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Updated Compositing (markdown)

yvecai 2012-12-01 12:52:25 -08:00
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Compositing.md

@ -72,6 +72,15 @@ The default (if no `comp-op` is set on a Style) is to skip the creation of a tem
As of Mapnik 2.1 you can also set an `opacity` property at the style level. This will also trigger the usage of an internal, blank canvas which all features of the style will be rendered against. When the temporary canvas is blended back into the main canvas the `opacity` of the temporary canvas will be set on the fly to allow for very consistent opacity of features (this can avoid artifacts for overlapping features). In this case the `src-over` compositing operation will be used if no other `comp-op` is set.
### Example of "erasing stroke"
This style will erase what was previously drawn on layers below :
```xml
<Style name="eraser" comp-op="dst-out">
<Rule>
<LineSymbolizer stroke-width="3" stroke="#FFFFFF" />
</Rule>
</Style>
```
## Grouped compositing