implementation of this algorithm available in the OpenCV library [@opencv_library].

The indices of the contour pixels of each void space are then uploaded to GPU memory. Moreover, we also upload to GPU memory the buffer in which, for each pixel, the identifier of the void spaces are stored. The last step of the pipeline renders a screen-aligned quad and computes the depth values for all pixels belonging to void spaces. To do so, we exploit a fragment shader, which determines the identifier of the void space adjacent to each pixel and iterates over the list of contour pixels of this space to compute the interpolated depth value by using Equation \[eq:isd\]. Then the interpolated depth is finally used to determine the color of the pixel based on the desired color scheme, chromadepth or pseduo-chromadepth, before we generate the iso-lines. Additionally, our fragment shader reconstructs a 3D position and normal in viewspace with which the illumination is computed.

Performance measurements
------------------------

We evaluated the performance of our GPU implementation on a computer with the following configuration: Intel i7 at 3.6 GHz, with 16 GB of RAM, and a GeForce GTX980, using a screen resolution of $1280\times720$ pixels. We measured the milliseconds 