  The question of the collimation of relativistic jets is the subject of a
lively debate in the community. We numerically compute the apparent velocity
and the Doppler factor of a non homokinetic jet using different velocity
profile, to study the effect of collimation on the appearance of relativistic
jets (apparent velocity and Doppler factor). We argue that if the motion is
relativistic, the high superluminal velocities are possible only if the
geometrical collimation is smaller than the relativistic beaming angle
$\gamma^{-1}$. In the opposite case, the apparent image will be dominated by
the part of the jet traveling directly towards the observer resulting in a
smaller apparent velocity. Furthermore, getting rid of the homokinetic
hypothesis yields a complex relation between the observing angle and the
Doppler factor, resulting in important consequences for the numerical
computation of AGN population and unification scheme model.
