Intro and Overview/FrameRates¶
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There are two different values which both get called ''frame rate''. The first is the number of frames a graphics card draws to its framebuffer per second. The second is the number of frames per second drawn on a monitor from the graphics card. The frame rate of the monitor is a constant set by video drivers, and the frame rate of drawing to the framebuffer is determined by how long it takes for a particular program to complete all the elements in a scene. Obviously, it is a waste of resources to draw more frames to the framebuffer than will be displayed on the monitor, although in some cases there may be reasons for doing so. (For example, if program control and drawing are done in the same loop, the control latency will be shorter at faster frame rates. Benchmarking 3D games or graphics cards is another reason why one might want to do this.) For the rest of this document, ''frame rate'' will refer to the refresh rate of the monitor. The absolute upper limit of the monitor's frame rate is specified by the ''maximum vertical refresh frequency''. In typical professional displays this value may be 160 Hz. Several support 180 Hz, and a few (such as the LGE Flatron 915 FT Plus) go up to 200 Hz. Unfortunately, your video driver may only allow certain pre-defined frame rates. See below and the installation details page of your platform for platform-specific information about setting monitor refresh rates. What frame rate do you want? If your stimulus involves high-speed motion, more frames per second is better, simply because it reduces temporal aliasing. At a minimum, your monitor's flicker rate (equal to frame rate on a CRT) should exceed the flicker fusion frequency of your subject. If your stimulus involves slow motion and your system can only move on-screen objects in integer pixel increments (this is not the case with the Vision Egg), you again want a high frame rate to maximize the smoothness of motion if an object must alternately move an integer number of pixels and then remain in place. With these arguments in favor of high frame rates, why might you want slower frame rates? It is easier on the (typically analog) electronic circuitry involved in the production and amplification of the video signal, and therefore square edges on waveforms will be closer to truly square, producing better spatial crispness. You may be able to draw more pixels per frame at a slower frame rate because of limitations in your monitor's ''horizontal refresh frequency'' and ''pixel bandwidth'' in addition to your video card's ''RAMDAC/pixel clock''. Platform-specific tools to create custom video modes ---------------------------------------------------- XFree86, the video driver for linux, lets the user specify exactly the timing parameters of the video signal in the ''modeline'' section of the XF86Config file. For Windows, RefreshForce_ (freeware) is very useful. PowerStrip_ (shareware) allows arbitrary setting of video mode. Furthermore, `Refresh Rate Fixer`_ may be of use. On Mac OS X, switchResX_ may provide a way to set the frame rate other than the default values. SGI IRIX has its own video mode tools such as setmon which allow arbitrary video modes to be produced. The `XFree86 Video Timing HOWTO`_ is an excellent resource for understanding the intricacies of the timing of video signals, even if you don't use X windows. Multi-tasking operating systems and dropped frames -------------------------------------------------- The simplest way to guarantee that your monitor is updated on every frame is to put the control and drawing routines in a loop which cannot be interrupted. This can be done with special graphics cards which have an on-board processor separate from the computer's main CPU, or in operating systems which give programs complete control of the CPU. Unfortunately, OpenGL graphics cards must be controlled from the CPU, and most modern operating systems have a kernel which can suspend any program. There is the (currently untried) possbility of running the Vision Egg on a system with a realtime OS which would also solve the problem completely. We believe the Vision Egg, or another OpenGL solution which has the ability to move objects in sub-pixel increments, provides the absolute best way to produce smooth motion. .. _RefreshForce: http://www.pagehosting.co.uk/rf .. _PowerStrip: http://www.entechtaiwan.com/ps.htm .. _`Refresh Rate Fixer`: http://www.radeon2.ru/refreshfix_eng.html .. _switchResX: http://www.madrau.com/html/SRX/indexSRX.html .. _`XFree86 Video Timing HOWTO`: http://www.tldp.org/HOWTO/XFree86-Video-Timings-HOWTO