You can use the from_framestore utility with 12-bit clips. By default, when exporting a 12-bit clip, from_framestore creates 16-bit SGI image files with the 12 bits of data from the framestore in the 12 most significant bits of each 16-bit colour component. You can override this behaviour with the -l command line option, which forces the data to be stored in the 12 least significant bits of each 16-bit colour component.