Full matrix for a grid of 6 × 6 pixels (left), approximated by 9 blocks of 2 × 2 pixels (middle) and 4 blocks of 3 × 3 pixels (right). The cost increases rapidly with the number of pixels N, as the inversion of the resulting matrix scales  ∝ N³. In the upper right corner, the spatial distribution of the pixels is indicated by the color of the block they belong to. The full matrix has in addition the cross-correlation function, Y, inserted in the lower left corner.