[Qgis-developer] min/max for rasters
radim.blazek at gmail.com
Mon Dec 16 01:30:57 PST 2013
On Sat, Dec 14, 2013 at 7:17 AM, Paolo Cavallini <cavallini at faunalia.it> wrote:
> Il 13/12/2013 20:18, Radim Blazek ha scritto:
>>> Can you describe some examples where 2-98% is a problem (data type,
>>> number of bands, map content, features/phenomena represented by those
>>> 2+2%,...) so that we can think about it better?
> Example #1 (less problematic): dtm and their legend are always shown
> wrong; newbies do not understand why
> Example #2 (more serious): rasterizing sparse vectors (e.g. rivers)
> results in a black rectangle, as the number of pixels with valid data is
We can try to distinguish discrete from continuous data by number of
values, but what will be the threshold? 2, 10, 50, 100...?
> In fact, I think we should help users more, e.g. by applying non linear
> colour scaling (log, exp) in case of very skewed raster values
> distribution: if data are more or less normally distributed, no cut is
> applied, and linear scaling is used; if they are badly skewdw or with
> outliers, apply a non linear colour scaling. With some thinking, this
> should solve most if not all user cases, without asking a normal user to
> understand much about raster stats.
Can you define precisely "more or less normally distributed"?
In general, applying more sophisticated/sensitive decision can catch
correctly more cases but it will become more difficult to be
communicated to a user. Until we hide it under single "intelligent"
> However, in my case the general setting "use min/max" does not seem to
> be working.
Contrast enhancement "Stretch to min/max" is not applied to new raster
layer? For me it works.
> Thanks for your thoughts.
> Paolo Cavallini - www.faunalia.eu
> QGIS & PostGIS courses: http://www.faunalia.eu/training.html
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