[GRASS-user] Problem with display resolution / resampling / smoothing

Glynn Clements glynn at gclements.plus.com
Fri Jul 8 00:00:36 EDT 2011


sleepysebi wrote:

> since April I'm working with GRASS and I think by now I got the basics.
> Now I'm working with high resolution satellite data, looking for small
> structures in ice.
> My Rasterimages have got a resolution up to 2.5 meters per pixel, but most
> of the time
> I don't zoom in that far.
> So the Problem is, when zoomed out GRASS doesn't displays the raster the way
> I want it to.
> To me it seems that it's a problem with resampling the image when
> displaying, but I'm not shure if I'm missing something.
> To make it clear I made a little screenshot:
> 
> http://imageshack.us/f/225/resamplecomparison.png/
> 
> So when at the same zoom lvl in EOG, it does some kind of 'smoothing' which
> looks way better.
> 
> I know I can use *r.resamp.interp to resample, using lanczos for example,
> but this is more for the final output, not for working inside GRASS
> (vectorizing,...).

The GRASS library resamples all input maps to the current region using
nearest-neighbour resampling. d.rast then resamples the data to the
display (image or window) size, again using nearest-neighbour
resampling.

If you want optimal output, the display size needs to match the
current region, and the input map needs to be explicitly resampled to
the current region using e.g. r.resamp.interp or r.resamp.filter.

Either that, or render the map at its native resolution, then resample
using an image-processing program.

-- 
Glynn Clements <glynn at gclements.plus.com>


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