[Mapnik-users] Re: mapnik & patents
kevin at tackledesign.com
Thu Mar 9 17:16:00 CET 2006
Sorry - I did a rather poor job explaining what I'm trying do. It's actually
quite simple in principle:
I'd like to create a map feature (say a polygon) that the color varies
depending on the value of one its related data properties but it does so as
a gradient not quantized increments. For example, I'm creating a map that
shows patents/population for a given region and I've stored that value in
the table along with the geometry. I'd then like to set up a range of colors
say a gradient from red to blue that varies across that range of values in
the patents/population field. It's very similar to what you've demonstrated
in your CO2 map example on the website, but rather than having fixed steps
the color varies across a spectrum as the data varies.
This is a fairly common feature in GIS software for visualizing data (I've
used it in ArcGIS, etc) but I realize the underlying complexity involved. I
was just curious if this kind of thing might be possible with the Mapnik
architecture as it stands now.
Otherwise, how many filtered items are feasible? Assuming the gradient isn't
possible I'd like to go ahead a quantize the data value (like in the CO2
example) but perhaps with a few more gradations.
Thanks for your thoughts on this,
On 3/8/06, Artem Pavlenko <artem at pavlenko.uklinux.net> wrote:
> Kevin ,
> > One quick question I do have: is there an easy way to render colors
> > of style as a gradient function of a data property in the GIS data?
> > For example, I'd like to make a gradient of colors based on a density
> > value and have it range between two colors. I realize that's a bit
> > tricky to implement but was wondering if there's a good strategy for
> > doing that sort of thing, aside from creating a set of filters like is
> > done in the Tutorial 2 example.
> No, creating filters for the big ranges won't be a good idea. I need
> some more info to understand what are you trying to achieve exactly.
> Have you got any screens?
> > If that's not possible, as it's designed now, does the symbolizer
> > render get access to the data related to the feature (such as
> > associated data fields in the DB)?
> Yep,it does. What kind of geometries you're trying to symbolize? If
> you have a bunch of points , you could write a specialized
> datasource/featureset that would take vector data and create raster on
> fly (using your density algo)?? I have done something like that in the
> Anyway, would be nice to have this functionality in Mapnik. Give me a
> bit more info and we'll figure something out.
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