Correction

The formula in the last two sentences of Section 2. "Texture Representation"
misses two signs for partial derivations.

original:

$Y_{r}^{(k)} = \lbrack {\partial Y_{r}^{(k)}\over r_1},{ \partial Y_{r}^{(k)}\over } \rbrack^T$

formula correction:
$Y_{r}^{(k)} = \lbrack {\partial Y_{r}^{(k)}\over \partial r_1},{ \partial Y_{r}^{(k)}\over \partial r_2} \rbrack^T$