A comment on “A novel approach for the registration of weak affine images”

• Mapping models used in image registration should form a group.
• We show that so-called weak affine transform is neither invertible nor closed.
• We show there exist affine subgroups which can serve as restricted affine mapping instead of weak affine transform.

In the recent paper (Li and Zhang, 2012) a mapping model of weak affine transform (WAT) was proposed for registering images of flat scenes. We show that such a model may cause inaccuracies when registering images with non-uniform scaling differences, even if they seemingly behave according to this model. Mathematical reason of this is that WAT is neither invertible nor closed. We explain why mapping models forming a group should be preferably used for registration and show some examples of models belonging to affine subgroups.