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How does ARToolKit work?
Basic Principles

ARToolKit applications allow virtual imagery to be superimposed over live video of the real world. Although this appears magical it is not. The secret is in the black squares used as tracking markers. The ARToolKit tracking works as follows:

  1. The camera captures video of the real world and sends it to the computer.
  2. Software on the computer searches through each video frame for any square shapes.
  3. If a square is found, the software uses some mathematics to calculate the position of the camera relative to the black square.
  4. Once the position of the camera is known a computer graphics model is drawn from that same position.
  5. This model is drawn on top of the video of the real world and so appears stuck on the square marker.
  6. The final output is shown back in the handheld display, so when the user looks through the display they see graphics overlaid on the real world.
The figure below summarizes these steps. ARToolKit is able to perform this camera tracking in real time, ensuring that the virtual objects always appear overlaid on the tracking markers.
Windows Camera Config

There are some limitations to purely computer vision based AR systems. Naturally the virtual objects will only appear when the tracking marks are in view. This may limit the size or movement of the virtual objects. It also means that if users cover up part of the pattern with their hands or other objects the virtual object will disappear.

There are also range issues. The larger the physical pattern the further away the pattern can be detected and so the great volume the user can be tracked in. Table 1 shows some typical maximum ranges for square markers of different sizes. These results were gathered by making maker patterns of a range of different sizes (length on a side), placing them perpendicular to the camera and moving the camera back until the virtual objects on the squares disappeared.

Pattern Size (inches) Usable Range (inches)
2.75 16
3.50 25
4.25 34
7.37 50

Table 1: Tracking range for different sized patterns.
This range is also affected somewhat by pattern complexity. The simpler the pattern the better. Patterns with large black and white regions (i.e. low frequency patterns) are the most effective. Replacing the 4.25 inch square pattern used above, with a pattern of the same size but much more complexity, reduced the tracking range from 34 to 15 inches.

Tracking is also affected by the marker orientation relative to the camera. As the markers become more tilted and horizontal, less and less of the center patterns are visible and so the recognition becomes more unreliable.

Finally, the tracking results are also affected by lighting conditions. Overhead lights may create reflections and glare spots on a paper marker and so make it more difficult to find the marker square. To reduce the glare patterns can be made from more non-reflective material. For example, by gluing black velvet fabric to a white base. The 'fuzzy' velvet paper available at craft shops also works very well.

You will find more informations on computer vision principle at this page, or more informations on performance at this page.