t="" produce="" a="" movie="" simulating="" fight="" on="" the="" screen="" in="" real="" time”，="" says="" weidhaas.="" there="" is="" way="" around="" problem:="" two="" movies="" have="" been="" made="" using="" still="" pictures="" generated="" by="" computer="" as="" individual="" frames.="" “the="" results="" were="" impressive,”="" lie="" says,="" “bat="" it="" was:="" cumbersome="" to="" do.="" at="" twenty-four="" frames="" per="" second,="" takes="" fourteen="" hundred="" separate="" images,="" make="" one-minute="" film.”="" another="" limitations:="" can="" access="" only="" enough="" memory="" cover="" 15-mile="" square="" area.="" an="" “observer”="" high="" up="" will="" see="" blank="" spaces="" beyond="" those="" limits.Weidhaas wants to add information about what overlies the terrain-cities, vegetation, roads; and so on. “Making the image as realistic as possibly will make our advice more effective he says, “and might lead to uses we haven't thought of yet.”1.As used in the first paragraph, terrain most clearly means （ ）.2.Livermore’s computer map, in combination with weather report, might be useful in predicting （ ）.3.The information used by the computer to make its detailed maps （ ）.I. was gathered by the Defense Department and the U.S. Geological SurveyII. shows points roughly 200 feet apart.III. involves altitude measurements.4.Which of the following is the best description of ray tracing?5.Information about cities, vegetation, and road overlying the terrain（ ）.'>
Imagine an accident in which a nuclear power plant releases radioactive gas. The cloud starts moving with the wind. Clearly the authorities will want to evacuate anyone in its path, but what is that path? Local wind information is meaningless without information about terrain; a mountain range or series of valleys can divert both wind and gas in unpredictable directions.To make “downwind” a useful term, scientists at Lawrence Livermore National Laboratory have put United States on a computer, the entire United States — every hill, every valley, every mile of seashore. Within minutes of a disaster, they can give meteorologists a context for weather data, and thus the ability to predict how toxic gases might spread.The database for this computer map is a series of altitude measurements, made over many years by the Defense Department and the U.S. Geological Survey. They represent the height above sea level of over a billion separate points — a grid of points 200 feet apart, spanning the country. Armed with these data, plus a program that manipulates them, a Cray-1 computer can produce an image of any piece of terrain, seen from any angle, illuminated by an imaginary sun atany time of day placing the “observer” at any altitude from zero to 40,000 feet.“We use a technique called ray tracing” says Patrick Weidhass, one of the Livermore computer scientists who wrote the program. The computer is told where the observer is. The program traces an imaginary ray from there outward until it “ intersects" with one of the points of altitude reworded in the machines memory. The computer them puts a lot of color at the proper place on the screen, and the program traces another ray.At its highest resolution of 2,000 horizontal and 1,700 vertical dots per picture, the computer has to trace several million rays. Even on the Cray, the most powerful computer in the world, this takes about a minute. Reducing the resolution to 400- 300 (a TV screen has 800 x 700)speeds it up to about eight seconds. "We can't produce a movie simulating fight on the screen in real time”， says Weidhaas. There is a way around the problem: Two movies have been made using still pictures generated by the computer as individual frames. “The results were impressive,” lie says, “bat it was: cumbersome to do. At twenty-four frames per second, it takes fourteen hundred separate computer images, to make a one-minute film.” Another limitations: The computer can access only enough memory to cover a 15-mile square area. An “observer” high up will see blank spaces beyond those limi
相关问题推荐
-
There is concern that police use the law to（ ）assets from people who have committed minor offences.
A.confiscate B.offset C.submit D.intensify -
With many ethnic groups represented, the student union is very （ ）.
A.homogeneous B.heterodox C.heterogeneous D.heterogeneousness -
One effect of the （ ）economy of 2009 on state and local governments is a decrease in income, sales, and property tax revenues.
A.prosperous B.sluggish C.substantial D.pervasive -
sleep="" alternation="" with="" some="" 16-17="" hours'="" wakefulness="" and="" that,="" broadly="" speaking,="" the="" normally="" coincides="" hours="" of="" darkness.="" our="" present="" concern="" is="" how="" easily="" to="" what="" extent="" this="" cycle="" can="" be="" modified. The question is no mere academic one. The ease, for example, with which people can change from working in the day to working at night, is a question of growing importance in industry where automation calls for round-the-clock working of machines. It normally takes from five days to one week for a person to adapt to reversed routine of sleep and wakefulness, sleeping during the day and working at night. Unfortunately, it is often the case in industry that shifts are changed every week; a person may work from 12 midnight to 8 a m one week, 8 a m to 4 p. m the next, and 4 p. in to 12 midnight the third and so on. This means that no sooner has he got used to one routine than he has to change to another, so that much of his time is spent neither working nor sleeping very efficiently. The only real solution appears to be to hand over the night shift to a number of permanent night workers. An interesting study of the domestic life and health of night-shift workers was carried out by Brown in 1957. She found a high incidence（发生率）of disturbed sleep and other disorders among those on alternating day and night shifts, but no abnormal occurrence of these phenomena among those on permanent night work. This latter system then appears to be the best long-term policy, but meanwhile something may be done to relieve the strains of alternate day and night work by selecting those people who can adapt most quickly to the changes of routine. One way of knowing when a person has adapted is by measuring his body temperature. People engaged in normal daytime work will have a high temperature during the hours of wakefulness and a low one at night; when they change to night work the pattern will only gradually go back to match the new routine and the speed with which it does so parallels, broadly speaking, the adaptation of the body as a whole, particularly in terms of performance. Therefore, by taking body temperature at intervals of two hours throughout the period of wakefulness it can be seen how quickly a person can adapt to a reversed routine, and this could be used as a basis for selection. So far, however, such a form of selection does not seem to have been applied in practice. 1.Why is the question of “how easily people can get used to working at night” not a mere academic question? 2.The main problem of the round-the-clock working system lies in （ ）. 3.The best solution for implementing the 24-hour working system seems to be （ ）. 4.It is possible to find out if a person has adapted to the changes of routine by measuring his body temperature because （ ）. 5.Which of the following statements is NOT TRUE?'>
We all know that the normal human daily cycle of activity is of some 7-8 hours' sleep alternation with some 16-17 hours' wakefulness and that, broadly speaking, the sleep normally coincides with the hours of darkness. Our present concern is with how easily and to what extent this cycle can be modified.The question is no mere academic one. The ease, for example, with which people can change from working in the day to working at night, is a question of growing importance in industry where automation calls for round-the-clock working of machines. It normally takes from five days to one week for a person to adapt to reversed routine of sleep and wakefulness, sleeping during the day and working at night. Unfortunately, it is often the case in industry that shifts are changed every week; a person may work from 12 midnight to 8 a m one week, 8 a m to 4 p. m the next, and 4 p. in to 12 midnight the third and so on. This means that no sooner has he got used to one routine than he has to change to another, so that much of his time is spent neither working nor sleeping very effic
-
The extremely （ ）trade in endangered species is proving irresistible to countries in grave currency crisis.
A.lucrative B.approximate C.ambitious D.prospective