[Qgis-developer] Expression Based.. Styling

[haubourg]

Hi Andreas, 
maybe I missed something, please tell me. 

This is how I see the matter: 
Rule based styling allows to create rules based on *existing* fields with
SQL queries, but, *Refine assistants only have access to existing fields*.
Once refined rules on single or multiple fields are created, you then have
the possibility to use expression evaluator to modify each rule, reorder,
one after each other. This is awesome for many use cases, but not for thoses
ones :

* Example 1:classify and normalize numeric data*  You want to apply a sqrt
function on a numeric field and classify it with graduate value. Having the
expression builder accessible instead of fields list allows it directly. 
Mapinfo allows you to classify a expression (sqrt(myfield)/max(myfield) *
100 for example).  *Doing it in QGIS requires to create a new field. So the
user need to have rights on it or duplicate it.* I don't like that users
duplicate data, since I am data administrator. Modifying data for
representation purposes is bad too. 


*Example 2: Calculate advanced rotation and size fields : *I have waste
water plants to map. I want them red symbols for dysfunctionning plants and
green for the others. I want a point size varying based on size (~number of
inhabitants). If I want a correct representation, I want point size to vary
between 40 mm on screen and 0.1. With an expression instead of existing
fields, this can be easy. *I do know how to do that in postgis or
spatiallite view by myself. Common users don't *
This would be even easier with a dedicated gui to help user choose an d
preview Sqrt, exp , or Ln dynamic. 

*Example 3: generate unique values for legend labels*I need to categorize
data on a field, but with a display order (40 classes). I have a code for
classe number, and a label field.  In QGIS, I have two choices: Classify on
code and manually change labels (long and painful), or classify on label
field with ruled based labeling, and then order classes with priority order.
Long and painful too. With an expression, I could evaluate "code" || ' - '
|| "label" , and have it sorted and readable directly. Not perfect, but
fast. *This is what we need when having a meeting with authorities and only
3 hours to prepare a draft map. *

Did I miss something? I think QGIS still need some work to achieve all
cartographic representation rules (
http://www.scribd.com/doc/33408233/SUG243-Cartography-Proportional-Symbol
http://www.scribd.com/doc/33408233/SUG243-Cartography-Proportional-Symbol )

Are you interested if my organisation supports such a functionnality? 


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