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In the paper they write:

"To calculate the best harmonic scheme of an image, we use Brent’s algorithm [Press et al. 1992] to optimize the orientation α (Eq. 2) for each template. H".

PRESS, W. H., TEUKOLSKY, S. A., VETTERLING, W. T., AND FLANNERY, B. P. 1992. Numerical Recipes in C: The Art of Scientific Computing. Cambridge University Press, New York, NY, USA.

My guess is that you can start by just trying angles in a course to fine scheme until convergence.