[MapProxy] transparent cache without source

[Rode Biet]

Dear all,
I used gdal2tiles.py generate tiles from imagery. I added these tile as a cache in mapproxy without source.
Now, I want to render these imagery tiles on top of openstreetmap.
I does work, but where the imagery is supposed to be transparent, it will draw a gray color.
If I look at the imagery layer alone in QGIS it is transparent.
In some sources it is possible to specify that it is transparent, but you cannot do that for caches.
Does anybody now how to make a cache without source transparent?
Fragement of my configuration file:
layers:  - name: osm    title: Open Street Map    sources: [osm_cache]  - name: imagery
    title: Imagery    sources: [imagery_cache]  - name: imagery_with_openstreetmap    title: Imagery with Openstreetmap    sources: [imagery_with_openstreetmap_cache]
caches:  osm_cache:    grids: [webmercator]    sources: [osm_source]    cache:      type: geopackage
      filename: openstreetmap.gpkg      table_name: tiles  imagery_cache:
    grids: [oefengrid]    sources: []    cache:      type: file      directory_layout: tms      directory: /some_directory/tiles  imagery_with_openstreetmap_cache:    grids: [webmercator]    sources: [osm_cache, imagery_cache]

sources:  osm_source:    type: tile    url: 'http://a.tile.openstreetmap.org/%(z)s/%(x)s/%(y)s.png'    grid: webmercator    transparent: true     grids:  webmercator:    base: GLOBAL_WEBMERCATOR    srs: EPSG:3857  oefengrid:    base: GLOBAL_WEBMERCATOR    srs: 'EPSG:3857'    origin: 'sw'

Thanks in advance.
Rode Biet.
 
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/mapproxy/attachments/20170407/23c4d01c/attachment.html>