The smallest 256x256 single-color PNG file, and where you've seen it (2015)

Re:The smallest 256x256 single-color PNG file, and where you... by tristanc on Thursday April 21, 2022 @11:17PM (#31117811)

The PNG format already allows grayscale 16bits / channel images. I regularly use this when rendering out depth maps from blender and ffmpeg seems to handle reading from these PNGs just fine (detecting the gray16 pixel format).

However I don’t know of any DEM tile apis that provide these sorts of PNGs but it sounds like a fun project!

Edit: I found this StackExchange post which shows how to generate 16-bit PNGs with gdal https://gis.stackexchange.com/questions/246934/translating-g...

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Re:The smallest 256x256 single-color PNG file, and where you... by tppiotrowski on Friday April 22, 2022 @04:08PM (#31127372)

I hacked up a quick script and converted https://s3.amazonaws.com/elevation-tiles-prod/terrarium/0/0/... to 16-bit (2 channel) PNG.

Any value below sea level I set to 0. Any value above sea level I converted using the following formula:

scaled = elevation * Math.floor[(256 * 256) / (8849 - 0)];

top_byte = scaled / 256;

bottom_byte = scaled % 256;

This provides accuracy to 1/7th of a meter compared to 1/10th of a meter with Mapbox but the tile size went from 104KB -> 25KB. For applications that can ignore elevations below sea level, this is a huge savings.

Edit: the top level tile has a lot of ocean so the size savings are better than average. On a tile with no water, the savings appear to be around 50%.

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