Direct rendering in the manner SGI describes in this paper is not the only option for implementing direct rendering. And direct rendering is not a necessity for production OpenGL implementations. The IBM OpenGL implementation described in [7] does not utilize direct rendering. Most currently available OpenGL implementations do not support direct rendering.
Previous to IBM's support for the OpenGL standard, IBM licensed IRIS GL from SGI for 3D hardware in the original RS/6000. Their IRIS GL implementation uses virtualized, direct rendering much like SGI's IRIS GL implementation [16,5].
Hewlett-Packard provides direct rendering support for their Starbase Graphics Library [2] with an approach that is different from SGI's direct rendering mechanism. Hewlett-Packard's approach acquires a fast lock to be held during rendering to the graphics engine. This locking allows clipping to be coordinated in software via shared memory window clip serial numbers and proprietary X requests to query the current clip of a window. While such a system avoids the complex hardware and operating system support involved in SGI's virtualized, direct access rendering mechanism, it forces explicit, fine-grain locking to arbitrate access to the graphics hardware.