[OSM-dev] PNG type for Mapnik map
falstaff at deheime.ch
Tue May 27 16:58:54 BST 2008
I found this E-Mail OSM-dev history:
> > I tried it out and the png256 stuff in Mapnik doesn't do the right
> > thing. You'll end up with tiles not fitting together because the
> > colors
> > as slightly off. The change is large enough to see in some places.
> Yes, I had the same experience. Not very often, but there'll be some
> tiles that don't match (background colour usually)
> > Imagemagick convert seems to better with this. The algorithm it
> > uses is
> > described on http://www.imagemagick.org/script/quantize.php . But I
> > could imagine that there are pathological cases where it also breaks.
> Yes, as with any quantization. AFAIK ImageMagick is using some kind
> of octree algorithms and so is Mapnik.
> Thanks for the link and I'll check it out.
> > I haven't looked into the algorithm that Mapnik uses, but maybe it can
> > be improved by making sure that all colors mentioned in the Map file,
> > i.e. all colors that we explicitly asked for, are retained. That way
> > only the "artificial" colors created by the antialiasing will be
> > changed.
> > This should make sure that tiles will always fit together because all
> > the larger colored areas will use the same colors.
> The above is pretty much what I'm thinking of trying. We can collect
> colours distribution from *.xml file.
> It would easy for solid strokes and fills and not so easy for image
> based symbols, which might have thousands of colours.
> I guess we can quickly quantize them as well. Also, we can just have
> a predefined set of colours, might work even better. There is
> probably some kind of cartographic rule - good map should have no
> more then 24 different prime colours or something.
> Another thing I'd like to investigate is using different colour
> spaces. I had more correct (visually on macbook) palette while
> quantizing in YCbBr.
> Anyways, could you send me those tiles, where colours don't much,
> please ? With original RGBA versions if possible.
Im using now Mapnik 0.5.1 and trying to work without Imagemagick. I
still have this different colours described above. The difference is
very small, but its not just different between Imagemagick/Mapnik, with
Mapnik there are also differences between area with roads and emtpy areas...
I found nothing on Mapnik Trac about this. Is there a solution on the
way? I've here some tiles made with Imagemagick and the same tile made
with Mapnik itself, if needed...
Tel. +41 41 340 08 56
falstaff at deheime.ch
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