[MapProxy] transparent layers get an opaque background after merging

[Tobias Wendorff]

Hi there!

I have a problem concerning the merging of two transparent layers: After
merging, the displayed layer gets an opaque white background. The
non-merged layers are displayed transparently - so these should be fine.

The layers are based on MBTiles coming straight out of QGIS. I can't
tell if the PNGs have an alpha channel or "palette transparency".

Enabling a storage for "cache_polygons_patterns_mixed" didn't change the
background.

My Pillow is at version 6.2.2 (last version for Python2). MapProxy and
all its dependencies and system libraries are up to date.

Before filing a bug in Github, am I doing anything wrong?
This might be associated with https://github.com/mapproxy/mapproxy/issues/2

Best regards,
Tobias

layers:
  - name: polygons_solid
    title: polygons_solid
    sources: [cache_polygons]
  - name: patterns
    title: patterns
    sources: [cache_patterns]
  - name: polygons_patterns_mixed_layer
    title: polygons_patterns_mixed_layer
    sources: [cache_polygons, cache_patterns]
  - name: polygons_patterns_mixed_cache
    title: polygons_patterns_mixed_cache
    tile_sources: [cache_polygons_patterns_mixed]

caches:
  cache_polygons_patterns_mixed:
    sources: [cache_polygons, cache_patterns]
    grids: [osm_grid]
    disable_storage: true
  cache_patterns:
    sources: []
    grids: [osm_grid]
    cache:
      type: mbtiles
      filename: /srv/mapproxy/data/patterns.mbtiles
  cache_polygons:
    sources: []
    grids: [osm_grid]
    cache:
      type: mbtiles
      filename: /srv/mapproxy/data/polygons.mbtiles

grids:
  osm_grid:
    base: GLOBAL_MERCATOR
    srs: 'EPSG:3857'
    origin: nw

globals:
  cache:
    base_dir: '/srv/mapproxy/cache_data'
    lock_dir: '/srv/mapproxy/cache_data/locks'
    tile_lock_dir: '/srv/mapproxy/cache_data/tile_locks'
    concurrent_tile_creators: 2
    link_single_color_images: true
  image:
    paletted: true
    formats:
      custom_format:
        format: image/png
        mode: P
        colors: 32
        transparent: true
        encoding_options:
          quantizer: fastoctree