The two ways to control visablility are through the Visibility/Opacity Contols (as mentioned above) and through an Alpha mask (8bit image file) applied to an object. This is a way to control some pretty cool special effects. It sounds a little more advanced than what you need - which would be the first method, but you should play arround with it.
- Take an two overlapping objects (start with spheres), apply different surface textures. Apply an alpha map to outer object. This will make one object the reveal the second (UV mapping allows position persision of the map) this can be used for "Cut-away" views of a model (for example). (Note: The usage all depends on the situaltion and your creativity.)
This method allows for only parts of the object to be disolved.-
A simple use of this is to use a 32bit image/mov file and map it on a plane. Now you have a video that you can add to you 3d that is fully integrated into the scene. This used to be called a "Clip Map" as the alpha clips the plane to reveal the desired portion of the image.
We used this in 2003 animation to include a student in a space suite floating is space (suite and space are all 3D) and in 2005 to include me inside a house viewed through a window as the camera pulled out. as well as for the student's head in the bed (sleeping)
shsrobotics.org Check out the media page to see what I mean.