The alternative here to achieve similar visual effects would be use multiple layers, correct? I could see this being more broadly popular so we really should expose this functionality via the mapfile/mapscript as well. I think that means exposing the layer RENDERMODE property and then the associated values. I guess that would mean on supporting something like:

RENDERMODE FIRSTCLASS|ANYCLASS

@geographika might have some opinion in terms of syntax. ANYCLASS is probably more descriptive than painter mode.

Would a possible option be to have it as a PROCESSING directive? E.g.

PROCESSING "RENDERMODE=FIRSTCLASS|ANYCLASS"

I think this would then already be supported by MapScript looking at this test, without need for modifications.

A couple of other options for the option naming (agree MS_PAINTERS_MODEL may be too technical:

FIRSTCLASSMATCH|ALLCLASSMATCH

Or slightly more literal but longer:

FIRSTMATCHINGCLASS|ALLMATCHINGCLASSES

With regards to using multiple classes from the same layer for a single feature, it is possible using GROUP in the class and then (at least with WMS) passing in style=group1,group2.

We don't have a good definition on when to add a full-on attribute vs using a processing directive. I agree that this feels more like a the latter...

You read my mind! Exposing layer RENDERMODE property was something I thought of when developing the feature for SLD. I will be more available on september for implementing this. I let you choose the syntax details that best fit existing usage.

I wonder what will happen if CLASSes have STYLEs with OPACITY. What is the result when RGBA 200 0 0 128 is rendered over RGBA 0 200 0 128?

@geographika, @sdlime: As suggested, I have added a processing directive:

PROCESSING "RENDERMODE=FIRST_MATCHING_CLASS|ALL_MATCHING_CLASSES" 

@jbo-ads cool feature, thanks!