Question

I’m trying to make a small XNA-based game, and I need to be able to draw a single texture inside multiple mobile circles around the screen, as if they were ‘spotlights’ revealing parts of a bigger picture.
While searching for how I would be able to do that, I found that stencils might be able to help me accomplish that, but I have no idea on how I’d use the stencils to do that.
If anyone has any information or ideas on how I can do that, I’d be very grateful.

Edit: I forgot to mention the game is in 2D.

Answer 1

To start with, you need a mesh in the shape of the desired stencil, in this case a circle. XNA doesn’t support many primitives, so you will need to approximate the circle with triangles.

Next, you render that mesh almost as normal but with:

graphics.DepthStencilState.DepthBufferFunction = CompareFunction.Never;
graphics.DepthStencilState.StencilEnable = true;
graphics.DepthStencilState.ReferenceStencil = 1;
graphics.DepthStencilState.StencilPass = StencilOperation.Replace;

Now you have a stencil with the holes.

Then, you render the texture through the stencil, with normal settings but with:

graphics.DepthStencilState.StencilEnable = true;
graphics.DepthStencilState.ReferenceStencil = 1;
graphics.DepthStencilState.StencilFunction = CompareFunction.Equal;

For more information see the reference for the DepthStencilState class.

This is based on my knowledge of 3D. You may have to do more stuff if you want to use sprites.