Using image filters and compositing together

Following up last week’s post about smart halos, I wanted to share some ways of using compositing and filters together to achieve interesting effects.

As a starting point we use the smart halos map from the previous posting. You can find the features used in this post in the compositing branch Mapnik 2.1 release. Just be aware that these new features are work-in-progress and some syntax might change in future releases.

Deep water

Here is the original style:

Let’s look at water bodies and land. Solid fills are looking good, but what about adding some texture to the map? One way to add ‘texture’ is to use

<PolygonPatternSymbolizer/>

Let’s grab one of the lovely watercolour patterns and apply to ‘water’ features.

Very nice so far, perhaps a bit flat. Now time to add some depth. One of the very useful image processing tools is the ability to apply various blurs. In this case I use ‘agg-stack-blur’ with both x/y radius' set to 10 pixels:

<Style name="water-shadow" image-filters="agg-stack-blur(10,10)" comp-op="dst-over">
    <Rule>
        <Filter>([HYC]=8)</Filter>
        <PolygonSymbolizer fill="lightblue" fill-opacity="0.4" smooth="0.7" transform="translate(0,0)" />
    </Rule>
</Style>

And lastly I add a bit of texture to the land fill to get that ‘paper’ feel to the final map :

I’m not sure the last step adds much to the cartography but it’s good to try different things. So go ahead and grab the latest source from compositing branch Mapnik 2.1 and have fun!

Ooh, one more.. I really like ‘blur’ filters, they seem to be so versatile. Here is a water-floating-shadow style using a combination of ‘emboss’ and ‘agg-stack-blur’ filters, and also applying transform on the shadow symbolizer

transform="translate(10,10)"

Floating water

Note, that while the water is floating in the air, all roads are on top of it – funky :)