[Qgis-developer] Bug with floating point gains

Nyall Dawson nyall.dawson at gmail.com
Thu Mar 5 20:24:43 PST 2015


On 5 March 2015 at 03:17, Matthew Hanson
<mhanson at appliedgeosolutions.com> wrote:
> Hello,
>
> I filed a bug report on this some weeks ago:
> http://hub.qgis.org/issues/11573
>
> But it hasn't gotten the required attention.    It's a rather egregious bug,
> which currently is restricting us to QGIS v2.2, which is becoming more of a
> problem the more outdated it gets.
>
> In short, when using a floating point gain with integer images the datatype
> is not promoted, so you end up with an image that is rounded to the nearest
> integer.  It's fairly common practice to store indices, like NDVI, as an
> Int16 with a gain of 0.0001 as it is half the space. But in QGIS, these
> images end up being rounded to mostly 0.
>
> The feature of auto-applying the gain is nice in practice, but without
> promoting the data type of the output it is counter-productive.   It would
> be good to either revert this feature or properly promote the datatype.
>
> I'd be happy to do more digging and try fixing this and issue a PR if
> someone could get me started by pointing me in the general direction of the
> code responsible for this.


This should be fixed in master now - can you please test and confirm?
If so, I'll backport to 2.8.

Nyall


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