Stencil.Buffer

State
Cited Tutorial
http://www.learnopengl.com/#!Advanced-OpenGL/Stencil-testing

Pipeline

Once the fragment shader has processed the fragment a so called stencil test is executed that, just like the depth test, has the possibility of discarding fragments. Then the remaining fragments get passed to the depth test that could possibly discard even more fragments. The stencil test is based on the content of yet another buffer called the stencil buffer that we're allowed to update during rendering to achieve interesting effects.

Stencil-Test

A stencil buffer (usually) contains 8 bits per stencil value that amounts to a total of 256 different stencil values per pixel/fragment. We can then set these stencil values to values of our liking and then we can discard or keep fragments whenever a particular fragment has a certain stencil value.

A simple example of a stencil buffer is shown below:

The stencil buffer is first cleared with zeros and then an open rectangle of 1s is set in the stencil buffer. The fragments of the scene are then only rendered (the others are discarded) wherever the stencil value of that fragment contains a 1. 

Stencil buffer operations allow us to set the stencil buffer at specific values wherever we're rendering fragments. By changing the content of the stencil buffer while we're rendering, we're writing to the stencil buffer. In the same (or following) render iteration(s) we can then read these values to discard or pass certain fragments. When using stencil buffers you can get as crazy as you like, but the general outline is usually as follows:

• Enable writing to the stencil buffer.

• Render objects, updating the content of the stencil buffer.

• Disable writing to the stencil buffer.

• Render (other) objects, this time discarding certain fragments based on the content of the stencil buffer.

By using the stencil buffer we can thus discard certain fragments based on the fragments of other drawn objects in the scene.

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