[OSM-dev] NOT-saving empty Mapnik tiles
igor.podolskiy at vwi-stuttgart.de
Mon Aug 15 09:08:51 BST 2011
> unfortunately, this line did not work:
>> im.save(buf, 'png256')
> I tried im.tostring() too, did not work either. This is the error message:
> Traceback (most recent call last):
> File "./mapnik_tile.py", line 143, in<module>
> im.save(buf, 'png256')
> Boost.Python.ArgumentError: Python argument types in
> Image.save(Image, instance, str)
> did not match C++ signature:
> save(mapnik::Image32, std::string)
> save(mapnik::Image32, std::string, std::string)
> Do you have an idea what went wrong?
Yes - I've again missed what mapnik.Image can and can't :( The problem
is that mapnik.Image doesn't support saving to a buffer. The tostring()
method return the raw data as a string, not the PNG data, so testing
this for length is useless. You can use that to convert a mapnik.Image
to a PIL image which is much more versatile. I've got it to work now
with the following code:
w, h = 100, 100
mapnik_image = mapnik.Image(w, h)
# <render to the image>
pil_image = PIL.Image.fromstring(
'RGBA', (w, h), mapnik_image.tostring())
buf = StringIO.StringIO()
# PIL supports saving to a buffer
print buf.len # yields 119 on my machine
# check the length, write the image to disk etc.
I don't really know how it behaves performance-wise on your application.
As of mapnik 0.7.1, it seems to me to be the only way to get a
mapnik.Image into a buffer.
BTW, now that you converted the mapnik.Image to a PIL.Image, you can use
all the PIL methods to analyze it in some other way, of course.
Hope that helps
More information about the dev