Differences between PTB-3 versions for OS X, Windows and Linux

The implementations of Psychtoolbox-3 for Microsoft Windows and GNU/Linux are derived from the Mac OS X implementation. As of October 2012, all versions are mostly compatible to each other and equally capable in core functionality. The different operating systems differ in their capabilities and limitations. You will have to take this into account if you want to write portable scripts. The following information should allow you to write scripts that are as portable as possible and to assess if PTB on a specific operating system is suitable for you.

The information on this page is periodically updated (i.e., at any point in time slightly outdated). It always refers to the state of the current beta flavor of PTB.


Status of different core features and how to handle differences

Matlab M-File functions

All functions that are implemented as Matlab M-Files should work without any differences between system platforms, as long as they don’t depend on system dependent MEX files. M-Files are tested with the Matlab 7.x series and the GNU/Octave 3.2.x series.

If you need to write scripts that have to behave differently on the different platforms, the following functions may be helpful:

  • IsOS X returns 1 when the script is executed under Mac OS X, 0 otherwise.
  • IsWin returns 1 when the script is executed under Microsoft Windows, 0 otherwise.
  • IsLinux returns 1 when the script is executed under GNU/Linux, 0 otherwise.
  • IsLinux(1) returns 1 when the script is executed under GNU/Linux with a 64 bit version of Matlab or Octave, 0 otherwise.
  • Is64Bit returns 1 when the script is executed in a 64-Bit version of Matlab or Octave, 0 otherwise.
  • IsOctave returns 1 when the script is executed in GNU/Octave, 0 if the script is executed in Matlab.
  • OSName returns the name of the operating system platform.
  • AssertOpenGL aborts execution of your script if someone tries to execute it in PTB-2 instead of PTB-3.
  • AssertOS X aborts execution of your script if someone tries to execute it on a operating system other than Mac OS X.
  • AssertGLSL aborts execution of your script if it is executed on graphics hardware that does not support the OpenGL Shading language GLSL.

Serial port access

Our IOPort driver provides high quality, flexible serial port support (also for USB serial ports) with high timing precision on all operating systems, so use of IOPort is strongly recommended over less optimal solutions like Matlabs serial command.

Joystick support

The OS X and Linux versions support joysticks via the new GamePad function. We provide some simple joystick support for Windows via WinJoystickMex.

A functional implementation of the Eyelink toolbox is included in the PsychHardware folder of PTB for all operating systems. It is reported to work well, although it’s not polished yet, e.g., the documentation and some of the demos are not yet updated and cleaned in all parts. You must install the runtime libraries from SR - Research on your system for this to work. They come bundled with your Eyelink and are also downloadable from their website.

Snd command for sound output

Sound output via the old and deprecated Snd command is currently implemented as a wrapper around the PsychPortAudio driver for decent cross-platform compatibility. However use of Snd is deprecated for all but the most simple use cases, e.g., providing simple auditory feedback to a subject. For research grade auditory stimulation, use PsychPortAudio directly, or you will be doomed!


OpenGL for Matlab/Octave support MOGL

Support for OpenGL low level commands is identical for all systems. Well, almost: Support for specific functions depends on the capabilities of your graphics hardware and OpenGL implementation, so some of the most recent features of OpenGL may not be supported on old graphics hardware. The solution to this problem is simple: Upgrade your graphics drivers or your hardware. We currently support the OpenGL 2.1 API and earlier versions, including various extensions.

PsychHID and the DAQ toolbox

PsychHID for control of USB HID devices is implemented on all systems since the end of 2011. You may encounter differences in the information provided about connected HID devices when calling PsychHID('Devices') due to differences in the underlying operating systems. If you write code that uses device enumeration to detect your devices, make sure to test on different operating systems to take these differences into account.


The GamePad function is only available on Mac OS X and Linux for now.


The Fonts command for flexible query and handling of character fonts only exists on Mac OS X with no plans to ever implement it on Windows or Linux.

ListenChar, CharAvail, GetChar, FlushEvents

In Matlabs regular GUI mode, functions for character input are implemented as a Java class via Matlabs built-in Java virtual machine (JVM). Usage on all platforms is identical, no known principal differences, except that it doesn’t work well on MS Windows Vista and Windows-7 (see the FAQ section about Vista problems for more info). Differences, if any, would be due to differences in the implementation of the JVM in different versions of Matlab. We need JVM version 1.4.2 or later. If Matlab is run without Java support or GUI (matlab -nojvm mode or matlab -nodesktop mode selected at startup), or if you are using Octave instead of Matlab, a different implementation is used, which works but is more limited in its capabilities. Read help GetChar and help ListenChar for more info. The GetKbChar function is often a viable and more robust replacement for GetChar.

KbCheck, KbWait, KbName

Usage, behaviour and timing of KbCheck, KbWait and KbName should be identical / similar on all systems - no known relevant differences.

The mapping of keyboard scancodes to key names used to be different in the Psychtoolboxes for OS 9, OS X, Windows and Linux. E.g., the left arrow cursor control key used to be named left on Windows, LeftArrow on Mac OS X and Left on Linux. This is annoying if one wants to write portable scripts. When writing new code, you should add the command KbName('UnifyKeyNames'); at the top of your script. This will make sure that KbName accepts and returns identical keynames on all operating system platforms, allowing you to write portable code without extra effort. However, mapping of some special or exotic keys may be incomplete, so you should probably avoid those keys for portable scripts.

% Add this at top of new scripts for maximum portability
% It unifies the names for keyboard keys across systems

SetMouse, GetMouse, ShowCursor, HideCursor, GetSecs

No known relevant differences in behaviour or quality, tell us if you find some! GetSecs timestamps are accurate at sub-millisecond level on all systems. The mouse and cursor handling functions on Linux allow to query and control multiple mice or mouse cursors individually. Only one mouse or mouse cursor is supported on OS X and Windows, multiple mice are “merged” into one mouse input.

Priority and WaitSecs

The range of values accepted by the Priority command differs between OS X, Windows and Linux. Priority(0) disables realtime scheduling on all operating systems. On Linux, the Priority command can utilize some special system facilities to make realtime mode more effective and robust than on the other operating systems. On Linux you can also optionally install a low-latency or hard-realtime operating system kernel and various other tweaks to the system configuration if you require especially precise or robust realtime behaviour. This is generally not needed for most experimental setups, but if you choose to do it, it can provide realtime behaviour and timing precision that is orders of magnitude better than what Windows or OS X can possibly achieve. See the Linux specific setup pages on the Wiki for more information.

Use the MaxPriority function to query maximum allowable priority levels for realtime scheduling (help MaxPriority) in order to keep your code portable.

% Select maximum allowable realtime priority
% for current operating system

WaitSecs accepts values with microsecond precision. The robustness and accuracy of the real waiting time compared to the requested waiting time depends on the scheduling accuracy and timing jitter of the underlying operating system. My experience is that scheduling on a well configured Linux system or Mac OS X Tiger / Leopard system is very accurate and robust, whereas Microsoft Windows systems are significantly worse on many tasks. For best possible precision, use a Linux system with low-latency or realtime kernel installed.


The implementation of Screen is mostly identical on all systems, providing the same capabilities and performance on all operating systems. The following differences are known to exist. These differences are mostly unavoidable and therefore unfixable due to different designs and limitations in the underlying operating systems.

  • Multi display support: On single display setups, Screen behaves identically. On multi display setups, enumeration of displays is handled slightly different:
    • Mac OS X: Screen('Screens') will return a vector with one numerical screen id for each connected display device, e.g., [0 1] for a dual-display system. Display 0 corresponds to the main display (the one with the Dock), Display 1 corresponds to the secondary display.
    • Linux: Each X-Windows system x-screen will be enumerated as a separate Psychtoolbox screen. A x-screen can cover multiple connected physical display devices, depending on how you configured your setup in the xorg.conf file. On a single display setup you only have one x-screen, which maps to Psychtoolbox screenid 0. On a multi-display setup you would group physical displays together into one or multiple x-screens. Each x-screen maps to a Psychtoolbox screenid and is either covered and displayed to by a separate fullscreen Psychtoolbox onscreen window for stimulus presentation, or not used by Psychtoolbox, e.g., for operator control displays. The individual physical display devices (aka outputs) attached to a Psychtoolbox screen can be individually controlled via the Screen('ConfigureDisplay') command and the optional physicalDisplay parameter of various Screen subcommands, e.g., the LoadNormalizedGammaTable command for setting individual gamma tables per display.
    • Windows: If your system is switched to multi-monitor mode, Screen('Screens') will report n+1 displays for a setup with n displays. The display with index zero is special: It corresponds to the display area of all connected displays. If you open an onscreen window on display zero, it will occupy all connected display devices. This is useful for presentation of stereo stimuli or other binocular stimuli. The screens with indices 1, 2, ..., n correspond to the display areas of display devices 1, 2, ..., n, so specifying these screen ids allows you to open onscreen windows on individual displays or to query and set properties of individual displays, e.g., refresh rate, resolution, color depth, size or the hardware color lookup tables. If you have an unusual display arrangement, you can also explicitely specify a rect argument to the Screen('OpenWindow', screenid, color, rect) command to create a window that occupies the specified rect on the desktop, partially covering multiple monitors.
  • Window placement and size:
    • Mac OS X: Screen('OpenWindow', screenid, color, rect) correctly handles the rect argument - You can open windows that don’t cover the full display area for debugging purpose. However, timing precision and stimulus onset timing is strongly impaired in windowed mode due to limitations of MacOS X. By default, an onscreen window always covers the full area of a specific display device.
    • Windows: Onscreen windows cover the full display area as on OS X by default. If you provide a rect argument, Psychtoolbox will open a onscreen window that only covers the area specified by the rect parameter. E.g., Screen('OpenWindow', 0, [], [0 0 800 600]) would open an onscreen window on display device zero, with its top-left corner at (0,0) and its bottom-right corner at (800,600). The window will have the usual decorations, title bar and buttons, so it can be closed, minimized/maximized, resized and dragged around on the screen. This is useful for debugging. You can also open multiple windows on the same display device.
    • Linux: ditto.
  • Color depth setting pixelsize: This parameter is mostly there for backwards compatibility. It should be left at its default setting in almost all cases.
    • Mac OS X: Screen tries to switch a display device to the requested color depth before opening an onscreen window on it. Psychtoolbox may work at a setting of 24 bpp or 16 bpp with reduced functionality, i.e., some alpha blending modes are not supported at a depth of 24 bpp and color resolution is drastically reduced at 16 bpp. Color index mode with 8 bpp is not supported on OS X.
    • Windows: The color depth setting is ignored. You’d need to change the settings in the display properties control panel of your system. However, any setting except the default of 32 bpp will cause abortion of your script with an error message.
    • Linux: Works with 24 bpp or 32 bpp, depending on graphics card. Simply leave out this parameter so PTB can choose the correct default.

    In any case, you have to run your displays at a color depth of 32 bits per pixel - which is the default setting - to take full advantage of Psychtoolbox alpha blending functions.

  • Stereo presentation modes stereomode:
    • Mac OS X: All modes are supported with high performance. For dual-display stereo you need to use a different setup code than on Windows. On Windows you’d use stereo mode 4, whereas on Mac OS you’d use stereo mode 10 and a slightly different setup code. Look at ImagingStereoDemo on how to do this. Caution: The different displays on a Mac OS X system are not synchronized wrt. their display refresh cycles! This can cause tearing artifacts on one of the two displays if you use dual-display mode for binocular stimulation and show stereo movie animations.
    • Windows: Mode 1 frame sequential stereo for driving shutter glasses is emulated in software by PTB on consumer hardware, so actual quality of frame-sequential stereo will highly depend on your system. The professional line of gfx-cards (ATI FireGL and NVidia Quadro series) should support this mode at high quality. All other modes (Anaglyph stereo, dual display stereo, free fusion) should work on all hardware with high performance.
    • Linux: Mode 1 frame sequential stereo for driving shutter glasses is emulated in software by PTB on consumer hardware, so actual quality of frame-sequential stereo will highly depend on your system. The professional line of gfx-cards (ATI FireGL and NVidia Quadro series) should support this mode at high quality. All other modes (Anaglyph stereo, dual display stereo, free fusion) should work on all hardware with high performance.

    In general, it is beneficial to use the stereo modes with the PTB imaging pipeline enabled. See ImagingStereoDemo for an example of use. On OS X this is crucial for dual display stereo. On all operating systems if you use Anaglyph stereo presentation, it will allow for very flexible control over gains and other parameters via SetAnaglyphStereoParameter().

  • Quicktime/GStreamer support for audio- and movie playback:
    • Mac OS X: Movie playback is fully supported via GStreamer, but on 32-Bit Matlab, Quicktime is used by default.
    • Windows: Movie playback is fully supported via GStreamer.
    • Linux: Movie playback is fully supported via GStreamer at a performance that generally exceeds the performance of either Windows or Mac OS X.
  • Drawtext and TextBounds functions for drawing text: No difference in functionality or quality, only in speed.
    • Mac OS X: Apples ATSU text renderer is used. Provides very flexible, very high quality text rendering at a comparably low speed. Unicode character strings are supported. Text bounding boxes are highly accurate.
    • Windows: By default, a text renderer based on the Windows GDI is used: It provides accurate text bounding boxes, anti-aliased text (although not at the superb quality level of OS X, but still good), and Unicode support, just as on OS X with moderate speed. An older text renderer can be optionally selected, which is based on OpenGL display lists: Less flexible, significantly lower text quality (no text Anti-Aliasing), only the standard ASCII character set, but very fast. Limitations of the fast optional renderer: Computation of text bounding boxes doesn’t take descenders of letters into account, due to some Microsoft implementation brain-damage. The text size setting is not correctly handled due to some unresolved bug. Text shows up at a size approximately 2/3rd of what was requested.
    • Linux: Fast text rendering of high quality, anti-aliased, unicode text with true type text fonts is fully supported. The speed of the Linux text renderer is usually significantly higher than the speed of the Windows or OS X renderer, while retaining their quality, functionality and flexibility.
  • Stimulus onset timestamps reported by Screen('Flip'):
    • Mac OS X: On many Intel Macs, the timestamps are guaranteed to be sub-millisecond accurate, regardless of system load or scheduling jitter. Synchronization of stimulus onset to the vertical retrace is guaranteed to be microsecond accurate on properly working hardware. However, on some recent versions of the operating system with ATI/AMD or Intel graphics cards, installation of the PsychtoolboxKernelDriver is required for reliable timestamping to compensate for various operating system bugs. Without this driver installed, a different set of workarounds so far provided at least millisecond accurate timestamping, but this workarounds are ineffective due to new additional bugs introduced in OS X 10.7 “Lion”, so on 10.7 and later, we cannot guarantee timestamp precision or correctness if you don’t use the kernel driver.
    • Windows: Accuracy of returned timestamps is the same as on OS X on NVidia and ATI hardware, but not on older Intel onboard graphics hardware. There it will be millisecond accurate in most cases, but no guarantees can be made due to technical limitations of the Windows + Intel graphics combo. Stimulus onset is microsecond accurate wrt. vertical retrace, as on Mac OS X.
    • Linux: Stimulus onset is microsecond accurate wrt. vertical retrace, as on Mac OS X. Currently you need to run the PsychLinuxConfiguration script once and then reboot once, if you use the proprietary graphics drivers from NVidia or ATI/AMD to get the same timestamping precision as on a well working OS X or Windows systems. However, this function is automatically executed after each installation or update of your Psychtoolbox, so it rarely needs to be called manually by you. If you choose to use the free graphics drivers for Intel, ATI/AMD or NVidia graphics cards instead of the proprietary drivers, timestamp precision and visual stimulus onset precision will be even better and more robust than on any other operating system or graphics hardware without any need for additional setup steps or use of root mode.
  • Color lookup tables, gamma tables, CLUT animation:
    • Mac OS X: OS X supports up to 16 bit DAC precision on graphics hardware that supports this. CLUT animation works, but updates to the CLUTS can’t be synchronized to Screen('Flip').
    • Linux: Has the same capabilities as Mac OS X, the status of CLUT synchronization is hardware dependent.
    • Windows: Windows supports up to 16 bit DAC precision on graphics hardware that supports this. CLUT animation is mostly impossible due to extreme brain-damage in the Windows CLUT implementation. Synchronization of CLUT updates to the execution of Screen('Flip') is possible.
    • Most older graphics cards are restricted to 8 bits DAC precision and even the latest models from ATI or NVidia have at most 10 bits of DAC precision, so the 16 bit limit imposed by the operating systems is a theoretical limit, not a practical one!

    On recent graphics hardware, the PsychImaging('AddTask','General','EnableCLUTMapping') function allows to restore clut animation in a portable and reliable way on all operating systems. This needs hardware with fragment shader support. This method of clut animation also allows for perfectly frame-accurate animation.

    However, we consider clut animation mostly an obsolete, ancient feature - There are almost always better ways to create animated stimuli with all the new PTB drawing functions.

  • Screen('Computer') command:
    • Mac OS X: The command returns lots of information about the host system, many of which seems to be pretty uninteresting for any practical purpose.
    • Windows: Reports only bare minimum.
    • Linux: Reports only bare minimum.