A 3x3 median filter cut OCR errors on a book page, but the same filter backfired on a half-size scan

A developer tested whether a 3x3 median blur could stay as a fixed step before OCR, sweeping kernel sizes on one page of H. G. Wells's The Time Machine. At full size, the 3x3 filter cut Tesseract's character errors from 11 to 2, while larger kernels became harmful. When the same page was scaled to half width, the same 3x3 filter raised errors from 4 to 28, and no kernel helped. The author suggests the effect depends on how the filter's window compares with letter stroke width, though this is a hypothesis from a single page and single run. The results indicate the right preprocessing depends on scan resolution rather than being a constant.