let diff_q1q2 = diff!(dst, offset - 2 * step, offset - 3 * step);
let diff_p1p2 = diff!(dst, offset + step, offset + 2 * step);
let delta = if weak {
let diff_q1q2 = diff!(dst, offset - 2 * step, offset - 3 * step);
let diff_p1p2 = diff!(dst, offset + step, offset + 2 * step);
let delta = if weak {