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Updated Compositing (markdown)

ThomasG77 2011-12-09 16:10:26 -08:00
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Ideas_Compositing.md

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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.) [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.)
* Note: see also artem's test images (generated with AGG): http://trac.mapnik.org/wiki/Compositing * Note: see also artem's test images (generated with AGG): http://trac.mapnik.org/wiki/Compositing
* Note: see also the GSOC page on related ideas: http://trac.mapnik.org/wiki/GSOC2010/Ideas?version=17#LayerComposites * Note: see also the GSOC page on related ideas: http://trac.mapnik.org/wiki/GSOC2010/Ideas?version=17#LayerComposites
* 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 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. * 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"?> <?xml version="1.0"?>
<Composite srs="..."> <Composite srs="...">
<!-- first, render the areas --> <!-- first, render the areas -->
<Map href="areas.xml"/> <Map href="areas.xml"/>
<!-- next, composite hillshading twice: --> <!-- next, composite hillshading twice: -->
<!-- first, darkened and using the screen blend mode --> <!-- first, darkened and using the screen blend mode -->
<Map type="raster" href="hillshade.tiff" blend="screen"> <Map type="raster" href="hillshade.tiff" blend="screen">
<Transform type="level">70,90%,0.8</Transform> <Transform type="level">70,90%,0.8</Transform>
</Map> </Map>
<!-- then, lightened and using the multiply blend mode --> <!-- then, lightened and using the multiply blend mode -->
<Map type="raster" href="hillshade.tiff" blend="multiply"> <Map type="raster" href="hillshade.tiff" blend="multiply">
<Transform type="level">0,80%,1.0</Transform> <Transform type="level">0,80%,1.0</Transform>
</Layer> </Map>
<!-- (at this point, we should have a hill shaded area map) --> <!-- (at this point, we should have a hill shaded area map) -->
<!-- next up, the features --> <!-- next up, the features -->
<Group id="features"> <Group id="features">
<!-- render the mapnik layer first --> <!-- render the mapnik layer first -->
<Map href="features.xml"/> <Map href="features.xml"/>
<!-- then replace the alpha channel of the resulting buffer with that of <!-- then replace the alpha channel of the resulting buffer with that of
the rendered labels: --> the rendered labels: -->
<Channel src="A" dst="A"> <Channel src="A" dst="A">
<!-- render the labels into the buffer then blur, invert, and level --> <!-- render the labels into the buffer then blur, invert, and level -->
<Map href="labels.xml"> <Map href="labels.xml">
<Filter type="blur">0,2.0</Filter> <Filter type="blur">0,2.0</Filter>
<Transform type="invert"/> <Transform type="invert"/>
<Transform type="level">5,8%</Transform> <Transform type="level">5,8%</Transform>
</Map> </Map>
</Channel> </Channel>
</Group> </Group>
<!-- finally, composite the non-shaded areas and labels --> <!-- finally, composite the non-shaded areas and labels -->
<Map href="noshade-fills.xml"/> <Map href="noshade-fills.xml"/>
<Map href="labels.xml"/> <Map href="labels.xml"/>
</Composite> </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. 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: 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). * `<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](http://www.imagemagick.org/script/command-line-options.php) semantics, but they could be expressed otherwise. * `<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.
* `<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?). * `<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. Recursively rendered Mapnik stylesheets should inherit their SRS from the `<Composite>` element, if provided.