To refine new features and capabilities in GEGL - GIMPs new engine - the gegl binary has a commandline and accompanying viewer / parameter tweaking UI that has been a useful development and validation aid. The best way to find bugs in libraries is to write software using them, and with a flexible editor we can do more flexible testing, many issues of weird interactions have already been discovered; and worked on as the interactions of this UI become more complete.
This tools has in the last month started evolving into a useful editor in its own right, permitting testing and development and testing of features like CMYK processing and mipmap previews.
This UI is both a proof of concept for GEGL (and for mrg, the immediate mode toolkit used), if all works well - most or all might eventually be rewritten in lua instead of C. Already now the constraints it is iterating within is both touch screen editing and keybinding friendliness.
Edit Jan 29/Feb 2/Feb16: more screenshots at various stages,
In the last ones has per-operation a live-editable lua scripts as can be seen in https://gitlab.gnome.org/GNOME/gegl/tree/master/bin/lua/ to build GEGL with this enabled dependencies needed luajit and development headers and mmm, mrg and mrl from https://github.com/hodefoting/mrg and similar.