I think you may want to start implementing your own labelling system.
The renderer implementations delegate the label handling out to an instance of
LabelCache; there is one instance provided by GeoTools and applications
sometimes implement their own. As an example uDig implements one that gathers
up labels from a number of renderer instances that are running in different
threads.
The thing is you can make a function (such as you are doing) but at best it is
going to provide a label and a position that is fed into the label cache
implementation. The label cache implementation does have some hints to control
it (so you can have more important labels take priority); but since you are
talking about your own clusters of features - you may want to buckle down and
implement a label cache that knows about your clusters of features.
Cheers,
Jody
On 13/07/2010, at 12:48 AM, LSA wrote:
> Hi all,
>
> I need to have my own custom labeling, which will place label near each
> feature using information about clusters of features (known to my
> application). In order to do it, I use my own function expression:
> public class MyExpression extends FunctionExpressionImpl {
> private Set lineIDs = new HashSet();
>
> public MyExpression() {
> super("MyExpression");
> }
>
> @Override
> public int getArgCount() {
> return 0;
> }
>
> @Override
> public Object evaluate(Object objFeature) {
> SimpleFeature feature = (SimpleFeature)objFeature;
> Object attr1 = feature.getAttribute("Attr1");
> return "Attr1="+attr1;
> }
> }
> Then I create my text symbolizer:
>
> MyExpression myExp = new MyExpression();
> TextSymbolizer textSym = sf.createTextSymbolizer(
> sf.createFill(ff.literal(Color.BLACK)), new
> Font[]{sf.getDefaultFont()}, null, myExp , placement, null);
>
> The problem that I run into is about feature passed to evaluate contains only
> geometry, no attributes at all (except FeatureId). So,
> feature.getAttribute("Attr1") will always return null. However, in order to
> display my clustering information I need certain attributes of each feature.
>
> Is this possible to do?
>
> As a horrible workaround, I pass Map<FeautureId, SimpleFeature> to
> MyExpression, and use these attributes. Downsides of this solution are
> obvious.
>
> Also I have another one question:
> Is it possible to override behaviour of the default labeling in order to show
> only one label per cluster of features? To what properties should I pay my
> attention?
> Are there some examples of creating custom labeling?
>
> Any help would be appreciated....
>
> Thanks in advance,
> Sergey
>
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