On Mon, Dec 16, 2013 at 8:41 AM, Vincent Schut <[email protected]> wrote: > On 12/14/13 07:17, Paolo Cavallini 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%. >> >> 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. >> >> However, in my case the general setting "use min/max" does not seem to >> be working. >> Thanks for your thoughts. >> > Imho, a lot could be derived from the image metadata: datatype, number of > bands, 'photographic interpretation', etc. > > - for 3 bands, 8 bit, assume rgb and do not stretch > - for 4 bands, 8 bit, assume rgba and do not stretch > - more bands and/or datatypes >8bit or float: usually means satellite > imagery. I'd say, start with a stretch of 2-98%. Users of satellite imagery > mostly know what to do when their image then still appears black, I think? > - 1 band data, >8bit: probably conitnuous data, possibly a dtm/dem, apply a > stretch, maybe also 2-98 %? Of course all nodata pixels should be excluded > from the 2-98% calculations. > - 1 band data, 8bit: probably sparse or 'class' data: discrete, might have > lots of actual nodata values. A good default strategy might be to apply a > random or default colormap.
Have you tried to run QGIS 2.0? I think that most of what you are suggesting is default and you can change the defaults if you want in: Settings > Options > Rendering > Rasters Missing is identification of discreet data. > Personally, I would refrain from non-linear stretch types as default. When I > open a raster, I do not want too much 'intelligence' to happen. Also, I > usually find the x-y% type of stretching more useful than the -x +y stdev > type of streching, but that might have to do with the datasets I usually > work with. > > Problems of course arise when people/programs do not correctly set nodata > values, etc. (e.g. in the case of rasterizing sparse vectors, all other > pixels should be nodata). I'd say, do not build in too much intelligence to > correct for things like that, in the end it only makes things more > complicated. I also hate programs which "intelligently" do something what I don't want. OTOH I also hate application which are not able to do some very simple task like render a raster so that I can see more than black rectangle. Unfortunately such simple task appears to be quite complicated. In this situation I would prefer first to try to improve style decision instead of resign completely. Radim > My 2 cents. > > Best, > Vincent Schut. > > _______________________________________________ > Qgis-developer mailing list > [email protected] > http://lists.osgeo.org/mailman/listinfo/qgis-developer _______________________________________________ Qgis-developer mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/qgis-developer
