[Qgis-developer] Re: classification of rasters

[Peter Ersts]

I was going to suggest that there be something like hypsometric tints 
anyway so this is very timely. It should not be a problem as it is 
really just adding another block of code and a couple of tweaks to the ui.

-pete

Tim Sutton wrote:
> 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
>>
>
>


-- 
====================================
Peter J. Ersts, Project Specialist
American Museum of Natural History
Center for Biodiversity and Conservation
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Tel: (212)-496-3488 or (212)-769-5742
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