Drawing a Circle on a plane, Boolean Subtraction - OpenGL

Question

I'm hoping to draw a plane in OpenGL, using C++, with a hole in the center, much like the green of a golf course for example.

I was wondering what the easiest way to achieve this is?

It's fairly simple to draw a circle and a plane (tutorials all over google will show this for those curious), but I was wondering if there is a boolean subtraction technique like you can get when modelling in 3Ds Max or similar software? Where you create both objects, then take the intersection/union etc to leave a new object/shape? In this case subtract the circle from the plane, creating a hole.

Another way I thought of doing it is giving the circle alpha values and making it transparent, but then of course it still leaves the planes surface visible anyway.

Any help or points in the right direction?

Answer 1

I would avoid messing around with transparency, blending mode, and the like. Just create a mesh with the shape you need and draw it. Remember OpenGL is for graphics, not modelling.

Answer 2

There are a couple ways you could do this. The first way is the one you already stated which is to draw the circle as transparent. The caveat is that you must draw the circle first before you draw the plane so that the alpha blending will blend the circle with the background. Then when you render the plane the parts that are covered by the circle will be discarded in the depth test. The second method you could try is with texture mapping. You could create a texture that is basically a mask with everything set to opaque white except the circle portion which is set to have an alpha value of 0. In your shader you would then multiply your fragment color by this mask texture color so that the portions where the circle is located are now transparent. Both of these methods would work with shapes other than a circle as well.

Answer 3

I suggest the stencil buffer. Use the stencil buffer to mark the area where you want the hole to be by masking the color and depth buffers and drawing only to the stencil buffer, then unmask your color and depth, avoid drawing to the stencil buffer, and draw your plane with a stencil function telling OpenGL to discard all pixels where the stencil buffer "markings" are.