Support UOM in the rendering process (a.k.a. variable size symbolizers)
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Key: GEOT-2964
URL: http://jira.codehaus.org/browse/GEOT-2964
Project: GeoTools
Issue Type: New Feature
Components: core render, core styling
Affects Versions: 2.6.2
Reporter: Milton Jonathan
Assignee: Andrea Aime
Fix For: 2.6.3
Attachments: geotools-tecgraf.2.6.x.uom_rescaling.patch
The provided patch adds support for rescaling symbolizers according to their
Unit of Measure (UOM). This feature allows for symbolizers to have a
"real-world" size (e.g., size given in meters or feet), so that they grow as
you zoom in.
There are several issues in the implementation that may require a discussion
before commiting:
- The approach used was to implement a UomRescaleStyleVisitor class for
rescaling Symbolizers according to their Unit Of Measure (i.e., compute their
sizes in pixels), and use that in SLDStyleFactory whenever a Style2D is
created. In order to do that, the current map scale is needed (i.e., how screen
units relate to real-world units, or how many meters are covered by each
pixel). This was extracted from the SLDStyleFactory.createStyle's ScaleRange
parameter, which was seen to be set to (scaleDenominator,scaleDenominator) in
the renderers. However, maybe it would make more sense to pass on the
scaleDenominator directly.
- Methods were added to RendererUtilities.java to help compute the pixels per
meter ratio (in order to convert sizes in meters/feet/etc to pixels).
- In SLDStyleFactory, both createStyleInternal and createDynamicStyle were
modified to rescale the given symbolizer. It was not clear whether
createDynamicStyle is needed or where it is used.
- Style2D objects created by SLDStyleFactory should always be rescaled
according to the UOM, but for one case: in Drawer.java (used for legend icons),
the rendering must be done WITHOUT rescaling the symbolizer, since a legend has
no scale. This was currently implemented adding a createStyleNoUOMRescaling
method, because as such less GeoTools code would be changed. But maybe it would
be better if SLDStyleFactory never rescaled anything and classes such as
StreamingRenderer explicitly rescaled the symbolizer to pixels and then called
SLDStyleFactory normally.
- The UomRescaleStyleVisitor could be discussed as well. For simplicity,
overriding visit() methods to add rescaling was done only on the Symbolizer
level, where the UOM parameter is available. The alternative would be to
implement visit(Graphic), visit(Stroke), etc. and have the UOM parameter be
included in the visitor's state.
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