ARGOS Virtual Pointer Accuracy

Last update: Monday 22 August 1994
The most common question about the virtual pointer is "How accurate it is?" That isn't a simple question to answer. There are a great many factors that must be considered before answering.

The simplest answer is: "People can position it within 1 pixel." But that just begs the question: "Well, how much is one pixel?" For the paper I published on my first experiment with the virtual pointer, I had my cameras in the following configuration:

Camera Configuration
11.8 cm separation between the cameras
96.5 cm convergence distance
8 mm lens
2/3 inch CCD
Display
12 inch Commodore Amiga Display
0.39 mm pixel separation
70 cm viewing distance
1 pixel disparity corresponds to 0.0317 degrees, or 1'54" of arc
Given that camera configuration, and the fact that we know subjects will almost always position the pointer within one pixel, we can calculate the change in distance that a one pixel error makes.

Using the values chosen for the experiment, we find the following:

	Pixel    Distance      Distance    % of D for 1
	 Sep     for Pixel    for Pixel-1   Pixel Error
	          Sep (D)      Sep  (D1)     (D-D1)/D
	----     ---------    -----------   -------------
	 -15       0.821         0.813         0.97%
	   0       0.954         0.965         1.14%
	  20       1.259         1.241         1.43%
	  40       1.810         1.771         2.15%
	  60       3.212         3.092         3.74%
	  80       5.237         4.927         5.92%
The first column is the separation (measured in pixels) of the left and right images of the virtual pointer as displayed on the monitor.

The second column gives the distances in the remote world (as viewed by the cameras) corresponding to those pixel positions.

The third column represents the distance corresponding to a pixel separation of 1 less than the second column.

The fourth column gives the percentage error of the total distance corresponding to a one pixel underestimation of the position of the object at the camera site.

For the given camera configuration, pixel separations of 80 are getting close to the limits of fusability.

These data can be presented graphically:

where the distance of the object from the cameras is measured in metres, naturally.

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