[OSM-dev] .bpg tiles

[Martin Koppenhoefer]

2015-03-17 12:21 GMT+01:00 andrzej zaborowski <balrogg at gmail.com>:

> 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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