Stencil buffer VS primitive tesselation

I am learning opengl es and am planning to make a program which will have a shape which can be cut into a smaller shape by removing a part of the shape dynamicly. The constraint is I must be able to tell if an object is inside or outside the cut shape.

The option I thought of are: 1) use a stencil buffer made up of just a black and white mask. This way I can also use the same map for collision detection. 2) the other option is to dynamicly change my mind renderd primitive an then tesselating it. This sounds more complex and is currently开发者_StackOverflow社区 my least favorite option. It would also make the collision detection more difficult.

PS I would like the part of the shape removed to be fall of in animation, I am not sure how choosing any of these methods will affect the ease of doing so. Please express your opinion.

What are your thoughts on this? Keep in mind that I am new to opengl an might be making mistakes without realizing it.

Thanks, Jason

It is generally considered a good idea to issue only write-commands to the graphics card. Basically that is "dont use glGet* commands at all", because the latency of those commands might be somewhat high.

That said option 1) is great if you just want to mask out stuff. As you are trying to make the cut part fall off this is really not an option, as you have to retrieve/reconstruct the vertices of that part.

I don't quite get the "tesselation" part of your second option, but if your primitive is a polygon and your cuts are straight lines, it is easy to calculate the 2 polygons after the cut. In fact the viewport clipping routine in OpenGL does that all the time and there is a lot of literatur, for example http://en.wikipedia.org/wiki/Sutherland-Hodgman

In the long term it is often way better to first build a (non-visual) model of what is going on in the application before visualizing.