Function kernel_lim 

pub fn kernel_lim(rdf: &[f64], mask: &[f64], grid: &Grid, b0: f64) -> f64

kernel_lim.m: the shadow threshold t in radians (see the module docs).

mask is the brain mask as 0/1. Where kernel_lim.m would loop forever (the residual can never reach tmin) this returns tmin, which flags no voxel, as any value kernel_lim.m could return would not; and the search stops once the sphere is larger than the volume.