Processing/GeoPackage, Set Z Value, expression based input does not show fields

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Author Name: Richard Duivenvoorde (@rduivenvoorde)
Original Redmine Issue: 17267
Affected QGIS version: master
Redmine category:processing/core

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Having a building geopackage with an attribute 'gemiddelde hoogte' == mean height, I thought to use the 'Set Z Value' (or set M Value) from the Processing Toolbox.

Setting it to a fixed value, like 50.0 works like a charm.

BUT if I want to set the Z value to the value of one of the fields of the feature (as seen in the screenshot) the 'Fields and Values' dropdown just shows NULL.
While if I use the 'Expression Dialog' to show a label, all Fields are shown.

I will attach the dataset (a GeoPackage) too.

OR am I missing a clue, like having a 2d crs while a 3d crs is mandatory?

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• MissingFields.png (Richard Duivenvoorde)
• ExpressionDialogOk.png (Richard Duivenvoorde)
• 3dAmsterdam.gpkg (Richard Duivenvoorde)

[qgib]

Author Name: Nyall Dawson (@nyalldawson)

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It's because that "expression" button is a bit misleading - it's there only for "instant" calculations. So you can use it to do something like "500/3", but this expression is evaluated immediately and the result placed in as the parameter value. Then the algorithm operates using just that static value.

That's why there's no fields available there - it's not calculated per feature, so field values have no meaning.

I think the button needs changing to a "calculator" icon or something to help clarify this.

But, for what you're ACTUALLY after:

While processing in 3.0 has support for "data defined" parameters (e.g. taking the parameter value from a feature based calculation, such as a field value), this hasn't yet been exposed in the ui, and accordingly no algorithms support it yet. Something to tackle for 3.2....

[qgib]

Author Name: Richard Duivenvoorde (@rduivenvoorde)

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Ok, Nyall, thanks for the clarification. I think this is more a UI-thing then?

Not sure if only changing the button to a calculator will fix this. As the dialog is identical, so I think people like me will think they can use is as they use it on other places?
Not sure what a viable solution for this would be though.

Close this one then?

[qgib]

Author Name: Giovanni Manghi (@gioman)

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Close this one then?

I would suggest at least to change the title and description of this ticket. Thanks!

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• status_id was changed from Open to Feedback

[qgib]

Author Name: Richard Duivenvoorde (@rduivenvoorde)

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• assigned_to_id removed Victor Olaya
• status_id was changed from Feedback to Closed