What you need to track motion
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What you need to track motion |
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You need 3 things to track motion:
There are different reasons to track motion of course, but for most performance or installation applications, I recommend either an analog camera and a frame grabber, or a GigE camera (the reasons are explained below.) Software There are a half dozen softwares now available to track motion. The most useful (in my opinion) are EyeCon and EyesWeb. Both run on windows. The two are quite different conceptually. EyesWeb is an object-oriented programming environment. EyeCon, while less powerful, is easier to use, quick to set-up and a good tool for performers. For any typical uses a performer might have, and for many installation applications as well, both will work fine. EyesWeb is free, and EyeCon costs 250 euros (30-day free). Students of our workshops get a 50% discount. Computer PC vs MAC Any PC or laptop with windows and a CPU speed of ca. 1GHz or more should be fine. The latency issues relate to the buses (or connections between camera and computer), not the CPU. I will come back to this shortly. As you may know, windows runs very well on MAC computers as long as they are equipped with Intel processors and have something called bootcamp. The software called "Parallels" also works, but you need to select the camera device in the parallels menu to use it. Desktop vs Laptop Both will work, but the desktop has the advantage of a PCI slot which accepts an industrial framegrabber (lower latency and other advantages). Digital Cameras It would be nice if there was an easy way to get a good quality video image into a computer using a digital camera, via usb or firewire. There is not. The key word here is "quality". That is, getting the image is not the problem, latency and image resolution are. (see more discussion about this below). Ethernet cameras do offer an interesting alternative. In fact we use them in our motioncomposer. As far as I know though, among the motion tracking software systems generally available, only the motioncomposer accepts these cameras (without needing special programming). Analog Camera + Frame Grabbers Analog cameras have a number of advantages. They do, however, require framegrabbers. Framegrabbers are the hardware devices that digitize the video image. You can use the camcorder you already have via the analog output of the camera. Almost all camcorders have this, though it may be difficult to find and require a special cable. You need to turn off the Auto-focus and , if you can, the Auto-aperture features. A better option is a surveillance camera, such as a CCD camera. It is better for a few reasons, including field of view and resolution. Field of view means how wide angle an image can be. The larger the chip, the wider the field of view (see figures 5 and 6). Yes, you can widen the field of view with lenses, but this comes at the cost of distortion and a lower resolution. Resolution is a complicated issue. Most people speak of resolution as the number of pixels. Pixel count however is only one factor in image quality (and this is what I mean by "resolution": how much useful information the picture contains). Increasingly, algorithms are used in camera chips which calculate (estimate) light and color values instead of actually measuring them pixel for pixel. I.e. an image from a new camcorder may look better, but actually have lower resolution! The consumer camera industry has been busily exploiting this trick in its effort to make ever-cheaper cameras with ever better-looking pictures. These developments may be good news for consumer camera prices, but they are bad news for motion tracking. For motion tracking we need true resolution, not pretty pictures. If you get a 1/2" CCD camera (black and white) e.g. from JAI -- see their A50 or A60 figure 4, and zoom lens that goes from 4 to 12 mm (see figure 7) you will have a good system. They cost around 500 new. or 50-100 in ebay. As I said, if you use an anlog camera, you need a framegrabber: framegrabber types
Digital Cameras (do not need framegrabbers)
Extras Manfrotto camera clamp and arm (see 8 and 9). Resolution and Latency The other major technical issue (besides resolution) is latency. This means the delay time between the action and the media response. Even a small latency weakens or ruins the sense of interaction. Noticeable latency means that rather than beguiling moments of synaesthesia, you get a kind of "cause and affect". This may seem like a small point, and may be it is for you, but for me it goes to the heart of what motion tracking is all about. New devices and connections are being made today with higher and higher data rates. But data rate is rarely the problem. The problem is latency and this is not the same thing as data rate! Questions, More Info... robert@palindrome.de (I answer all my mail, but please be patient).
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