Hey, I just started with the book and I have a question on formula 2.68. Here the constraint
D^TD=I_l
is stated. But this would mean that D has to be a orthogonal matrix with n=l. And this would miss the whole point of the compression. At the beginning of section 2.12 it is explained that l<n, columns of D are orthogonal and
||D_{:,i}||_2 = 1
Is the constraint above a mistake?