[luatex] Get area of a glyph?

[Arno Trautmann]

Hi all,

again a rather unusual question: I'd like to get the black area of a 
glyph. Why? To estimate it's greyness: For a “–” e.g. I want to get 
something like “5%” (of the whole box surface), while a “m” would give 
some “30%”, and a black box of course would result in 100%. Of course I 
would also be happy with any absolute value like “0.1 square inches” or 
whatever. So: Is there any chance to read such a value from inside 
LuaTex? Or is this information not available?

In the end, I want to estimate the greyness of a document based on the 
amount of “ink” used instead of Paul's (great!) ansatz in his TUGboat 
article based on the stretching and shrinking of glue.

cheers
Arno