Data Files
©
2016-2017 SR Research Ltd.
79
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 participant's
head moves. Even when a chinrest is used to support the participant'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. See the formula below. Remember to multiply the result by 57.296
to get the angle in degrees.
angle a
f
x
x
y
y
f
x
y
f
x
y
=
+
×
+
×
+
+
×
+
+
cos(
(
) (
)
)
2
1
2
1
2
2
1
2
1
2
2
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. These 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
f
x
y
f
y
=
×
+
+
+
0 01745
2
2
2
2
2
.
yres
f
x
y
f
x
=
×
+
+
+
0 01745
2
2
2
2
2
.
4.4.2.3 GAZE
Gaze position data reports the actual (x, y) coordinates of the participant's gaze
on the display, compensating for distance from the display. The units are in
actual display coordinates (usually pixels) which can be set in the EyeLink
configuration file PHYSICAL.INI. The default EyeLink coordinates are those of a
1024 by 768 VGA display, with (0, 0) at the top left.
The resolution data for gaze position data changes constantly depending on
participant head position and point of gaze, and therefore is reported as a
separate data type (see below). A typical resolution is about 36 pixels per degree
for an EyeLink setup in which the distance between the participant's eyes and
