After changing the output type of my wms layers from png to gif, I noticed strange artifacts in one of my background layers. Are these the result of the dithering algorithm, or is it a bug?
Here are some getmap requests that show what I mean:
the image as it should be (png):
http://www.sarvision.nl/geoserver/wms?bbox=29,-1.8,29.2,-1.6&styles=raster&Format=image/png&request=GetMap&layers=srvn:ies_greatlakes_background2001&width=300&height=300&srs=EPSG:4326
the same request but as gif:
http://www.sarvision.nl/geoserver/wms?bbox=29,-1.8,29.2,-1.6&styles=raster&Format=image/gif&request=GetMap&layers=srvn:ies_greatlakes_background2001&width=300&height=300&srs=EPSG:4326
If you look closely, you'll see strange wave-like thin greenish lines in the blue parts. Now indeed these blue parts do represent water, but I'd rather add the waves myself if I wanted them to be there 
I also tried rendering a blue polygon as a reference, and depending on fill color I think I do see some stripy features, but I never managed to get them as clear as in the gif above.
Now probably Andrea is just going to tell me it's the built-in gif palette plus dithering, but I just want to be sure (it's kind of ugly, imho).