2015-03-17 12:21 GMT+01:00 andrzej zaborowski <[email protected]>: > Hi, > > BPG is an image format by Fabrice Bellard that was in the news a few > months ago as "the JPEG replacement". Its lossy compression mode > seems to work well for OSM tiles because the artifacts are of a > different type than in JPEG. Here are some results for a small test > area for different compression ratios (percentages of size saved > against PNG). > >
Thank you for pointing to this. > 51% at default level (-q 28) -- I could see no artifacts in the test area > > 62% at -q 32 -- minor deterioration can be seen, you can browse a > small area at c.tile.openstreetmap.pl/viewer.bpg.xhtml > I can see some aliasing e.g. in the casing, but am not sure if this is from the rendering or the compression, it would be useful to have a direct comparison (i.e. the same tiles in png and bpg and maybe also jpeg). This is announced as jpeg replacement, so I guess lots of colours are not a problem? When using sat-images, hillshading or other kinds of gradients (e.g. from blurring or rastersymbolizer) you will get a lot of shades and png will compress worse compared to the relatively few colours you get for instance with the standard style. Generally I think loosing image quality but saving 50% of space is likely not a tradeoff that OSM wants to accept, or the tiles would already be compressed in jpeg and not png. cheers, Martin
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