[MapProxy] link_single_color_images not working with mapnik source

[Peter Sprague]

Hello,

I'm using MapProxy to seed a CouchDB with tile images on a FreeBSD server.

My workflow is Tilemill -> mbtiles export -> mapproxy seed -> CouchDB. 
For zoom levels 10 - 17, rendering takes ~ 2-3 days, but produces a 
quite small db file @ ~2mb.  I attribute this to the large spaces with 
no data and the use of link_single_color_images: true

In an effort to speed the rendering process up, I have tried rendering 
mapnik.xml instead of the mbtiles produced by Tilemill.  All good with a 
tremendous speed up, but at the cost of a database that is 20x larger 
due to MapProxy producing a tile for every z/x/y.  Most of the thousands 
of tiles produced are transparent with no data.

Aka link_single_color_images: true does not seem to be working with 
Mapnik source, but does with mbtiles as a source.

Is there a way to use link_single_color_image with Mapnik?

Thanks,
-- 
Peter Sprague MSc.
Trails North America Mapping Ltd.
peter.sprague at trailsnorthamerica.org