if (v > 0 && u > 0) {
mesh.addFace(curr[v - 1], vert, prev[v - 1],
dUV.scale(u, v - 1), dUV.scale(u, v),
dUV.scale(u - 1, v - 1));
mesh.addFace(vert, prev[v], prev[v - 1], dUV.scale(u, v),
dUV.scale(u - 1, v), dUV.scale(u - 1, v - 1));
}
curr[v] = vert;
}
prev = curr;
if (u == 0) {