Always passes.
Passes if (referenceValue & readMask) = (stencil buffer value & readMask).
Passes if (referenceValue & readMask) > (stencil buffer value & readMask).
Passes if (referenceValue & readMask) >= (stencil buffer value & readMask).
Passes if (referenceValue & readMask) < (stencil buffer value & readMask).
Passes if (referenceValue & readMask) <= (stencil buffer value & readMask).
Always fails.
Passes if (referenceValue & readMask) != (stencil buffer value & readMask).
Specifies the stencil test function. The initial value is "Always".