[windowPtr,rect]=Screen(‘OpenWindow’,windowPtrOrScreenNumber [,color] [,rect][,pixelSize][,numberOfBuffers][,stereomode][,multisample][,imagingmode][,specialFlags][,clientRect][,fbOverrideRect][,vrrParams=]);
Open an onscreen window. Specify a screen by a windowPtr or a screenNumber (0 is
the main screen, with menu bar). “color” is the clut index (scalar or [r g b]
triplet or [r g b a] quadruple) that you want to poke into each pixel; default
color is white.
If supplied, “rect” must contain at least one pixel. “rect” is in screen
coordinates (origin at upper left), and defaults to the whole screen. (In all
cases, subsequent references to this new window will use its coordinates: origin
at its upper left.). Please note that while providing a “rect” parameter to open
a normal window instead of a fullscreen window is convenient for debugging,
drawing performance, stimulus onset timing and onset timestamping may be
impaired, so be careful.
“pixelSize” sets the depth (in bits) of each pixel; default is to leave depth
unchanged. You should usually not specify such a bit depth, the system knows
what it is doing.
“numberOfBuffers” is the number of buffers to use. Setting anything else than 2
will be only useful for development/debugging of PTB itself but will mess up any
“stereomode” Type of stereo display algorithm to use: 0 (default) means:
1 means: Stereo output via OpenGL native quad-buffered stereo on any stereo
hardware supports this.
2 means: Left view compressed into top half, right view into bottom half of
window for frame-doubling stereo.
3 means left view compressed into bottom half, right view compressed into top
half for frame-doubling stereo.
4 and 5 allow split screen stereo display where the left view is shown in left
half, the right view is shown in the right half of the display, e.g., for
mirrorscope/haploscope setups, or dual-display stereo devices.
A value of 5 does the opposite (cross-fusion), exchanges left and right eye
Values of 6,7,8 and 9 enable Anaglyph stereo rendering of types left=Red,
right=Green, vice versa and left=Red, right=Blue and vice versa.
A value of 10 enables multi-window stereo: Open one window for left eye view,
one for right eye view, treat both of them as one single stereo window.
A value of 11 enables our own frame-sequential stereo mode for driving shutter
glasses and similar devices on display hardware and operating systems which do
not support frame-sequential stereo natively (like mode 1).
A value of 12 enables stereo processing within separate streams of the imaging
pipeline, followed by some custom display method for the end results of that
separate stream processing. This is usually used for stereo output to special
display devices like Virtual reality head sets, instead of output to a normal
onscreen window or display monitor.
See StereoDemo.m for examples of usage of the different stereo modes. See
ImagingStereoDemo.m for more advanced usage on modern hardware.
“multisample” This parameter, if provided and set to a value greater than zero,
enables automatic hardware anti-aliasing of the display: For each pixel,
‘multisample’ color samples are computed and combined into a single output pixel
color. Higher numbers provide better quality but consume more video memory and
lead to a reduction in framerate due to the higher computational demand. The
maximum number of samples is hardware dependent. Psychtoolbox will silently
clamp the number to the maximum supported by your hardware if you ask for too
much. On very old hardware, the value will be ignored. Read ‘help AntiAliasing’
for more in-depth information about multi-sampling.
“imagingmode” This optional parameter enables PTB’s internal image processing
pipeline. The pipeline is off by default. Read ‘help PsychImaging’ for
information about typical use and benefits of this feature.
“specialFlags” This optional parameter enables some special window behaviours if
the sum of certain flags is passed. A currently supported flag is the symbolic
constant kPsychGUIWindow. It enables windows to behave more like regular GUI
windows on your system. See ‘help kPsychGUIWindow’ for more info. The flag
kPsychGUIWindowWMPositioned additionally leaves initial positioning of the GUI
window to the window manager. The flag kPsychExternalDisplayMethod marks this
onscreen window as not an actual visual stimulation surface, ie. actual visual
stimulation is provided by some other external display mechanism, e.g., Vulkan
or some VR compositor or such. This tells Screen() to suppress certain warnings
or checks which would be prudent if the window were the primary and critical
means of visual stimulation. The flag kPsychDontUseFlipperThread prevents use of
the internal background flipper thread, and thereby of any functionality
depending on it, e.g., frame-sequential stereomode 11 and async flips.
“clientRect” This optional parameter allows to define a size of the onscreen
windows drawing area that is different from the actual size of the windows
framebuffer. If set, then the imaging pipeline is started and a virtual
framebuffer of the size of “clientRect” is created. Your code will draw into
that framebuffer. At display time, the content of this virtual framebuffer will
get scaled to the size of the true onscreen window, a process known as
panel-scaling or panel-fitting. This allows to decouple the size of a stimulus
as drawn by your code from the actual resolution of the display device. The
feature is mostly useful if you need to run the same presentation code on
different setups with different native resolutions. See the ‘help PsychImaging’
section about ‘UsePanelFitter’ for more info.
“fbOverrideRect” This optional parameter allows to override the true size of the
onscreen windows framebuffer for the purpose of image processing operations with
the imaging pipeline. While the true size of the windows framebuffer is defined
by the standard “rect” parameter, internal processing will instead use the given
override size. This usually only makes sense in combination with special output
devices that live outside the regular windowing system of your computer, e.g.,
special Virtual reality displays.
“vrrParams” This optional parameter allows to control the method for scheduling
visual stimulus onset. By default, if the parameter is omitted, or set to a mode
of 0, standard presentation with fixed refresh rate is used. Visual stimuli will
present at the start of a new video refresh cycle of fixed duration, ie. timing
is quantized to multiples of refresh duration.
Non-zero values ask to use a more fine-grained technique to schedule stimulus
onset than the classic fixed refresh interval scheduling on suitable hardware
and operating systems. This may allow to more often achieve a visual stimulus
onset exactly at or close to the ‘when’ onset time asked for in Screen(‘Flip’),
instead of only at the closest frame boundary of a fixed duration frame. This
needs a suitable operating-system, display driver and graphics hardware, as well
as a suitable display device that can run at a non-fixed variable refresh rate.
Selecting a mode other than zero on unsuitable system hardware+software
configurations will abort ‘OpenWindow’. Fine-grained stimulus onset scheduling
(ie. non-zero mode) is currently only supported on Linux with some hardware.
Settings other than mode 0 may require passing a vector with parameters instead
of just a mode selection scalar. E.g., instead of vrrParams = mode, it could be
vrrParams = [mode, styleHint, minDuration, maxDuration].
The ‘styleHint’ parameter describes or hints to the style of visual stimulation
timing to be expected for the session. It gives the scheduling algorithm some
high level hint that may allow to optimize for higher precision and robustness.
The only supported styleHint at the moment is styleHint 0 for ‘don’t know / none
/ default’. Future versions of Screen() may support more specific styleHint
values for common visual stimulation paradigms.
‘minDuration’ and ‘maxDuration’ would define the minimum and maximum duration of
a video refresh cycle that the given display is capable of in VRR mode, e.g., in
situations where this can’t be auto-detected reliably by Screen().
If mode is set to 1, Screen() will auto-select the strategy based on the given
hardware setup, operating system and display drivers, ‘styleHint’ and other
vrrParams to provide more fine-grained visual stimulus onset timing. See ‘help
VRRSupport’ for hardware and software requirements and setup instructions for
VRR on your system.
If mode is set to 2, Screen() will use VRR technology in the straightforward
naive way, efficient, but of limited timing precision and stability: If a ‘when’
target time is given in Screen(‘Flip’, …), Screen will simply wait until that
time and then submit the flip request to hardware. Immediate flips will be
submitted to hardware immediately. Special constraints of the specific operating
system, display driver, graphics card or display model are not taken into
account, jitter in hardware or software is not compensated for in any way.
If mode is set to 3, Screen() will use its own more sophisticated implementation
of a VRR scheduler on top of the simple VRR mechanism provided by the operating
system or graphics driver. It will try to take information about current display
system state, e.g., last vblank or flip completion time, minimum and maximum
possible refresh rates etc., into account, in order to do a better job at
hitting desired ‘when’ target times than a naive implementation.
Future versions of Screen() may bring additional fine-grained presentation
timing modes of higher sophistication or with different performance vs precision
vs reliability tradeoffs.
Opening or closing a window takes about one to three seconds, depending on the
type of connected display. If your system has noisy timing or flaky graphics
drivers it might take up to 15 seconds to open a window.
COMPATIBILITY TO OS-9 PTB: If you absolutely need to run old code for the old
MacOS-9 or Windows Psychtoolbox-2, you can switch into a compatibility mode by
adding the command Screen(‘Preference’, ‘EmulateOldPTB’, 1) at the very top of
your script. This will restore Offscreen windows and WaitBlanking functionality,
but at the same time disable most of the new features of the OpenGL
Psychtoolbox. Please do not write new experiment code in the old style!
Emulation mode may contain significant bugs, as it gets virtually no testing, so
use with great caution!