The operation of DrawPixels was described in section 3.6.3,
except if the format argument was STENCIL_INDEX.
In this case,
all operations described for DrawPixels take place, but
window 
coordinates, each with the corresponding stencil index,
are produced in lieu of fragments.
Each coordinate-stencil index pair is sent directly to the per-fragment
operations, bypassing the texture, fog, and antialiasing application
stages of rasterization.
Each pair is then treated as a fragment for purposes of the pixel ownership
and scissor tests;
all other per-fragment operations are bypassed.
Finally,
each stencil index is written to its
indicated location in the framebuffer,
subject to the current setting of StencilMask .
The error INVALID_OPERATION results if there is no stencil buffer.