  Direct methods are widely used for alignment of models to images, due to
their accuracy, since they minimize errors in the domain of measurement noise.
They have leveraged least squares minimizations, for simple, efficient,
variational optimization, since the seminal 1981 work of Lucas & Kanade, and
normalized cross correlation (NCC), for robustness to intensity variations,
since at least 1972. Despite the complementary benefits of these two well known
methods, they have not been effectively combined to address local variations in
intensity. Many ad-hoc NCC frameworks, sub-optimal least squares methods and
image transformation approaches have thus been proposed instead, each with
their own limitations. This work shows that a least squares optimization of NCC
without approximation is not only possible, but straightforward and efficient.
A robust, locally normalized formulation is introduced to mitigate local
intensity variations and partial occlusions. Finally, sparse features with
oriented patches are proposed for further efficiency. The resulting framework
is simple to implement, computationally efficient and robust to local intensity
variations. It is evaluated on the image alignment problem, showing
improvements in both convergence rate and computation time over existing
lighting invariant methods.
