[Tilecache] Customize TileCache get image routine

[Linda Rawson]

Thanks for the information.

*>>>>1.  Check if the image already exists in the cache.

>>>>>I believe this is handled by TileCache at the 'Service' level,
depending
>>>>>on what you mean by 'cache'.
*
Basically from the class MyDGLayer(Layer):

I only want to "question" the service if I don't have the image cached on my
disk already.  What method would I call to check that?  They don't have one
call to question and retrieve.  Basically you call one url to ask if they
have an image then I will call the WMS service if they do.  Their WMS
service returns an image, right or wrong, always.  So I could eliminate two
hits on their server if I knew I had the image on disk already.

Thanks in advance,
Linda


On 10/15/07, Christopher Schmidt <crschmidt at metacarta.com> wrote:
>
> On Mon, Oct 15, 2007 at 09:49:19AM -0600, Linda Rawson wrote:
> > Basically I need to do the following:
> >
> > 1.  Check if the image already exists in the cache.
>
> I believe this is handled by TileCache at the 'Service' level, depending
> on what you mean by 'cache'.
>
> > 2.  If not then check for the image with the provider by utilizing a url
> > that returns a value.
> > 3.  Check the value and If still no image then try another WMS service
> > provider.
> > 4.  If still no image then return a noimage tile.
>
> Right. You'll need to add something like:
>
> class MyDGLayer(Layer):
>    def renderTile(self: tile):
>        bbox = tile.bbox()
>        u = urllib.urlopen('http://digitalglobe.com/tile?bbox=%s' % bbox)
>        tile.data = u.read()
>        if not tile.data:
>            u = urllib.urlopen('http://wms.example.com/tile?bbox=%s' %
> bbox)
>            tile.data = u.read()
>        if not tile.data:
>            tile.data = open("/my/local/no_image.png").read()
>        return tile.data
>
> > Can you direct me to the best python resource for these things?
>
> You'll probably just want to look at basic Python documentation for
> 'loading data from remote servers' or 'loading data over http' or
> something like that. I never looked at any resources other than Google
> for learning Python, so I can't help specifically.
>
> Sticking that at the end of the Layer.py file, and then creating a:
>
> [digitalglobe]
> type=MyDGLayer
>
> Should get you started.
>
> Regards,
> --
> Christopher Schmidt
> MetaCarta
>
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