[Qgis-user] nodata and raster calc

[Havard Tveite]

nodata + 1 = nodata
nodata * 1 = nodata
nodata AND 1 = nodata
nodata OR 1 = 1

Håvard

On 11. feb. 2019 03:11, Nyall Dawson wrote:
> Hey all,
> 
> I'm seeking some feedback on whether our current handling of nodata
> pixels in the raster calculator is correct, or (my suspicion) annoying
> and limiting.
> 
> Currently, we treat nodata pixels the same as a "NULL" value in SQL --
> so ANY function or operation with nodata as an input results in a
> nodata output.
> 
> This means:
> 
> - nodata + 1 = nodata
> - nodata * 1 = nodata
> - nodata AND 1 = nodata
> - nodata OR 1 = nodata
> 
> To me this is a mistake. I don't think we should treat nodata the same
> as NULL, and I think for rasters then "nodata OR 1" should return 1.
> 
> The end result of the current behaviour is that ANY nodata input
> blocks processing of that pixel. So you can't do something like take
> two raster files, and create an output showing where either input has
> a 1 value (which I would expect raster1 at 1 OR raster2 at 1 to do).
> 
> Otherwise, we need to have some concept of "coalesce" in the raster
> calc. So then at least you could do:
> 
>      coalesce(raster1 at 1, 0) OR coalesce(raster at 1, 0) ....
> 
> 
> Thoughts?
> 
> Nyall
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