The default value is 1.0, which results in a medium contrast image. Values larger than 1 lighten the image and reduce contrast, while values less than 1 darken the image, increasing contrast. All flags can be abbreviated to their shortest unique prefix. The -gamma option isn't really necessary since you can achieve the same effect by piping the output from pgmcrater through pnmgamma . However, pgmcrater performs an internal gamma map anyway in the process of rendering the elevation array into a graymap, so there's no additional overhead in allowing a user-specified gamma.
Real craters have two distinct morphologies.