An objective criterion for stopping light–surface interaction. Numerical validation and quality assessment

In most algorithms of global illumination, light–surface interaction terminates declaring that result at some point is close enough to some reference ground truth data. The underlying principle of such criterion is to minimize the processing time without compromising the (subjective) visual perception of the resulting image. We introduce an objective-driven condition for stopping the simulation of light transport. It is inspired by the physical meaning of light propagation. Besides, it takes into account that computations are performed in finite precision. Its main feature is the definition of the threshold establishing the maximum number of pixels that are completed in finite precision. Its value is computed at run time depending on the brightness of the image. As a proof of concept of the validity of this approach, we employ the stopping condition in a light tracing algorithm, propagating light that is generated by the light source. We assess the quality of the computed image by measuring the Peak Signal-to-Noise Ratio and the Structured Similarity Index error metrics on the standard scene of the Cornell Box. Numerical validation is performed by comparing results with the output of the NVIDIA ®® Iray render whose stopping condition is based on Russian roulette and on the elapsed time.