[Qgis-developer] Information about Progress in diagram branch

[Tim Sutton]

Hi Marco

Ah I been wondering what youve been up to in that branch! So can you
generate pie charts based on the attributes in a layer? Any chance of
a screenie? :-)

Regards

Tim

2007/2/17, Marco Hugentobler <marco.hugentobler at karto.baug.ethz.ch>:
> Dear qgis developers,
>
> You may have noticed my activities in the diagram branch in the last time. My
> progresses are quite good and I hope that it can be merged into svn head
> before 0.9 release (whenever this will be). If you like to try out the new
> feature, you may download the sources with:
>
>  svn co https://svn.qgis.org/repos/qgis/branches/diagram-branch
>
> I'd like to give some information here about what I'm doing there. I'm also
> preparing a pdf with detailed technical description and I'll send a link to
> it to the devel list as soon as it's available.
>
> The main technical change in the branch is the class QgsVectorOverlay. A
> vector overlay is anything on top of a vectorlayer which relates to the
> position of the features. An example of a vector overlay are diagrams placed
> on a map (for which there is an early implementation in the branch). Another
> example would be labels (but the branch still uses the current label
> implementation).
>
> In the diagram branch, there is the interface QgsVectorOverlay and a subclass
> of it QgsDiagramOverlay. A vector overlay stores a multimap of
> QgsOverlayObjects (basically position, bounding box and feature geometry) and
> is able to draw those objects (which is specific to subclasses). It is a
> multimap because multitype objects may have several overlay objects.
>
> To manage the positioning of the overlay objects, there is the interface
> QgsOverlayObjectPositionManager and the subclass
> QgsCentralPointPositionManager. The idea of the position manager is that it
> may evaluate the overlay objects of one or more vector overlay together with
> the feature geometries to achieve an optimal placement. E.g. if there is a
> diagram overlay and labels at the same time, it would be great if they do not
> overlap each other and not the labels/diagrams of other features. The current
> implementation in the class QgsCentralPointPositionManager implements the
> simplest behaviour by placing the objects on the center point of a feature
> (without considerating other vector overlays). But the architecture in the
> diagram branch makes it possible to implement sophisticated placement in the
> future.
>
> The implementation of the diagram overlay in the branch is done as plugin. For
> this, a new plugin type has been introduced (QgisPlugin::VECTOR_OVERLAY).
> Vector overlay plugins are subclasses of QgsVectorOverlayPlugin and provide a
> method for the vector properties dialog to request an input dialog suitable
> for the specific vector overlay type. So QgsVectorLayerProperties checks if
> there are any overlay plugins loaded and inserts a new tab with the specific
> dialog for each. Vector overlays could of course also be implemented as part
> of the core.
>
> The current implementation of diagrams provides support for pie- and barcharts
> and for linear scaling of the diagram size according to a classification
> attribute. These behaviours are also extendable by implementing interfaces,
> but I don't want to bore you here with the exact details. The implementation
> does not yet support loading/saving to/from project file and does not yet
> generate an entry in the qgis legend. Once these points are solved, I will
> try to request permission for a merge to svn head. From my explanations above
> you can see that the impact of the changes on qgis core stability are minimal
> because most of the actual actions are done in the diagram plugin.
>
>
> All the best,
> Marco
>
>
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-- 
Tim Sutton

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