[Qgis-developer] Re: classification of rasters

[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
>


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