[Qgis-developer] Re: classification of rasters

[Marco Hugentobler]

Hi Tim,

Great! I entered a line into the wiki site of the branch and will bother you 
again with more concrete ideas and/or some code in a few days.

Regards,
Marco

Am Montag, 5. März 2007 12:59 schrieb Tim Sutton:
> Hi Marco (cc Peter Ersts & Gary)
>
> 2007/3/5, Marco Hugentobler <marco.hugentobler at karto.baug.ethz.ch>:
> > Hi Tim,
> >
> > For the project I'm working on, I need to have the possibility to colour
> > a raster layer (e.g. a DEM) by building classes and assign a color to
> > each. Afaik there is no possibility to do so in QGIS. Can I implement it
> > in the raster transparency branch (even though it is not related to
> > transparency)?
>
> I think this is fine - any problem with this Peter? I am afraid if you
> do the work outside the branch, we will have difficulty merging
> everything later as we are making some quite big changes to raster -
> so your idea to work in the transparency branch is a good one.
>
> > I wonder what the best way is to implement this behaviour. UMN mapserver
> > preclassifies the expected values in a color table to make lookup faster
> > (http://mapserver.gis.umn.edu/docs/howto/raster_data). But it also seems
> > that their expressions for classifications can be more complicated than
> > simple comparison operations. So I'm probably going to start by
> > evaluating pixel values against a set of classes and see how the
> > performance behaves.
>
> The stuff I wrote for pseudocolour rendering of grayscale  uses a
> similar approach - evaluate each pixel before rendering and decide
> algorithmically what colour to assign it (i.e. no lookup table). Radim
> also did some work in raster on lookup for palettes. Both aproaches
> seem to offer good performance - I think recent (>=QGIS 0.8) slowdowns
> in QGIS rendering relate to canvas changes.
>
> One of the things I am planning to do in tis branch is refactor raster
> into a bunch of smaller dedicated-purpose classes. We are keeping a
> list of our plans here:
>
> http://wiki.qgis.org/qgiswiki/RasterTransparencyBranch
>
> Having raster classification will be a great addition to QGIS btw!
>
> Will be good if you can do the same.
>
> Regards
>
> Tim
>
> > What do you think?
> >
> > Regards,
> > Marco
>
> --