Subtract the mean rotation of a vector field.
SF = subsbr(F,R0) subtracts the mean rotation of the vector field
(or field array) F, centered on R0 = [X0 Y0] in physical units (mm).
For each field F, the mean rotation is determined as (half) the curl of
F averaged over space. If R0 is not specified, the central point of the
field is taken.
subsbr is useful for processing vector fields originating from a camera
or device with a residual background rotation (solid-body rotation).
spaverf(vec2scal(subsbr(V,R0),'rot')) gives a zero scalar field for all
V and R0.
If no output argument, the result is displayed by showf.