It has been observed that filter banks from a certain popular separable 2D wavelet class always appear to have a higher coding gain with respect to a separable image model than with respect to an isotropic one. This behavior is examined in detail and an explanation for it is offered. As it turns out, for such filter banks, it is highly improbable (if not impossible) for the isotropic coding gain to exceed the separable coding gain.
Michael D. Adams