This is stub article and needs to be expanded by somebody with pictures etc.. please go ahead.
Barrel distortion is the opposite of pincushion distortion.
Tools to correct barrel distortion
- The original PTStitcher can be scripted to batch process images with known a, b & c parameters. It can also be operated with one of the GUI tools such as PTGui, PTAssembler, hugin or PTMac.
- The Correct filter in the Panorama Tools Plugins for the gimp or photoshop uses the same a, b & c parameters as PTStitcher. Note that it doesn't know about d & e parameters and uses 'd' as an overall scaling factor instead.
- PTLens is a Photoshop plugin and a stand-alone Windows tool that uses the same a, b & c parameters and comes with a database of popular lenses.
- Gimp wideangle plugin uses a different formula altogether to correct distortion.
- Gimp phfluuh plugin is another tool that corrects lens distortion using yet another formula.
- CamChecker is a tool for automatically determining lens distortion and generates a different set of parameters from everything else.