Function sample_intensities_fsl 

fn sample_intensities_fsl(
    data: &[f64],
    nx: usize,
    ny: usize,
    nz: usize,
    point_mm: &[f64; 3],
    normal: &[f64; 3],
    voxel_size: &[f64; 3],
    t2: f64,
    t: f64,
    tm: f64,
) -> (f64, f64, bool)

Sample min/max intensities along inward normal (matches FSL-BET2’s step_of_computation)

FSL samples from 1mm to d1 (7mm) for Imin, and up to d2 (3mm) for Imax. Initial values: Imin = tm, Imax = t Final clamps: Imin >= t2, Imax <= tm

point_mm: vertex position in mm coordinates normal: unit normal vector (outward pointing)

Returns (i_min, i_max, success) where success=false means sampling failed and the caller should use f3=0 (like FSL does).