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Helicopter Flight Simulation Motion Platform Requirements(46)

时间:2011-11-12 12:15来源:蓝天飞行翻译 作者:admin
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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
d = 172309 . ( . )i I = 0 1 61 (19)
, ,...,
i
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 band decreases with increasing altitude.
 
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