Compositing

TopOSM is pretty inspirational. How about an XML dialect for specifying the composition of multiple Mapnik-rendered layers into a single map. This example is based on TopOSM's compositing steps, which also incorporate gdalwarp-generated hill shadings. (These could be integrated using a RasterSymbolizer, or a thin wrapper could be generated at runtime.)

Note: see also artem's test images (generated with AGG)
Note: see also the GSOC page on related ideas part Layer Composites
Note: Research possibility of use RPN (reverse polish notation) for specifying the rendering and compositing order. It would be hard to understand complex compositing schemes, but easy to implement
Note: Research multithreaded/multiprocess rendering and compositing of layers. In a nutshell, rendering of two different layers to be composed could be handled by different threads, all compositing should be multithreaded. Research and benchmark if it makes sense performance-wise.

        <?xml version="1.0"?>
        <Composite srs="...">
    
            <!-- first, render the areas -->
            <Map href="areas.xml"/>
    
            <!-- next, composite hillshading twice: -->
    
            <!-- first, darkened and using the screen blend mode -->
            <Map type="raster" href="hillshade.tiff" blend="screen">
                <Transform type="level">70,90%,0.8</Transform>
            </Map>

            <!-- then, lightened and using the multiply blend mode -->
            <Map type="raster" href="hillshade.tiff" blend="multiply">
            <Transform type="level">0,80%,1.0</Transform>
            </Map>
    
            <!-- (at this point, we should have a hill shaded area map) -->
    
            <!-- next up, the features -->
            <Group id="features">
                <!-- render the mapnik layer first -->
                <Map href="features.xml"/>
                <!-- then replace the alpha channel of the resulting buffer with that of
                 the rendered labels: -->
                <Channel src="A" dst="A">
                    <!-- render the labels into the buffer then blur, invert, and level -->
                    <Map href="labels.xml">
                        <Filter type="blur">0,2.0</Filter>
                        <Transform type="invert"/>
                        <Transform type="level">5,8%</Transform>
                    </Map>
                </Channel>
            </Group>
    
        <!-- finally, composite the non-shaded areas and labels -->
    
            <Map href="noshade-fills.xml"/>
            <Map href="labels.xml"/>
    
        </Composite>

The idea here is that the composition starts with an empty buffer and draws layers into it recursively, in the order that they appear in the XML. The areas.xml Mapnik stylesheet is rendered directly into the buffer, then the hill shading is applied twice: first darkened and composited using the "screen" blend mode; then lightened and composited using the "multiply" blend mode. The "features" group creates a new temporary buffer, into which the features.xml Mapnik stylesheet is rendered, then has its alpha channel ("A") replaced by that of the rendered labels.xml stylesheet (which, before being applied, is blurred, inverted, and leveled). The "areas" buffer is then composited onto the hill-shaded area map, followed by straight-up alpha composited noshade-fill.xml and labels.xml Mapnik styles.

Basically, each XML results in an image operation:

<Group> and <Layer> operations create new rendering contexts, the results of which are composited onto the buffer of the parent context (optionally specifying a blend mode).
<Transform> and <Filter> operations modify the buffer of the parent rendering context. The blur filter and color transforms in this example use ImageMagick semantics, but they could be expressed otherwise.
<Channel> operations render their child layers or groups into a temporary buffer, one or more channels of which are then applied to the parent context (optionally specifying a bitwise operator?).

Recursively rendered Mapnik stylesheets should inherit their SRS from the <Composite> element, if provided.