This page documents the Edit › Options › Compatibility command dialog.
Some commands have an associated ImageJ 1.x compatibility mode flag. The flag setting affects how some commands behave in ImageJ. The authors of ImageJ2 want to provide a mechanism whereby ImageJ 1.x behavior can be overridden when it makes sense. This allows breaks to be made with the past if necessary. Setting the mode to “Modern” will override some ImageJ 1.x behavior while setting it to “Legacy” will maintain ImageJ 1.x behavior as much as possible.
Commands that are affected by this setting:
The Invert command - in ImageJ 1.x the Invert command inverts all pixels with respect to the minimum and maximum data values present in an image. Though the behavior is different with respect to 8-bit images which always invert relative to 0/255. When the compatibility mode setting is “Legacy” this behavior is maintained. When its setting is “Modern” 8-bit images will also invert relative to minimum and maximum data values.