TIL from my wife in the context of checkbox detection! letters detection fourier transform – Google Suche

• Introduction to the Fourier Transform for Image Processing
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• crowoy/Letter-Recognition: Fourier Feature Analysis - Classifying letters [S,T,V] based on features in the Fourier Space

TL;DR you can use fourier transforms on letters, that then lead to differentiable results! Bright lines perpendicular to lines in the original letter etc.