DM, you are probably talking about boolean objects, I will try to wrap it in a nut shell for you
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When you create an object like a sphere, you get this list of icons next to it when you select it (the M for material, and then you have the A.)
When you press the A you get a screen with object proprety's. It should be neutral. if you put the neutral to positive.
Then create a second object, like a box, and select the A.
Put the object proprety to negative. when you put the box half way over the sphere, so you only see the top of the sphere select both of them, and press the G icon (this will group them.) now, if evrything works and you render the image, you should sea a half sphere.
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hope this helps...
