I agree Regis, it would add a new "dimension" to the rendering engine. +1 also from me.
giovanni 2012/7/9 Nathan Woodrow <madman...@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.haubo...@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. > > _______________________________________________ > > Qgis-developer mailing list > > Qgis-developer@lists.osgeo.org > > http://lists.osgeo.org/mailman/listinfo/qgis-developer > _______________________________________________ > Qgis-developer mailing list > Qgis-developer@lists.osgeo.org > http://lists.osgeo.org/mailman/listinfo/qgis-developer >
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