[QGIS-Developer] Processing 3.0: Possible change to the Convex Hull algorithm

Matthias Kuhn matthias at opengis.ch
Thu Jun 29 22:47:05 PDT 2017

On 6/29/17 10:21 AM, G. Allegri wrote:

>     1. Allowing multiple field selection for grouping
>     2. Keeping attributes of first feature when grouping
> I will accept this criteria, obviously, if it's the preferred solution
> for the mosts. 
> I just want to report that many users (partecipants to courses or
> customers) say they find having the "first feature value" misleading. 
> I agree with them. I would set the field values only for the grouping
> fields, having the same value within the group, and set null for the
> other fields.
> The problem raises during a long workflow. At some point you obtain a
> dataset with unconsisten field values, and it's not always obvious to
> know when and.which field value was set miningless.

In the long run the most flexible solution will be to allow specifying
the aggregate function applied to each field individually ("first/any",
"mean", "mode", "max"...).

No worries if it's not implemented right now, but please keep the code
modular enough to introduce this easily later down the road. Without
manually editing each algorithm that supports grouping.

A possible approach to this would be a parameter
"ParamterFieldAggregation" that offers a standard gui to configure the
aggregation behavior (fieldA: min; fieldB: mean; fieldC: any; fieldD:
mean(expression("numBirds2017 - numBirds2000")) ).

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