pnmcurve is a command line tool that applies tone
curves to pnm images. Curves are specified as a
set of control points of a cubic spline. pnmcurve
applies the resulting spline curve to the pnm
image read from stdin and writes the resulting
image to stdout. Command line switches allow the
user to apply the curve only to selected color
channels. A separate FLTK-based graphical curve editor helps to set the control points.