RawTherapee is an advanced program for developing raw photos and for
processing non-raw photos. It is non-destructive, makes use of OpenMP,
supports all the cameras supported by dcraw and carries out its calculations
in a high precision 32bit floating point engine.

By default, the SlackBuild bulding the gtk3 interface. If you want instead
to build the gtk2 interface download the gtk2 source and pass GTK2=yes to
SlackBuild.