[Qgis-developer] min/max for rasters

Radim Blazek 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
> <2%.

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"
style option.

> 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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