[MapProxy] Tiff images vertically swapped

[Martin Dirichs]

Hello,

I just did a fresh installation of MapProxy on an Ubuntu 16.04
server using the pre-packaged Mapnik 3.0.9. The Mapnik
configuration is based on raster images (Tiff) for the top zoom
levels. Higher zoom levels are based on geo data in a
PostgreSQL-PostGIS database.

Looking at the result with the MapProxy demo, I see this for
zoom level 0:

http://dynaserv.htwsaar.de/downloads/mapproxy-level-0.png

The Tiff image seems to be swapped vertically. This can be
seen in all levels that are based on the raster images, regardless
of the web service used (WMS / TMS). Higher zoom levels that
are rendered with the database contents are fine, however.
This is a configuration I habe been using previously with older
versions of OS / Mapnik / MapProxy, so I known that the Tiff
images are correct.

I am unsure where to look for the error (MapProxy / Mapnik /
some underlying library)? Any hint is greatly appreciated.

This is my mapproxy.yaml:
services:
  demo:
  tms:
  wmts:
  wms:
    srs: ['EPSG:3857']
    image_formats: ['image/png']

layers:
  - name: osm
    sources: [osm_cache]

caches:
  osm_cache:
    grids: [GLOBAL_WEBMERCATOR]
    sources: [osm_map]

sources:
  osm_map:
    type: mapnik
    mapfile: /opt/osm-mapnik/mapquest-config.xml

globals:
  cache:
    meta_size: [2, 2]
    meta_buffer: 80
    base_dir: /var/cache/mapproxy

  image:
    resampling_method: bilinear
    stretch_factor: 1.15
    max_shrink_factor: 4.0
    paletted: true
    formats:
      custom_format:
        format: image/png

Kind regards,
Martin Dirichs