[Qgis-developer] Data-defined symbol size is not backwards compatible to 2.8.2

[Régis Haubourg]

Hi, 
here are the ideas behind this work, Nyall (code reviewer) and Vincent
(author) could explain implentation choices more than me (funder):

- get a more consistent UI with data defined widgets, and not "advanced
fields". That way, size is in one place only.
- offer an assistant on size varying common expression. You will find it at
the bottom of the drop down widget. It computes max value from field or
expression and allows normal user to do what other GIS do.
- That assistant offer legend previsualisation, and generates a legend for
map and composer. I wish we have a legend for any expression...
- During implementation, we understood that symbol size was a multiplication
factor of size varying factor. That implied that it was impossible to
predict final screen size. The new implementation clears that up. 
- offer a size varying graduated renderer, allowing the use of
classifications algorithms
- offer a legend for diagrams (yes!)

IMHO, we need to read previous versions correctly, but that sometimes need
to read all features to retroengineer a size expression. Vincent have
planned to polish that now that feature freeze is made. 

All that needs testing of course. I'm not totally satisfied with the
assistant shortcut, hidden inside the size varying widget. If someone have a
better UI idea.. 

Hope that helps clarifying those changes. 
Cheers
Régis





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