[Qgis-developer] raster visualization

[Agustin Lobo]

This procedure would not work well for thematic maps (qualitative variable) but
I think it is reasonable as initial default. Nevertheles, a
qualitative scheme should be
ready for thematic maps (i.e. landcover, vegetation or soil maps). Maybe the
12 classes color table from colorbrewer could be a good first option:
http://colorbrewer2.org/index.php?type=qualitative&scheme=Set3&n=12

Also, median should be used instead of mean and mad instead of sd to calculate
the clipping (at least optionally) as raster layers do have outliers:
mad is just
the median of the absolute value of the differences to the median:

median(abs(x-median(x))

Agus

El día 20 de abril de 2012 14:46, Tim Sutton <lists at linfiniti.com> escribió:
> Hi
>
> On Fri, Apr 20, 2012 at 2:17 PM, Marco Hugentobler
> <marco.hugentobler at sourcepole.ch> wrote:
>> Hi
>>
>>
>>>BTW Freak out will go after resampler branch is merged....
>>
>> I've managed to convert freakout style to singleband pseudocolor in the
>> project file / qml conversion to the new format :-)
>>
>>> - visualizing monoband rasters could be improved:
>>>
>>> - why not loading it with a greyscale by default? or with a colour ramp?
>>> - why not adding a dtm colour table (we can take the one
>>
>>
>> I'd like to improve selecting colorramps in the raster resampler branch
>> (that would even allow us to add a freakout color ramp for trips down memory
>> lane).
>>
>> Default style: I'm not sure what would be a good default, since we don't
>> know the value range at the beginning (at least not without doing a time
>> consuming value scan). Do you have any ideas (maybe a partial raster scan to
>> speed things up?).
>>
>
> Yes - Julien had a really nice suggestion to implement the following logic:
>
> - on initial load of a raster, generate a quicklook that is the larger
> of 1/4 screen resolution or 500x500 pixels by sampling every nth pixel
> - generate a histogram from the quicklook
> - calculate clipped 2% - 96% range min max for each band
> - apply a histogram stretch based on the above
> - the histogram could be used for generating the graph in raster
> props, and the quicklook could be used to create thumbs and previews
> etc
> - ideally we should cache these quicklooks and only regenerate them if
> the underlying dataset has changed
>
> I believe if we do this we will have fast initial load and the images
> (grayscale and rgb) will 'look right' when first loaded (i.e with good
> contrast) and it would not be necessary to assign any color value to
> grayscales by default.
>
> Regards
>
> Tim
>
>
>> Regards,
>> Marco
>>
>>
>> On 20.04.2012 11:10, Tim Sutton wrote:
>>>
>>> Hi
>>>
>>> On Fri, Apr 20, 2012 at 10:47 AM, Giovanni Manghi
>>> <giovanni.manghi at gmail.com>  wrote:
>>>>
>>>> On Fri, 2012-04-20 at 10:19 +0200, Paolo Cavallini wrote:
>>>>>
>>>>> Hi all.
>>>>> Visualizing monoband rasters could be improved:
>>>>> - why not loading it with a greyscale by default? or with a colour ramp?
>>>>> - why not adding a dtm colour table (we can take the one
>>>>> in/usr/lib/grass64/etc/colors/terrain), possibly replacing the Freak out
>>>>> one?
>>>>
>>>> +100
>>>> much needed improvements
>>>>
>>> All good ideas but we are feature frozen so lets first focus on
>>> releasing 1.8 and then we can do these.
>>>
>>> BTW Freak out will go after resampler branch is merged....
>>>
>>> Regards
>>>
>>> Tim
>>>
>>>> -- Giovanni --
>>>>
>>>>
>>>>
>>>> _______________________________________________
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>>>> Qgis-developer at lists.osgeo.org
>>>> http://lists.osgeo.org/mailman/listinfo/qgis-developer
>>>
>>>
>>>
>>
>>
>> --
>> Dr. Marco Hugentobler
>> Sourcepole -  Linux&  Open Source Solutions
>> Churerstrasse 22, CH-8808 Pfäffikon SZ, Switzerland
>> marco.hugentobler at sourcepole.ch http://www.sourcepole.ch
>> Technical Advisor QGIS Project Steering Committee
>>
>>
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>> http://lists.osgeo.org/mailman/listinfo/qgis-developer
>
>
>
> --
> Tim Sutton - QGIS Project Steering Committee Member (Release  Manager)
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