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Updated Grouped rendering (markdown)

springmeyer 2011-12-11 22:57:41 -08:00
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Grouped-rendering.md

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Using "grouped rendering" on a layer renders all styles of one layer for all features that have the same value before proceeding to other features that have a different value. This is useful for rendering roads in the correct z-order:
Using "grouped rendering" on a layer renders all styles of one layer for all features that have the same value before proceeding to other features that have a different value.
Example syntax looks like:
```xml
<Layer srs=".." group-by="z">
<StyleName>casing</StyleName>
<StyleName>fill</StyleName>
...
</Layer>
```
This is useful for rendering roads in the correct z-order:
* The datasource (e.g. Postgres) sorts the data based on a field (e.g. z_order) into ascending order
* Mapnik paints the features as they come in, but stops when the next feature has a higher z-order than the current one. It then proceeds to paint other styles for the features it has retrieved so far.