[QGIS-Developer] nodata and raster calc

[Tim Sutton]

Hi


> On 11 Feb 2019, at 04:11, Nyall Dawson <nyall.dawson at gmail.com> 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


Yes this was by design. IIRC Marco Hugentobler implemented the raster calculator under contract to me a long time ago and we discussed this use case in particular. Our view was that nodal acts as a mask. To work around the case where you want to treat nodata as zero you would need to reclassify the raster to recode null to zero and then perform your calculation. To support your use case couldn’t we just add a zero_if_null(raster) function that you can wrap around layer references? Or use your coalesce idea below - though maybe zero_if_null will be more intuitive?

Regards

Tim


> 
> 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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—








Tim Sutton

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Ex Project chair: QGIS.org

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