Pre-filter （Ball-point pen filter）
The prefilter performs filtering before processing the Dynamic Thresholding.
The ordinary filter is the default.
Texts written by ball point pens actually have two lines per stroke. To the human eye this is not big problem, but OCR may
not recognize the text. The Ball Point Pen Filter averages these two lines into one solid line.
Example
| Ordinary Filter | Ball Point Pen Filter |
|---|---|
 |
 |
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