User manual

eye
(0,0)x
yline of
sight
f
(x,y)
HREF Plane
The (x,y) positions define a point in a plane at distance f (15000 units) from the
eye. The HREF units are independent of system setup, display distance, and
display resolution. The HREF coordinates are reported in integer values, with
260 or more units per visual degree.
The (0, 0) point in the coordinate system is arbitrary, as the relationship
between display positions and HREF coordinates changes as the subject's head
moves. Even when a chinrest is used to stabilize the subject's head, head
rotations of several degrees can occur. HREF coordinates are best used for
determining angles relative to a known eye position, or to measure eye-
movement velocities, as described below.
The eye rotation angles may be directly computed from the HREF (x, y) pairs.
There are several methods of specifying eye-rotation angles. The angular
distance (eye rotation magnitude) between any two HREF points is directly
computable. The C code to compute this angle is given below. Remember to
multiply the result by 57.296 to get the angle in degrees.
angle a
fxxyy
fxy fxy
=
+×+×
++ × ++
cos(
()()
)
2
1212
2
1
2
1
22
2
2
2
2
The HREF angular resolution may be computed as the first derivative of the rate
of change of HREF position with angle. It is sufficient to compute the resolution
separately for the x and y coordinate directions. This may be used to compute
true eye-movement velocities, by dividing computed velocity in HREF units by
the resolution for the sample. These formulas give the x and y resolution in
units of change in HREF position per degree of visual angle:
xres
fxy
fy
=×
++
+
0 01745
22
22
.
2
yres
fxy
fx
=×
++
+
0 01745
22
22
.
2
4.4.2.3 GAZE
Gaze position data reports the actual (x, y) coordinates of the subject's gaze on
the display, compensating for distance from the display. The units are in actual
Data Files
© 2005-2008 SR Research Ltd.
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