Rezen is written primarily in Python and uses the Enthought Tool Suite (code.enthought.com) extensively, in particular, Traits for the GUI, Chaco for line plots, bar charts and 2D plots and Mayavi for 3D. NumPy is used for fast and efficient array handling and SciPy for statistical and optimization functions. 

All these Python packages are open source projects with an extensive number of contributors. Visual Reservoir acknowledges this excellent work. 

Traits can use either Qt or wxPython as an underlying layer. Rezen uses the wxPython layer.

Mayavi uses the VTK visualization toolkit which itself uses OpenGL.

The Python code also interfaces to some Fortran and C legacy code.

A small amount of Tcl scripting is also used. 

A framework diagram is shown below.

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