paper source:
- https://www.researchgate.net/publication/318123092_Adaptive_method_for_multi_colored_text_binarization
- http://www.m.cs.osakafu-u.ac.jp/cbdar2007/proceedings/papers/O1-1.pdf
note: implementation differs from the papers
- Canny threshold 100, 100
- Discard edge box area < 4 (paper 15)
- Discard edge box width / height > 5 (paper 10)
- Inner edge box counted when area >= 20
- Point 5 using K = 5 (paper 20), accepts large text
