layer-structure.md

Tree-like layer structure

Layers can be arbitrarily nested, in other words layers can be made of another layers. This allows better layer organization and complex grouped compositing, see Layer level compositing.

Rendering rules

Layers are rendered in top-down order as they are defined. Rendering and even querying data by a layer is not proceeded in following situations:

• A layer is deactivated (by status property in XML).
• A layer is not visible for given scale denominator.
• Layer's extent doesn't intersect with map's extent.
• A layer has zero active styles.

If one of these conditions holds, all sub-layers are ignored.

The spatial reference system definition (srs)

If the srs of a layer is not defined explicitly, it is inherited from parent layer or from map in case of top-level layer.

Purely compositional and organizational layers

Let's call a layer with no Datasource, comp-op and opacity purely organizational layer. Such a layer can group another layers with no effect on rendering result. Only way how purely organizational layer can influence rendering is by status="off". If comp-op or opacity is used, layer will have its own rendering buffer.

An example

<Map>
    <Layer name="terrain">
        <StyleName>terrain</StyleName>
        <Datasource>
            ... some raster data ...
        </Datasource>

        <Layer name="contours">
            <StyleName>contours</StyleName>
            <Datasource>
                ... some vector data ...
            </Datasource>
        </Layer>
    </Layer>

    <Layer name="paths">
        ...

        <Layer name="level2">
            ...
        </Layer>
        <Layer name="level3">
            ...
        </Layer>
    </Layer>

    <Layer name="poi" status="off">
        ...

        <Layer name="poi2">
            ...
        </Layer>
        <Layer name="poi3">
            ...
        </Layer>
    </Layer>
</Map>