Why is the derivative of solid angle expressed as dvx dvy in Lightmann et al.?

[zn53]

hello
please see attached snapshot from the book of Ligthmann & al. (problem book in relativity and gravitation).
Can somebody please explain the expression for the derivative of the solid angle ?
here it is given as dv_xdv_y where the v's are speeds !
How come this is so ?
Thanks,

 

[dauto]

The solid angle is dv_xdv_y/v² but the problem is about photons so v = c = 1 in natural units.