bug in combining transparent wms servers?

[Steven M. Ottens]

Hi all,

I've got an UMN mapserver setup with several mapfiles which are used in
an educational website where students can add/remove layers and move
them up and down. I use different mapfiles so the data can be properly
categorised.
The website uses mapbuilder and it creates a seperate request for each
layer, so for 1 layer it gets 1 image, 2 layers; 2 images, etc. The good
part is that it is very easy to hide layers or move them relatively to
each other. The bad part is that they cannot save the map and add it in
their report. So I'm trying to write a print function which solves that
problem by creating one image which can be saved.
To do so I've written a mapfile which includes the other mapfiles as wms
servers (http://jana.geodan.nl/steveno/mapserver/base_combined.map) this
works if there are no transparent layers in the stack.
If I do have 1 transparent layer on top of a normal layer:
http://jana.geodan.nl/steveno/mapserver/2layersmapbuilder.png
it still works:
http://jana.geodan.nl/steveno/mapserver/2layerscombined.png
But if I add an extra layer between the bottom one and transparent one
which is a lines only layer:
http://jana.geodan.nl/steveno/mapserver/1layermapbuilder.png
I get the following look in mapbuilder:
http://jana.geodan.nl/steveno/mapserver/3layersmapbuilder.png
but mapserver combines it to:
http://jana.geodan.nl/steveno/mapserver/3layerscombined.png
Basically it only combines the transparent top layer with the layer
below, but not with other visible layers.

Is this an error in my base_combined.map file or a limitation/bug of
mapserver and is there a solution?

Many thanks,

Steven M. Ottens
www.edugis.nl