[QGIS-Developer] What to do about incorrect SAGA algorithms?

[Paolo Cavallini]

Hi Nyall,

On 04/03/19 12:11, Nyall Dawson wrote:
> Hi list,
> 
> Just raising the discussion about what we should do with SAGA
> Processing algorithms when the results generated by SAGA are known to
> be incorrect (i.e. there's a bug in SAGA itself).
> 
> Specifically, the SAGA difference and symmetric difference algorithms
> generate incorrect results. See https://issues.qgis.org/issues/21354,
> and the results from
> https://github.com/qgis/QGIS/blob/master/python/plugins/processing/tests/testdata/saga_algorithm_tests.yaml#L318
> . This seems to be a general problem with the algorithms, not isolated
> to particular input data.
> 
> My gut feeling is: we should "deprecate" these algorithms (which means
> they continue to work for existing models, but are hidden from the UI
> and cannot be run from the toolbox or added to new models). I don't
> see the value in exposing broken algorithms when we have robust
> alternatives (the native QGIS difference/symmetric difference
> algorithms are very fast, stable, and well tested), and potentially a
> lot of harm.
> 
> Thoughts?

thanks for raising this. I fully agree, and will push things a bit
further: IMHO we should hide all algs from other backends when we have a
robust alternative. This means qgis core alg should:
* cover the full range of options available in the alternative command
(often e.g. GRASS has a number of options that can be useful in special
cases)
* be robust even for huge data sets (e.g. GRASS is usually conservative
in the use of memory, and can complete analyses even when other algs fail)

* life is not perfect, so I think a power user should have the
possibility of using other algs, in case we overlooked a special option,
on temporarily if our alg is broken.

I therefore suggest to:
1. have a list of the potentially duplicated algs
2. find one or more volunteers checking thoroughly the above
3. hiding those who correspond to the criteria
4. design a last resort way of using hidden algs.
I'm available for 2.
All the best.
-- 
Paolo Cavallini - www.faunalia.eu
QGIS.ORG Chair:
http://planet.qgis.org/planet/user/28/tag/qgis%20board/