Real-Time “Photorealistic” Shading

One of the main techniques used by software renderers to produce stunningly realistic images is programmable shading—executing an arbitrarily complex program to compute the color at each pixel. Thus far, programmable shading has only been available on software rendering systems that run on general-purpose computers. Rendering each image can take from minutes to hours. Parallel rendering engines, on the other hand, have steadily increased in generality and in performance. We believe that they are nearing the point where they will be able to perform moderately complex shading at interactive rates. Some of the obstacles to this are imposed by hardware, such as limited amounts of frame-buffer memory and the enormous computational resources that are needed to shade in real time. Other obstacles are imposed by software. For example, users generally are not granted access to the hardware at the level required for programmable shading. This paper first explores the capabilities that are needed to perform programmable shading in real time. We then describe the design issues and algorithms for a prototype shading architecture on PixelFlow, an experimental graphics engine under construction. We demonstrate through examples and simulation that PixelFlow will be able to perform high-quality programmable shading at real-time video rates. We hope that our experience will be useful to shading implementors on other hardware graphics systems.