Add comment explaining why condition is necessary.
@@ -1472,6 +1472,9 @@ tsError ts_bspline_derive(const tsBSpline *spline, size_t n, tsReal epsilon,
k = i *dim + j;
l = (i+1)*dim + j;
ctrlp[k] = ctrlp[l] - ctrlp[k];
+ /* The new control point (ctrlp[k]) of
+ * a line (deg == 1) must not be scaled
+ * with `p / (u{i+p+1} - u{i+1})`.*/
if (deg > 1) {
kid1 = knots[i+deg+1];
ki1 = knots[i+1];
