option "input" i "Input image filename" string yes
option "output" o "Output image filename" string yes
-option "vf" - "Vector field filename" string yes
+
+defgroup "DVFoption" groupdesc="an option of this group is required" required
+groupoption "vf" - "Vector field filename" string yes group="DVFoption"
+groupoption "coeff" - "B-Spline coefficients filename" string yes group="DVFoption"
section "Options"