see the terms of use below (N.B. Satellite imagery is provided by and copyright © MapTiler © OpenStreetMap contributors).

If you aren't interested in the implementation details and would like a copy, you can skip the rest of the post and download the screensaver here:

Download

N.B. I am pretty sure I jumped through all the hoops I need to so that this screensaver can be installed without dire warnings, but please let me know if you aren't able to install it.

I wanted a full screen satellite map with the characters journeys (based on my latitude/longitude data) rendered on top. There are native libraries out there to do that kind of thing, but I thought it would be interesting to try and do some of this myself.

I started out hunting around for the Web Mercator Projection algorithm that GoogleMaps uses. There is a pretty good explanation of this in the Google Maps Javascript API docs. I do wonder though how that Javascript code translates to the projection formulas for x and y. I am missing something there.

Someone had kindly ported that code to Swift so I was able to use that to get latitudes and longitudes successfully projected onto the screen:

Next step was to get hold of a satellite imagery tileset (obviously based on the Mercator Projection) that was free and easy to embed in my native app (no dependancy on an Internet connection for me). Fortunately OpenMapTiles had exactly what I needed. I didn't really need the full 186Gb dataset (although if I had a spare pile of cash lying around it would be fun to play with) so I grabbed the low res tileset.

The download is basically just a SQLLite database file that you embed straight in your app. You can query a particular tile row,column and zoom level (the low res dataset has zoom levels 0-5) and extract the jpg data (the data format info is actually in the db so you can query that) into a byte buffer that you can pass stright into NSImage. Apart from some teething problems with the screensaver binary, this worked pretty well and I was able to get tiles rendering onscreen pretty quickly:

It's not particularly performant and I'm sure I could improve the animation by using GL or Metal directly, but I think it is good enough to warrant moving on to something new.

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 Tom Horn.  All rights reserved.

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