I have a 1 band DEM geotiff, and a formula to convert altitude -> RGB and RGB -> altitude (something like this: https://docs.mapbox.com/help/troubleshooting/access-elevation-data).
Using the formula (and GDAL/Python), I converted my geotiff to a 3 bands (R, G & B) geotiff, each band having values in the 0-255 range.
Using mapnik / mod_tile, I'm then serving my geotiff as PNG tiles to a web client. Everything is fine if I setup mod_tile to serve the tiles as 24 or 32 bits PNGs. But if I serve them as 8 bits PNGs (to reduce their size), then the decoded values are a bit off (I can't see the difference when looking at the image, but the RGB values are not exactly the same and it thus messes my decoded altitudes).
Am I right expecting to be able to do what I want (retrieving the exact RGB values) with 8 bits PNGs instead of 24/32, or there something I don't understand about 8 bits PNGs (if so, I'll have to dive into mod_tile code, I guess that when we ask 8 bits, it generates 24 or 32 and then compress)?
No, you are not right in expecting that you can compress any ensemble of 24-bit values losslessly to 8-bit values. If there are more than 256 different 24-bit values in the original, then some of those different values will necessarily map to the same 8-bit value.