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higher altitudes. In flight, objects invisible at high
altitudes become visible at low altitudes, as the angle that
the object subtends at the eye becomes larger. When this
subtended angle exceeds an optical resolution threshold,
the object can be seen. These changes in detail cannot be
represented accurately owing to the required computational
load in the image generator. Instead, visual databases,
which are sets of objects, are stored and then faded in and
out of the scene according to range. This fading is adjusted
so that the transitions between databases appear natural.
How an image generator accomplishes this fading of
objects is called “level-of-detail management.”
For this experiment, visual databases were constructed of
green polygons on a brown ground plane. These polygons
were randomly positioned at locations within a 430-ft
radius forward of the aircraft. Each database was associated
with a particular altitude band (2.5-ft band). These bands
occurred between altitudes of 5 and 45 ft. As the vehicle
moved between these 2.5-ft altitude bands, one database
smoothly faded out while the new database faded in.
For each altitude, whether or not a polygon was displayed
at that altitude depended on which of two visual thresholds
was used. The “small” visual threshold allowed objects to
be drawn when their vertical visual angle spanned at least
0.1°; the “large” visual threshold allowed objects to be
drawn when their vertical visual angles spanned 0.2°.
In either case, the polygons were all rendered in a common
set. That is, the polygons were all distributed on a ground
plane; whether or not they were then drawn depended on
the visual threshold used. Thus, at the 0.1° threshold,
more polygons were drawn than in the 0.2° threshold. The
polygons that would appear in the 0.2° threshold would
also appear at the 0.1° threshold. Four different ground
planes of polygons, two each for the two visual
thresholds, were used so that any effects found would not
be due to some unfortuitous and unusual random
placement of the polygons.
Three levels of “level-of-detail management” were
evaluated: high, medium, and low. For the high condition,
62 polygon sizes were distributed exponentially in the
database with diameters between 0.025 ft and 17.23 ft.
The exponential spread was determined according to the
relation
di I
= 17.23(0.9)i = 0,1,...,61 (19)
where di is the diameter of the ith polygon.
These 62 polygon sizes were placed in the common
database in the following manner. First, the largest
polygons were randomly placed until a specified portion of
the unfilled area was filled. Then, polygons of the next
largest size were randomly placed in the remaining area
until that same proportion of the unfilled area was filled.
This continued, allowing no overlapping regions, until
polygons of all 62 sizes were placed. Since the image
generator could display a maximum of 1350 polygons at
60 Hz, the proportion parameter was selected such that
this maximum was not exceeded.
For the medium condition, the polygon sizes were
distributed evenly, not exponentially, between 0.025 ft
and 17.23 ft. For the low condition, only a single database
was presented at all altitudes, so no fading in and out
occurred. For the low condition, two sets of only three
polygon sizes were used: one set had polygon diameters of
3, 5.23, and 7.46 ft. The other set had polygon diameters
of 6, 10.45, and 14.9 ft. These sizes were chosen using
the following rationale. First, the largest polygons in each
set spanned 0.1 and 0.2 visual degrees at the 45-ft altitude.
Second, the smallest polygon in the large set of sizes
spanned approximately twice as much visual angle as the
small polygon in the small set.
Analytical Evaluation of Visual Scenes
Several metrics were used to evaluate the efficacy of using
these procedures in constructing the databases. First, the
number of visible polygons is plotted for each of three 10°
elevation bands (in fig. 63). These three 10° elevation
bands between 0° and 30° below the horizon nearly span
the field of view represented by the cockpit windows. As
shown, in the high condition, the number of polygons
visible is nearly constant versus altitude for each elevation
band. However, it is difficult to compare the medium and
low condition in terms of the level-of-detail represented.
For the medium condition, the number of polygons
visible always increases with altitude, but in the low
condition, the number of polygons in the 0°–10° elevation
　
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