[Qgis-developer] Re: classification of rasters

[Tim Sutton]

Hi Marco

Sounds cool! I'm also in favour of a new tab since I dont like lots of
pop up windows ala arcmap.

Regards

Tim

2007/3/10, Marco Hugentobler <marco.hugentobler at karto.baug.ethz.ch>:
> Hi Tim,
>
> I did some experiments with classifications by evaluating value against a set
> of classes. It slows down rendering a bit, but speed seems still acceptable
> for a first version. I may return and try to enhance speed with color tables
> later.
>
> I'm trying to program a GUI for the classification now. I see two
> possibilities:
>
> A new tab called 'custom color map' in the raster layer properties, containing
> the GUI elements to create a classification.
>
> A button 'custom color map...' under the color map combo box. This would pop
> up the dialog for creating the classification.
>
> I'm more in favour of the first one, as it allows users to immediatly try out
> the symbology by pressing 'apply'. What do you think?
>
>
> 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
> >
> > --
>


-- 
-- 
Tim Sutton

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