MirrorMode

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MirrorMode -- Issues regarding mirror mode on dual display setups.

If you run your system in a dual-display or multi-display configuration,
you usually have the option of either running the displays as separate
entities or in mirror-mode (aka clone mode). In mirror mode, all displays
show the same visual content. They run at the same resolution and color
depths.

There are reasons pro- and against using mirror mode while running
Psychtoolbox. You'll have to choose your trade-off.

Good reason to use mirror mode: Better timing, higher performance

In mirror mode, all physical displays are driven by the same framebuffer,
the one controlled and occupied by the Psychtoolbox onscreen window.
This means that Psychtoolbox is in full control of the graphics hardware,
all VRAM video memory and all hardware ressources are devoted to visual
stimulus generation and presentation. This allows to cache the maximum
amount of textures in the onboard VRAM memory of the gfx card for maximum
image drawing speed and most reliable timing, because all/most textures
can be prefetched and kept resident in the fast VRAM, and even if PTB
needs to fetch textures from system memory, it can do so at maximum speed,
because the AGP or PCI-Express bus is allocted solely for Psychtoolbox
needs. You can also run the highest display resolutions and Anti-Aliasing
multisample levels in this configurations.

Good reason to use desktop spanning mode or separate displays: Display of
binocular stimuli on two different physical displays (binocular rivalry,
stereo stimuli, ...): Still high performance and good timing.

If you open a single stereo onscreen window in desktop spanning mode or
two/multiple separate Psychtoolbox onscreen windows on separate non-mirrored
displays, timing and performance will still be good, because Psychtoolbox
is still in full exclusive control of the graphics hardware. This allows it
to manage, and possibly share, the ressources between the different windows
in an intelligent way, trying to preserve as much texture caching as possible.

Bad reason to use non-mirror mode: Having one display with the Matlab window or
user interface, the other for Psychtoolbox.

The graphics hardware now has to maintain at least two separate framebuffers, one
for each display. It also has to maintain different memory buffers for textures,
3D geometry and context state for each single application that is visible: At
least one OpenGL context for the Matlab window, at least one context for the Aqua
user interface, at least one context for each other visible application, at least
one for Psychtoolbox. All these buffers take away precious VRAM memory that could
be used by Psychtoolbox for more efficient stimulus drawing or image caching.
Bus bandwidth needs to be shared between Psychtoolbox and all other applications,
hardware computation time is splitted (and thereby taken away from PTB) between all
running OpenGL applications. There is also significant overhead when multiple
applications are simultaneously updating the display and the rendering engine has
to switch between the different clients in a round-robin timesharing fashion.
All in all this can significantly reduce drawing performance and introduce
timing jitter in the order of multiple (dozens) milliseconds.

The severity of this degradation and timing jitter depends strongly on the exact
type of script run, the type and size of stimuli, what kind of applications are
running in parallel and on the speed and amount of VRAM of your graphics hardware.

Some studies may not be seriously affected, others will be seriously impaired.

Another reason for not using mirror mode could be because you must! Some laptops
seem to be incapable of properly running an external display with proper synchronization
when switched to mirror-mode. In that case it may be still a good idea to open a
onscreen window on the "unused" display, even if it does not show any stimulus. This
way, Psychtoolbox at least keeps full control over the gfx-hardware, minimizing
interference from other applications.

Choose your poison, live is full of tradeoffs ;-)