Creating photorealistic images is one of the ultimate goals of computer graphics. Previous work has shown that a material's microstructure plays a crucial role when trying to achieve photorealism. This is because a material's appearance depends on the roughness of its microstructure. Due to this dependence, effects such as masking and shadowing have to be taken into account, as these are capable of altering the effective reflectance of a material. Render engines typically use a mathematical expression, known as a visibility function, that aims to calculate the impact of these effects. However, even the best visibility function known is still an approximation; an exact solution doesn't exist. In order to evaluate the accuracy of visibility functions, an algorithm can be created that computes the correct output, such that the output of a given visibility function can be compared against it. Such an algorithm can be one of two types: approximative or exact. In this paper, we show that approximative algorithms are very capable and come close to their exact counterparts. However, there is still a non-negligible difference between them, meaning they aren't suitable for applications that demand very high levels of accuracy.