How to display form as partially transparent

I have a transparent layer on the top of a form. I need to display only some part of that transparent form.

How could I achieve this ? Thanks

------edited------

If you think of a map and traffic signal icons displaying on the map. There are a map image layer and a traffic signal icons layer, which is tranparent except icons.

if you move the map location, the traffic signal icons should move along with. if the certain area of map does not appear on the screen by moving, corresponding traffic signal icnos should disappear as well. This is what I am trying to do here. Moving the开发者_运维知识库 trasparent form is straightforward, but I need to hide some portion of that transparent form.

     [Map]
[Traffic sig]    here I need to show    "fic s"  part only. 

This is for a very small image. Is there any other better way to do so ?

Thanks,

Break up the layer into its component parts? Maybe a more specific example of what you're trying to accomplish would help. Is this a layer of display graphics that can be partially hidden by other window elements, or are these sets of controls that should be shown/hidden based on business rules?