The -gamma switch is used to lighten (higher values) or darken (lower values) the image the default is 0.0.
This command is a shell command, and is available from TCL in the same fashion that other UNIX commands are.
COMMAND SYNTAX: xdither [file] [-title] [-width] [-height] [-noreverse] [-gamma] [-dpi] [-thresh] [-sat] [file] Optional: File to put Postcript into (def: stdout) [-title] Optional: Title to print (def: gamma used.) [-width] Optional: width of paper in inches (def: 8.5) [-height] Optional: Height of paper in inches (def: 11.0) [-reverse] Optional: If present do reverse the image [-gamma] Optional: Lighten or darken the image (def: 0.0) [-dpi] Optional: Target resolution of printer (def: 300)