From the Open Planets Foundation Blog:
Automated identification of corresponding images and duplicate removal is a challenging task due to inconsistent quality of digitized book collections. Validating such collections based on qualitative criteria is a challenging endeavor according to the sheer amount of data that has to be processed. Traditional approaches seem to have peaked at a certain level. Scientists at the Austrian Institute of Technology developed a generalized method providing detailed comparison of corresponding images even on historical or handwritten texts.
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