Earlier quoted context omitted.
Patents are about processes and if you read this patent it is very specific. Your link even mentions that there have been multiple programs that do this for a long time.
Patents are not only about processes, they can also cover the final product amongst other aspects. The patent in question also does the same [1], as visible via claims 14, 29, 46, 51, whereas the claim 1 is about the "method". Reading the claim #1 of the said patent, I do not find it "very specific" as you said. It is quite broad within the purposes of making a mosaic image. One way of avoiding infringing claim 1 is…
Also matching by root mean squared error is pretty specific. It is possible to match based on other error criteria.