In the function
glStencilFunc(GLenum func, GLint ref, GLuint mask)
what is the role of mask , we are sending the ref value and the buffer content is being compared to the ref value than why do we need to send mask value ?
what i read about the mask value is "mask that is ANDed with both the reference value and the stored stencil value" but why do we need that?
It all depends on what you're doing with the stencil. Some stencil operations treat the stencil value as just an integer, incrementing and/or decrementing the stencil value for some effect or purpose. Other operations use it as a bitmask, where each bit or group of bits in the stencil value represents something in particular. That is, from a semantic perspective, you have multiple values that just so happen to be stored in the same 8 bits of space.
The mask value is for cases like that. You might have 8 separate masking regions, and you want to render some geometry against one of the region. You use the mask parameter to specify which bit (that specifies a region) to mask off a piece of geometry.