Updated Compositing (markdown)
ThomasG77
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[TopOSM](http://wiki.openstreetmap.org/wiki/TopOSM/Details) 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](http://wiki.openstreetmap.org/wiki/TopOSM/Details#Combining_images_into_a_final_composite), which also incorporate `gdalwarp`-generated hill shadings. (These could be integrated using a [RasterSymbolizer](http://trac.mapnik.org/wiki/RasterSymbolizer), or a thin wrapper could be generated at runtime.)
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* Note: see also artem's test images (generated with AGG): http://trac.mapnik.org/wiki/Compositing
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* Note: see also the GSOC page on related ideas: http://trac.mapnik.org/wiki/GSOC2010/Ideas?version=17#LayerComposites
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* 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
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* 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.
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* Note: see also artem's test images (generated with AGG): http://trac.mapnik.org/wiki/Compositing
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* Note: see also the GSOC page on related ideas: http://trac.mapnik.org/wiki/GSOC2010/Ideas?version=17#LayerComposites
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* 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
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* 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.
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<?xml version="1.0"?>
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<Composite srs="...">
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<?xml version="1.0"?>
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<Composite srs="...">
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<!-- first, render the areas -->
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<Map href="areas.xml"/>
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<!-- first, render the areas -->
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<Map href="areas.xml"/>
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<!-- next, composite hillshading twice: -->
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<!-- next, composite hillshading twice: -->
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<!-- first, darkened and using the screen blend mode -->
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<Map type="raster" href="hillshade.tiff" blend="screen">
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<Transform type="level">70,90%,0.8</Transform>
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</Map>
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<!-- first, darkened and using the screen blend mode -->
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<Map type="raster" href="hillshade.tiff" blend="screen">
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<Transform type="level">70,90%,0.8</Transform>
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</Map>
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<!-- then, lightened and using the multiply blend mode -->
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<Map type="raster" href="hillshade.tiff" blend="multiply">
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<Transform type="level">0,80%,1.0</Transform>
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</Map>
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<!-- then, lightened and using the multiply blend mode -->
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<Map type="raster" href="hillshade.tiff" blend="multiply">
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<Transform type="level">0,80%,1.0</Transform>
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</Layer>
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<!-- (at this point, we should have a hill shaded area map) -->
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<!-- (at this point, we should have a hill shaded area map) -->
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<!-- next up, the features -->
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<Group id="features">
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<!-- render the mapnik layer first -->
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<Map href="features.xml"/>
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<!-- then replace the alpha channel of the resulting buffer with that of
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the rendered labels: -->
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<Channel src="A" dst="A">
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<!-- render the labels into the buffer then blur, invert, and level -->
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<Map href="labels.xml">
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<Filter type="blur">0,2.0</Filter>
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<Transform type="invert"/>
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<Transform type="level">5,8%</Transform>
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</Map>
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</Channel>
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</Group>
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<!-- next up, the features -->
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<Group id="features">
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<!-- render the mapnik layer first -->
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<Map href="features.xml"/>
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<!-- then replace the alpha channel of the resulting buffer with that of
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the rendered labels: -->
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<Channel src="A" dst="A">
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<!-- render the labels into the buffer then blur, invert, and level -->
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<Map href="labels.xml">
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<Filter type="blur">0,2.0</Filter>
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<Transform type="invert"/>
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<Transform type="level">5,8%</Transform>
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</Map>
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</Channel>
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</Group>
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<!-- finally, composite the non-shaded areas and labels -->
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<!-- finally, composite the non-shaded areas and labels -->
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<Map href="noshade-fills.xml"/>
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<Map href="labels.xml"/>
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<Map href="noshade-fills.xml"/>
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<Map href="labels.xml"/>
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</Composite>
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</Composite>
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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.
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Basically, each XML results in an image operation:
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* `<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).
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* `<Transform>` and `<Filter>` operations modify the buffer of the parent rendering context. The blur filter and color transforms in this example use [ImageMagick](http://www.imagemagick.org/script/command-line-options.php) semantics, but they could be expressed otherwise.
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* `<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?).
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* `<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).
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* `<Transform>` and `<Filter>` operations modify the buffer of the parent rendering context. The blur filter and color transforms in this example use [ImageMagick](http://www.imagemagick.org/script/command-line-options.php) semantics, but they could be expressed otherwise.
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* `<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?).
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Recursively rendered Mapnik stylesheets should inherit their SRS from the `<Composite>` element, if provided.
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