when I posted the question I would never have imagined that we would get to Java code.
as it happens, I'm a Java programmer, so I can read your answer. To me this code seems to transform an image like this any white pixel becomes a space, anything else becomes a $. This would work for black and white images like the original apple logo
however, I like using mockofun's ascii filter because it seems to use different brightness levels associated with different characters. I read somewhere that they actually map each brightness level to a specific character according to the character's "coverage"
they seem to have really put some thought into the multiple types of ascii art like for ex. using the Newskool characters.
having said that, I'd like to thank you all for giving me a very good insight into this and helping me find out more about monospace fonts and ascii art