Some engines might automatically yield a correct Z buffer, in which case you have no problems at all. In general this is more complicated. Now we would like to animate our sprite. This can be done quite easily if we keep the triangles constant and just change the points. To do this, we will define 'frames'.
Every frame is in fact a different set of the same points.
Again we will try to explain with an example. Each frame contains the same four points but with other coordinates. In this particular case it is the fourth point that moves down from 'fr1' to 'fr6'. At run-time, there is always one frame active. To animate, we just select another frame.