[Qgis-developer] Expression Based.. Styling

[G. Allegri]

I agree Regis, it would add a new "dimension" to the rendering engine.
+1 also from me.

giovanni

2012/7/9 Nathan Woodrow <madmanwoo at gmail.com>

> Hey Régis,
>
> I think that would a good feature to add. It would add a lot of power
> to the rendering engines, including the rule based render. +1 from me
> for adding support for this if we can.
>
> - Nathan
>
> On Mon, Jul 9, 2012 at 6:18 PM, haubourg
> <regis.haubourg at eau-adour-garonne.fr> wrote:
> > 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?
> >
> >
> > --
> > View this message in context:
> http://osgeo-org.1560.n6.nabble.com/Expression-Based-Styling-tp4986594p4986858.html
> > Sent from the Quantum GIS - Developer mailing list archive at Nabble.com.
> > _______________________________________________
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