result = visualAngle(numpixels, viewdist, dpi, va)
Author: Xuemei Zhang 12/8/95
Purpose:
Utility to compute visual angle (or numpixels, viewdist, dpi) of a
display monitor.
The parameter to be computed variable's value should be set to -1.
The value will be computed from the other variables and returned as the
result.
If va (viewing angle) is not given, it is taken as the unknown.
Examples:
visualAngle(20, 12, 72)
returns the visual angle spanned by 20 pixels on a 72dpi display at 12
inch viewing distance;
visualAngle(-1, 12, 72, 5)
returns the number of pixels needed to span 5 degrees visual angle on a
72dpi display at 12 inch viewing distance.
Viewdist: inches.
Va: degrees (of angle).
Dpi: dots-per-inch [default = 90]