all the fragments and compare their depth information to ensure that
only fragments that will eventually be rendered to screen are
submitted to the fragment shading stage (aka Hidden Surface
Removal, HSR). Therefore, the fragment shading is not related to
number of vertices, but number of visible fragments instead. As
shown in Fig. 7 (a), the slope of the fragment shading time is
dependent to the fragment shader used. Setup cost is dependent on
the resolution shown in Fig. 7