1. A computer graphics system for generating a pixel value for a pixel in an image, the pixel being representative of a point in a scene, the computer graphics system generating the pixel value by an evaluation of an integral of a selected function, the ray trace the computer graphics system comprising: A. a sample point generator configured to generate respective sets of sample points each associated with one of a series of rays in a ray trace configured to have a plurality of trace levels, the ray at at least one level being split into a plurality of rays, each ray being associated with a ray instance identifier, the sample point generator being configured to generate the sample points as predetermined strictly-deterministic low-discrepancy sequence to which a selected rotation operator is applied recursively for the respective levels; and B. a function evaluator configured to generate a plurality of function values each representing an evaluation of said selected function at one of the sample points generated by said sample point generator and use the function values in generating the pixel value: