I am wondering if this is possible to assign a map/option for "opacity" of the materials. so that I could make a perforated metal sheet i.e.?
That should already be there unless you are talking about something else. You can plug a map into the material's opacity input.
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Also I may not be able to imagine that a metal/mirror material is emitting light.. is there any real-life example?
I don't think so, but artists might still want to use it. Is that something that bothers you? 😊 The implementation of emission is shared by all materials so that I can put it on every material at no extra cost.
Cheers,
Thomas