A typical hydrodynamic physics program may run up to 20 hours on a CDC 7600 computer and generate up to a billion numbers. The biggest problem facing the users of these programs is simply comprehending what has been calculated. Stacks of computer listings are typical outputs but serve as a poor communication medium. Static pictures are much better than listings but are limited to two dimensions. Through the use of movies, one adds a third dimension to the communication process. This paper will describe the evolution of computer generated movies at Los Alamos Scientific Laboratory and describe the techniques now in use.
"What results do we have from last night's production runs?" is a question asked daily by weapons designers at Los Alamos Scientific Laboratory. It is not a question that can be answered by a single number or a small set of numbers. A typical production run will take from 20 minutes to 20 hours of CDC 7600 time and may generate billions of numbers. Through the use of computer generated movies we are able to present a very good description of what has been calculated. The movies are not only an excellent source of information, but valuable as a teaching medium.
The production codes are used to model physical events and usually calculate physical variables as a function of time. The geometry of a problem is usually described by a mesh containing from 1,000 to 15,000 nodes. These two dimensional meshes are not necessarily rectangular and are, at times, quite distorted. A number of physical variables are associated with each node and each zone. Each problem will run several thousand cycles; thus generating billions of numbers. The presentation and interpretation of these numbers is a very complex problem.
The production codes exhibit their results in a number of ways. One method is to generate hundreds of pages of printed output. This is the most visual demonstration to the casual observer that the individual is doing some work as shown by the huge piles of listings occupying his office. They are of very limited use otherwise with only a limited nwnber of pages giving useful information. It should be pointed out that the designer is constrained to produce such long listings since he never knows precisely which parameters will be essential for his analysis. In a given case, however, he may use only a small percentage of the information printed.
A second form of output is a set of tapes to draw pictures on paper through the use of a mechanical plotter. The pictures are complicated, taking up to an hour each to plot. If the plotter is available, the tapes are readable, and the plotter does not malfunction, the designer will have one or two excellent plots late in the day - provided he chose the right set of parameters to plot. These plots are excellent to work from (very detailed, large scale, and may be written on) but are too time-consuming and awkward to convey enough information about a production run.
The main source of information has been and will continue to be computer output microfilm (COM). This is generally 35 mm roll film with an ever-increasing percentage being in color. The Laboratory will also have a microfiche capability in the near future. The results are displayed at regular intervals during the course of a problem and in many different ways. This film provides all the information that the designer was able to request before the problem was run. In most cases, hindsight indicates that a picture of the problems at a given time and a particular view was not requested and would have been invaluable in analyzing the results of the production run. The standard COM output gives little feel for the relationship between physical variables and time. However, the data record of the run is in a compact form on COM and is saved for later reference.
Movies give the viewer another dimension in viewing the results of the calculations. It gives a qualitative indication of the relationship between physical variables and time. We generated our first movie from computer-generated output in 1960. The problem was run on an IBM 704 computer and the geometry was plotted on a mechanical plotter at intervals during the problem. These drawings were then photographed on 16 mm film with each plot being exposed 10 times. The result was a poor movie, rather jumpy, but still valuable and indicated that movies would indeed serve a useful purpose in problem analysis.
During the early development of 16 mm film by the Laboratory's Central Computing Facility, the turn-around time for 16 mm film was from two to six weeks. When this was reduced to a one day turn-around time, the use of movies became a practical tool for the designers.
Several problems become immediately evident when attempting to make movies simultaneously during the running of a production code:
In item one (1) we have been able to reduce the number of frames needed by the use of variable speed projectors. These projectors have viewing rates of from one to 24 frames/second, both forward and reverse. This enables one to stop the film at various times, reverse it and view a portion of the film repeatedly. This has proved invaluable, not only in computer-generated movies but for other movies also.
A separate movie code, M2C (MAGEE Movie Code), was developed to solve some of these problems, M2C picks up data from a file written by the production codes. This intermediate file contains all designated variables at a number of problem times. These variables are represented by 20 bit words giving about four significant figures and a data range of 0.000015 to 65,536.0, both positive and negative. In most problems, several hundred problem times will adequately describe a problem. Often data are saved at large time intervals during certain phases of a problem and at small time intervals at other phases. This intermediate file structure is well defined and is now being generated by a majority of production codes.
M2C has the ability to generate a specified set of problem times from those given through interpolation. This enables one to make a movie with constant time increments with data that is sparse at some times and dense at others or even when given at unequal time intervals. The code will also generate new variables from those given; for example, one may convert from rectangular to polar coordinates. The modified data is then available for making movies.
At present, M2C contains eight different plotting modules. Each module is independent of the other modules and is easily modified for special plots. New modules will be added as required. Each plot module contains a number of easily specified options; giving the user complete control of the generated movie. Some of the options available are color, plot orientation, background grid control, selection of variables, dynamic tracking of a spccified node, partial mesh selection, and time-step control. The plot modules include mesh plots, interface plots, contour plots, several isometric plots, rotational plots, and several experimental plots.
M2C has, in addition, a number of options for titles. A movie can be completcly generated by M2C with titles, any number of individual movies, and leader at both ends. The generated film is ready for public showing upon processing.
The movie code is at present used for movie generation by a number of production codes at LASL and is being adapted by others to provide all COM plotting output. We intend to use the code as a basis for providing terminal graphics. It will be extended to provide more types of plotting and to give multiple plots per frame.
This is not the only code in use at LASL for movie generation. It is a special code designed to display the results of calculations involving two dimensional meshes. It has proved itself as a valuable tool in a production environment and is unequaled in presenting the results of two dimensional time-dependent mesh calculations.