int x= vi & XMASK; /* distance along strip */
int y= vi >> YSHIFT; /* distance across strip */
double u= y * 1.0 / (Y-1); /* SGT's u runs 0..1 across the strip */
int x= vi & XMASK; /* distance along strip */
int y= vi >> YSHIFT; /* distance across strip */
double u= y * 1.0 / (Y-1); /* SGT's u runs 0..1 across the strip */