Informally, a measure of the brightness of mid-level tones in an
image. Outside this specification, the term "gamma" is often used as
the exponent of a power function that is the transfer function of any
stage(s) of an imaging pipeline:
output = input ^ gamma
where both input and output are scaled to the range 0 to 1. Within
this specification, gamma refers specifically to the function from
display output to image samples.