Hi GeoTools Developers,
We have been using GeoTools for several years now and greatly appreciate
the community effort that makes it such a successful project. During a
recent project we noticed a GeoServer rendering anomaly and traced it back
to the ScreenMap optimizations in ShapefileFeatureReader.
We have implemented changes on a forked repository (
https://github.com/emiller-planalytics/geotools) which reproduce the
problem in the unit test FilteredScreenMapShapefileTest and also include a
conceptual solution with backward compatible modifications to
ShapefileFeatureReader and ShapefileFeatureSource. We have confirmed that
these changes do not adversely affect existing unit tests.
The crux of the problem is that the ScreenMap optimization currently
filters out (screen) coincident features without consideration for
subsequent filtering. This becomes apparent when ScreenMap filtering
accepts a feature that will be rejected by a subsequent filter and rejects
a subsequent, (screen) coincident feature that would be accepted by the
later filter. In this situation a feature should be rendered but is not.
The included test case uses several coincident points to illustrate the
problem, although we believe that nearly coincident (with respect to
original CRS) and polygon features could also exhibit this behavior.
The proof-of-concept solution pushes the layer filter to
ShapefileFeatureReader. When provided, the filter is evaluated before
assessing the ScreenMap filtering. Because the filter evaluation in the
forked repo is naive (it always assumes that attributes are required), the
new execution path effectively removes the benefits of the ScreenMap
optimization for any layers with filters. This could be improved by making
the filter evaluation smarter in cases where a DBF read is not required.
The changes do not include similar enhancements for
IndexedShapefileFeatureReader but it is expected that this should be
handled in a similar manner to ShapefileFeatureReader.
I look forward to your thoughts on this matter. Pleas let me know if I can
clarify anything. I will attempt to issue a pull request for reference.
Best regards,
Eli Miller
--
Eli Miller
Director of Development / Enterprise Architect, Planalytics
(W) 610.407.2930
(M) 484.467.7926
920 Cassatt Road, Suite 300
Berwyn, PA 19312
[email protected]
<http://www.planalytics.com/>
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