>Psychtoolbox>PsychGLImageProcessing

rval = kPsychNeedDualWindowOutput

Return a flag that you can pass to the ‘imagingmode’ parameter of
Screen(‘OpenWindow’) in order to request support for output of the same
visual stimulus to two different onscreen windows, but with separate
post-processing and output formatting for each window in the builtin
imaging pipeline.

FIXME: More info…

It is part of the Psychtoolbox imaging pipeline and only works on modern
graphics hardware, e.g., ATI Radeon 9600 and later, NVidia GeforceFX 5000
and later. Check the www.psychtoolbox.org Wiki for graphics hardware
recommendations and for a description of PTB’s image processing
capabilities.