[MapProxy] Image quality problem for 8 bit paletted png, ragged lines

[Anne Blankert]

Hello List,

I am having a (minor) problem with image quality. Cached lines are much 
more ragged than the original lines. Resolution and projection of source 
and cache are identical. I found a work-around (meta_buffer:0, 
meta_size[1,1]), but I think it would be better if the work- around 
would not be necessary?
Visual
1. cached 256x256 png image: 
http://research.geodan.nl/edugis/images/raggedlines.png
2. wms original 1184x1184 png image: 
http://research.geodan.nl/edugis/images/wmsoriginal.png

Explanation:

I am using mapproxy version 1.3.0, tried both Windows (64 bit) and 
Ubuntu (64 bit) versions, same result. See mapproxy.yaml below.

Reproducing the problem:
My mapping client sends the following request to mapproxy for a single 
tile of 256 x 256 pixels:
http://mymapproxy.example.com/mapproxy/service?LAYERS=agn&TRANSPARENT=true&FORMAT=image%2Fpng&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&Styles=&SRS=EPSG%3A900913&BBOX=543008.64525162,6863280.7649075,543161.51930816,6863433.638964&WIDTH=256&HEIGHT=256

The resulting 256x256 png image is at:
http://research.geodan.nl/edugis/images/raggedlines.png

The mapproxy cache on disk contains the same image, also displaying 
ragged lines.

In order to fill the cache, Mapproxy request a  1184 x 1184 png image 
from the orginal WMS server (mapserv version 6.0.1):
http://mywms.example.com/cgi-bin/mapserv?map=maps/geodan/agn.map&styles=&format=image%2Fpng&height=1184&bbox=542960.875795,6862774.37441,543667.918307,6863481.41693&transparent=True&layers=agn&service=WMS&width=1184&request=GetMap&srs=EPSG%3A900913&version=1.1.1

The resulting wms image is at:
http://research.geodan.nl/edugis/images/wmsoriginal.png

As you see, the original image does not show ragged lines (cached tile 
area is at top left of this wms image) . If you zoom in and compare 
these images, you will notice that the original image has more gray 
levels than the cached image. Therefore the problem seems not to be in 
the lines, but in the paletted color-table of the 8 bit png image.

The workaround is to configure the cache as follows:
meta_buffer: 0
meta_size: [1,1]

With these settings, the cached image and the wms image are identical, 
so there is no need for Mapproxy to process the image and the ragged 
line problem disappears.

Is there a configuration option to solve this problem? Maybe force 
MapProxy to use the same paletted color table for cache and original? I 
tried several image encoding options.

Thanks,

Anne Blankert
==

My mapproxy.yaml

services:
   demo:
   kml:
   tms:
     # needs no arguments
   wms:
     srs: ['EPSG:4326', 'EPSG:900913', 'EPSG:28992']
     # image_formats: ['image/jpeg', 'image/png']
     md:
       # metadata used in capabilities documents
       title: MapProxy WMS Proxy
       abstract: MapProxy service
       online_resource: http://mapproxy.example.com/
       contact:
         person: OSGIS beheerder
         position: Beheerders
         organization: Geodan.nl
         address: Geodan, President Kennedylaan 1
         city: Amsterdam
         postcode: 1079 MB
         country: the Netherlands
         phone: ...
         fax: ..
         email: ...
       access_constraints: none
       fees: 'None'
layers:
   - name: agn
     title: Adressen en Gebouwen Nederland (agn)
     sources: [agn_cache]

caches:
   agn_cache:
     grids: [GLOBAL_MERCATOR]
     sources: [agn_wms]
     disable_storage: false
#    meta_buffer: 0
#    meta_size: [1,1]

sources:
   agn_wms:
     type: wms
     wms_opts:
       featureinfo: true
     supported_srs: ['EPSG:900913']
     req:
       url: http://mywms.example.com/cgi-bin/mapserv?map=maps/geodan/agn.map
       layers: agn
       transparent: true

grids:
   global_geodetic_sqrt2:
     base: GLOBAL_GEODETIC
     res_factor: 'sqrt2'

globals:
   # # coordinate transformation options
   # srs:
   #   # WMS 1.3.0 requires all coordiates in the correct axis order,
   #   # i.e. lon/lat or lat/lon. Use the following settings to
   #   # explicitly set a CRS to either North/East or East/North
   #   # ordering.
   #   axis_order_ne: ['EPSG:9999', 'EPSG:9998']
   #   axis_order_en: ['EPSG:0000', 'EPSG:0001']
   #   # you can set the proj4 data dir here, if you need custom
   #   # epsg definitions. the path must contain a file named 'epsg'
   #   # the format of the file is:
   #   # <4326> +proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs <>
   #   proj_data_dir: '/path to dir that contains epsg file'

   # # cache options
   cache:
     # where to store the cached images
     base_dir: 'c:/temp/mapproxy/mp'
     # where to store lockfiles
     lock_dir: 'c:/temp/mapproxy/mplocks'

   # image/transformation options
   image:
       #resampling_method: nearest
       # resampling_method: bilinear
       resampling_method: bicubic
   #   formats:
   #     image/png:
   #        encoding_options:
   #          quantizer: fastoctree
   #     jpeg_quality: 90
   #     # stretch cached images by this factor before
   #     # using the next level
   #     # stretch_factor: 1.15
   #    stretch_factor: 1.2
   #     # shrink cached images up to this factor before
   #     # returning an empty image (for the first level)
   #     max_shrink_factor: 4.0