[Chameleon] How to specify 24bit output for a WMS layer within an xml context file?

Jeff McKenna jmckenna at dmsolutions.ca
Tue Jan 24 08:23:49 EST 2006

Have you tried setting Chameleon's "image_type" parameter to PNG24 in 


Jennifer Zeisloft wrote:
> Hello All,
> I have been having problems displaying raster imagery from web mapping 
> services in MapServer/Chameleon (poor visual resolution, essentially).  
> I first sought help from the MapServer list (pasted below), but have 
> discovered that I likely need Chameleon help.
> Specifically, I am wondering how I can specify that 24bit output be 
> generated from a layer in a web mapping context.
> In addition to the responses below, Frank Warmerdam also suggested 
> adding PROCESSING "DITHER=YES" to the mapfile if I do not specify jpeg 
> or png24 in the mapfile output.  This did help somewhat.  But again, the 
> problem is that I cannot refer to a mapfile in an xml context file (as 
> far as I know).  So, there must be some other means in Chameleon of 
> specifying that the output is 24bit rather than 8bit.
> I am grateful for any hints or ideas you can offer!
> Thanks!
> Jennifer Zeisloft
> UW Sea Grant
> ------------------------------------------------------------------------------------------------------------------
>  >From the MapServer List Discussion:
> Greetings All,
> We have been noticing that some (but not all) WMS raster images look 
> pretty terrible (extremely low resolution compared to what they should 
> be) when added to our MapServer application.  For instance, from 
> TerraServer (http://terraservice.net/ogcmap.ashx), USGS Digital Ortho 
> Quads are virtually useless regardless of scale, while USGS DRGs look 
> decent.  Daily MODIS imagery from NASA JPL World Map Service 
> (http://wms.jpl.nasa.gov/wms.cgi?) has also typically been a problem.  
> And there are others...
> I have recently been playing with GDAL to display our own rasters, but 
> also partially with the hope that it might somehow magically fix this 
> problem (of course it hasn't).
> I am wondering whether the display problem is local or if it is on the 
> data custodian's end, and whether this is typically the case.  Is there 
> any way to correct it?  If it is at the custodial end, what might be 
> done differently to increase the display resolution for WMS clients (so 
> that we don't make the same mistake)?
> I really appreciate any hints or ideas you can offer.
> Thanks much!
> Jennifer Zeisloft
> UW Sea Grant
> ------------------------
> Jennifer -
> You don't give us much information to go on, other than your dislike of 
> the results!
> If you're consistently seeing poor-quality results, then there may well 
> be something systematic you're doing in your map files or MapServer 
> configuration.  I'm assuming you're using MapServer to generate these 
> "horrible-looking" images.  If you are, for example, requesting 
> low-color output from high-color images, you're going to see blotchy 
> output.  DRGs are low-color images (only 13 unique colors) so they are 
> not as prone to being affected by a low-color output request.
> Could you provide an example of the map file and configuration you're 
> using to retrieve and display these images?  There's nothing special 
> about the WMS interface that inherently limits image quality - you can 
> get a perfect reproduction of the source imagery in many cases.
>     - Ed
> Ed McNierney
> President and Chief Mapmaker
> TopoZone.com / Maps a la carte, Inc.
> 73 Princeton Street, Suite 305
> North Chelmsford, MA  01863
> Phone: +1 (978) 251-4242
> Fax: +1 (978) 251-1396
> ed at topozone.com <mailto:ed at topozone.com>
> --------------------------------------
> Thank you for responding, Ed.
> Actually, these raster data sets are displayed as part of Chameleon 
> contexts (so there is no mapfile).  I added the USGS orthos to a mapfile 
> to see if the result was any different and it wasn't (the mapfile code 
> for this layer is pasted below in any case).
> The application can be found at 
> http://maps.aqua.wisc.edu/lscmp/viewmaps.phtml.  The example data sets I 
> am referring to can be found in the Wisconsin: Imagery context.
> I have read through the documentation on rasters in MapServer, as well 
> as searched list archives and have not been able to determine how I 
> would go about specifying low or high -color output.  So, perhaps this 
> is part of the problem?
> Again, any suggestions are greatly appreciated!!
> Jennifer Zeisloft
>   CONNECTION "http://terraservice.net/ogcmap.ashx"
>    "init=epsg:4326"
>   END # Projection
>    "wms_srs"             "EPSG:4326"
>    "wms_name"            "DOQ"
>    "wms_title"           "USGS Digital Ortho-Quadrangles"
>    "wms_onlineresource"  "http://terraservice.net/ogcmap.ashx"
>    "wms_abstract"        "NA - test"
>    "wms_keywordlist"     "NA -test"
>    "wms_server_version"  "1.1.1"
>    "wms_formatlist"      "image/png,image/jpeg,image/wbmp"
>    "wms_format"          "image/png"
>   END # Metadata
> END # layer
> -------------------------------------
> From: Frank Warmerdam <warmerdam at pobox.com 
> <mailto:warmerdam at pobox.com>>    Mailed-By: lists.umn.edu 
> <http://lists.umn.edu>
> Reply-To: Frank Warmerdam <warmerdam at pobox.com <mailto:warmerdam at pobox.com>>
> To: MAPSERVER-USERS at lists.umn.edu <mailto:MAPSERVER-USERS at lists.umn.edu>
> Date: Jan 23, 2006 10:57 PM
> Subject: Re: [UMN_MAPSERVER-USERS] Horrible Looking WMS Raster Imagery
> On 1/23/06, Brent Pedersen <bpederse at nature.berkeley.edu 
> <mailto:bpederse at nature.berkeley.edu>> wrote:
>  > have you tried using image/jpeg?
> In addition to fetching data from the remote WMSes as
> JPEG, it is also necessary for mapserver to generate 24bit
> output (JPEG or PNG24 for instance).  I don't know how to
> do that with Chameleon myself, but I'm sure there is a setting
> somewhere.
> I think Ed is right that the problem is 24bit to 8bit color
> conversion with correspondingly poor color resolution.
> Given sufficient blotchiness from this conversion it can
> even appear that spatial resolution is degraded (though it
> isn't actually).
> Best regards,
> ---
> I set the clouds in motion - turn up   | Frank Warmerdam, 
> warmerdam at pobox.com <mailto:warmerdam at pobox.com>
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | Geospatial Programmer for Rent
> ------------------------------------------------
> Brent,
> Thanks for the suggestion.  I switched the output from png to jpeg - but 
> no change (the jpeg might have looked a little worse??).  Still, the 
> crux of the matter is, I suppose, that these images are displayed 
> through a context file.  The rasters are coming straight from their 
> "original" (perhaps) source, which doesn't allow me to specify how the 
> image is displayed, but merely that an image is displayed (furthermore, 
> the context file is automatically generated by the "Save Context" widget 
> in Chameleon).
> And an addendum to my last message:  I did find output specifications in 
> the documentation 
> (http://mapserver.gis.umn.edu/docs/reference/mapfile/outputformat/), but 
> I don't have the opportunity in this instance to specify the output in 
> this sense.
> Jennifer
> -----------------------------------------------
> Hi Frank,
> Thanks for the response, Frank.  It is sounding more and more like this 
> is a question I ought to forward to the chameleon list - because I am 
> also not sure how to generate 24bit output with chameleon!
> I appreciate the much needed direction!
> Thanks all!
> Jennifer

Jeff McKenna
DM Solutions Group Inc.

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