The line printer then produces a picture, which is removed, shown to the customer, placed in a protective jacket, and the transaction concluded. An alternate procedure is to copy photographs or other art work, in which case the customer brings in original material which is placed under the camera and an appropriate printout made.
The simplest picture structure is the 12"x12" format containing perhaps 90x120 "points" of information. A typical example of one of these is shown in Figure 2 and uses a special symbol set to generate a pseudo grayscale. Figure 3 shows a different form of programming wherein a variable size dot structure is employed, resulting in the equivalent of a much enlarged newspaper "half tone". In either case the 7x9 matrix of the line printer gives a relatively coarse image and pictures must be viewed at some distance in order to produce a semblance of naturalness.
Reproduced picture quality is influenced by a series of factors, the first, of course, being reasonable flattering lighting. The TV camera is a key element as it will provide the initial limitation in picture quality. In most instances a high resolution camera will not be required and the important considerations will be freedom from noise and good grayscale reproduction. In connection with this, it is important to note that the standard vidicon camera employs an Antimony Trisulphide target which has semi- logarithmic characteristics, tending to compress the whiter portions of the picture. This is desirable when looking at a television monitor as it compensates for the anti-log characteristics of the CRT, but can have some negative aspects when a limited number of grayscale levels are digitized and when hard copy is generated. Camera tubes with linear characteristics are readily available, with the most common being the silicon diode target tube. This tube also has the advantages of exceptional ruggedness and freedom from accidental damage due to exposure to bright light. Estimated lifetime is approximately 10 times that of a conventional vidicon tube.
Inasmuch as the total "printing" process may take from 30 seconds to over a minute, it is highly desirable to have some means of picture storage so that the subject will not have to remain in a rigid pose for the entire process. Typical memories can include scan conversion tubes, video disc recorders, a digital field freeze, or the minicomputer memory itself, when used in a fast DMA mode. Using a memory prior to computer processing is convenient in that it allows the operator to preview the basic. image and insure that a pleasing result has been obtained. This can save a considerable amount of paper and/or customer dissatisfaction.
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