If the pixel depth is greater, it is assumed the span occludes the previous spans at this pixel. The pixel's color and texture values are written into the frame buffer and the new depth is written into the Z-buffer. Finally, during display generation the digital frame buffer is scanned and converted to video signals. Most GSs support a variety of video signals. To prevent annoying screen flicker of horizontal lines, a non-interlaced video signal of at least 60Hz should be supported.
GSs range greatly in capabilities and price.
For VR, GSs which are part of a general development platform are usually more suitable. For GSs of this type the SGI Reality Engine 2 and the Evans and Sutherland Freedom Series 3000 sit at the high end. SGI's Indy and Intel's ActionMedia are low end examples. In addition to price, the most useful metric is the number of shaded, textured, antialiased polygons that can be drawn in a second. When combined with the desired frame rate of the application, this metric defines the maximum complexity of a scene in the virtual environment.