GIServer - The WMS Format Parameter.

Original File	format.htm
Abstract	The WMS Format parameter and the implication of cascading requests in GIServer
Author	Pedro Pereira Gonçalves (email)
Implemented Functions
Last Change
History	2001-10-26 : File Created
Index Page	GIServer tutorial
Important Links:	W3.org HTML 4.01 Specification web page : home page and objects OpenGIS : home page and Web Map Server specification 1.1.0 (pdf , rtf)

All the WMS in GIServer work as cascanding requests that are launch in an equal number to the layers requested and all its parameters are sent to the different cascanding requests.
For example, when requesting more than one layer the FORMAT parameter is applied to all the layers requested. This means that for the :

  REQUEST=GETMAP&
  LAYERS=TERRA.PATHFINDER,TERRA.BORDERS&
  BBOX=-10,32,20,47&
  WIDTH=200&HEIGHT=100&
  FORMAT=JPEG

Both cascanding layers (TERRA.PATHFINDER,TERRA.BORDERS) will be requested with the JPEG format. However, in this case the second layer is a transparent vectorial, that when drawn in the JPEG format will loose part of its transparency characteristics.

The solution to this problem is to tell the GIServer to request that specific layer with the GIF format, but maintaing the final resulting image with the JPEG format. This is done adding the TERRA.BORDERS.FORMAT=GIF tag to our request.

  REQUEST=GETMAP&
  LAYERS=TERRA.PATHFINDER,TERRA.BORDERS&
  BBOX=-10,32,20,47&
  WIDTH=200&HEIGHT=100&
  FORMAT=JPEG&
  TERRA.BORDERS.FORMAT=GIF

Because this last request does not follows WMS specification it is necessary to create a new STYLE in that layer registration.

With this the style DEFAULT was created with the value FORMAT=GIF. So the request :

  REQUEST=GETMAP&
  LAYERS=TERRA.PATHFINDER,TERRA.BORDERS&
  BBOX=-10,32,20,47&
  WIDTH=200&
  HEIGHT=100&
  STYLES=,DEFAULT&
  FORMAT=JPEG

With this solution it is possible to maintain the request in a given format while informing the GIServer to deal with specific layers in other formats.

See also: