[MapServer-users] Changing opacity of one layer in a multilayer WMS request URL with MapServer 8

[Matthew Graber]

Hello all!

With MapServer 8, I'm wondering what the proper way to change the opacity
of a single layer in a multilayer WMS request is.

For example, the following URL excerpt shows the old way of accomplishing
this that worked while using MapServer 7:

SERVICE=WMS&VERSION=1.3.0&REQUEST=GetMap&FORMAT=image%2Fpng&LAYERS=test_a,test_b&map.layer[test_b]=OPACITY+50

My understanding is that this would have the same effect as adding "OPACITY
50" to the test_b layer in the mapfile. The mapfile containing test_a and
test_b did not have OPACITY included when they were written. Obviously,
this no longer works with MapServer 8, and now that request has the same
results as omitting the "&map.layer[test_b]=OPACITY+50" portion entirely.

Based on https://mapserver.org/fr/MIGRATION_GUIDE.html, I see that OPACITY
now belongs inside a COMPOSITE object. I tried the following modifications
to the request URL:

SERVICE=WMS&VERSION=1.3.0&REQUEST=GetMap&FORMAT=image%2Fpng&LAYERS=test_a,test_b&map.layer[test_b].composite=OPACITY+50
SERVICE=WMS&VERSION=1.3.0&REQUEST=GetMap&FORMAT=image%2Fpng&LAYERS=test_a,test_b&map.layer[test_b].composite[0]=OPACITY+50

I also tried this both with the mapfile left as-is, and with an empty
COMPOSITE object being included in each layer object, and each time I got
the same unsuccessful result as before.

Is there a way to accomplish this in MapServer 8?

Also, does this map.layer[layername]=... syntax still work in MapServer 8?
The documentation at
https://mapserver.org/cgi/controls.html#using-mapserver-version-8 suggests
it might no longer work for MapServer 8 (based on the heading "Using
MapServer version < 8"), but the documentation at
https://mapserver.org/pdf/MapServer.pdf shows the same examples with the
heading "Using MapServer version <= 8" on page 1065 of the PDF, so I'm
unsure if this is still intended to work in MapServer 8.

Thank you,
Matt
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