Abstract: Real-time graphics hardware is becoming programmable, but this programmable hardware is complex and difficult to use given current APIs. Higher-level abstractions would both increase programmer productivity and make programs more portable. However, it is challenging to raise the abstraction level while still providing high performance. We have developed a real-time procedural shading language system designed to achieve this goal. Our system is organized around multiple computation frequencies. For example, computations may be associated with vertices or with fragments/pixels. Our system's shading language provides a unified interface that allows a single procedure to include operations from more than one computation frequency. Internally, our system virtualizes limited hardware resources to allow for arbitrarily-complex computations. We map operations to graphics hardware if possible, or to the host CPU as a last resort. This mapping is performed by compiler back-end modules associated with each computation frequency. Our system can map vertex operations to either programmable vertex hardware or to the host CPU, and can map fragment operations to either programmable fragment hardware or to multipass OpenGL. By carefully designing all the components of the system, we are able to generate highly-optimized code. We demonstrate our system running in real-time on a variety of hardware.