borders. The contour of this area is composed of a high number of pixels that have to be iterated in order to properly interpolate the depth value in the VSSs, thus slowing down the performance. Table \[tab:perf\] also provides the maximum number of contour pixels for a single empty area and the number of contours for each visual configuration in parenthesis. As described before, the configuration in which the camera is placed far away from the object exhibits a high number of contour pixels compared with the other configurations, whilst the total number of enclosed areas is smaller.

Although we achieve interactive frame rates, when combined with other expensive rendering techniques, such as direct volume rendering and screen space ambient occlusion, we have investigated further approaches to increase the achieved frame rates. Thus, instead of iterating over all the contours pixels, the fragment shader can skip a user-defined number of pixels. As presented in Table \[tab:perf\], the algorithm can be sped up by more than a factor of 2 by simply increasing the step size to 3 or 5 during the iteration of the contour pixels. However, this optimization is not free of drawbacks. Skipping a high number of pixels 