@noiob If this is anything close to what you have in mind, this was produced with GraphicConverter, using the Picture: Levels facility. There, you have three pointers you can drag, corresponding to the min/max/median levels in an image, making it simple to bring all "sort of white" shades to just plain white.
https://lgbt.io/media/yiLmk5ccj3a-JDENnLM
https://www.lemkesoft.de/en/products/graphicconverter/download/
(Similar "level crushing" can be found in most graphics apps - GC's just a very handy tool I keep around)