[Qgis-developer] Legend for proportionnal symbols and expression based symbols

Denis Rouzaud denis.rouzaud at gmail.com
Wed Dec 4 03:34:06 PST 2013

Hi régis,

Martin Dobias will make a proposal soon for a legend refactoring.
You should see with him how to let those things happen in the future in 
the new legend.



On 04. 12. 13 12:08, Régis Haubourg wrote:
> Hi All,
> we have a long lasting problem with proportionnal legends in QGIS. It gets
> more problematic with V2, as those maps are so beautiful and popular.
> I'm starting some brainstorming to clarify fonctionnal specifications what
> we actually want to have legends in layers registry and composer working.
> Here are some exemples of what I would love to see in legends:
>   - [0] proportionnal circles or square with 1 to 5 classes
>   - [1] proportionnal symbols AND colors
> Now that we have expressions everywhere, we probably will have to handle
> legends differently than in other GIS, where legend is generated once, when
> user tune symbology. Let me explain:
> - user can now define expressions for size, color, rotation, border width...
> for any sub-marker (1 to n) of a symbol.
> - Moreover, we can use conditionnal expressions, and scale dependent
> conditions ($scale variable).
> - QGIS is also a server, we need to be able to generate legends working on
> really displayed datas.
> Conclusion, It now is impossible to correctly pre-generate the legend by
> analysing symbology properties.
> Does this sound possible to developpers to explore some other way? We could
> read real dataset to collect informations to build a legend, and eventually
> do some statistical classification to get discrete classes from continuous
> data?
> We could do that on live or on-demand update maybe.
> My idea was to collect rendered object in current canvas and current scale.
> BUT.. we can face discrete data - some kind of SELECT DISTINCT can do the
> trick - , OR continuous data - we need then to do some statistical
> classification to reduce the legend to 1 - 10 classes to get it readable.
> We also need some intelligence in those statistical classifications:
>   - round values to keep it readable
>   - display separate classes only if size or color distance is long enough so
> that human eye sees some difference on screen. ie: do not display for
> classes if symbol size varies from 0.5 to 0.7...
>   - add some options in GUI to choose number of classes, type of legend
> rendering..
>   - keep the classification process fast (subset of data with random
> sampling?)
>   - test all use cases, I'm mostly thinking of points symbols, but lines and
> polygons must not be forgotten.
> I'm sure plenty of scientists have already been theorizing all this. Any
> opinions? Could this be an idea for GSoC or other kind of training course?
> I'll be pleased to get feedback from the community.
> Cheers
> Régis
> [0] http://kelsocartography.com/blog/?p=2224
> [1] http://www.geoclip.fr/img/bicol.jpg
> --
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