I define the lower layers of Virtual Reality to be: the highly-accurate, real-time simulation by computer of the interaction of the physical world with human senses. My focus is on the visual system, the talk will desceribe the techniques used to perform this simulation in several running systems at Sun microsystems. These include: correct perspective viewing equations, correcting for the optics of both human eyeballs and glass CRT's, predictive head trackers and other hardware nasties. This talk is an expansion of my SIGGRAPH '92 talk to include details of the Virtual Portal, a 1K x 2K walk-in virtual display device.