COMPUTER GRAPHICS AND IMA(]E PI%OCESSING 6, 485-4.ql (1977)

NOTE

Line Printer Modification for Better Gray Level Pictures

l°I~IILI P I-IA~%IILL

Control Systems and tIuman Engineering Laboratory, l~ational Re~earch Council, Ottawa, Canada K1A OR6'

Received June 2, 1976

Pictorial output; from the standard line printer is convenient, inexpensive, and fast bu t still relatively crude and inefficient. Simple and practicable modifications would give improved pict~ure quality, easier programming, and faster output. I~ is suggested tha~ manufacturem and users would both profit if these modifications were made avail- able as a line printer option.

i. INTRODUCTION

Increasing activity in image processing and visual pattern recognition has gener'~ted ~ widespre'~d need in the computer world for a gray level display of pictures. The wu'iety of graphic devices available to meet this requirement in- eludes electrostatic printers, ink plotters, microfilm printe,'s, solid-stt~te light- emitting panels, c'~thode ray tubes, and line printers. The latter, because of its common availability at computer installations, h'~s been a popular choice for picture generation particularly at the quick-look working level. This is hardly surprising. The line printer is, in its own right, a valuable peripheral device in "my computer system and offers, in addition, effective hard copy gray level output at high speed and very low cost.

A number of publications [1-4J have dealt with the improvement of line printer gray level output. Among the problems treated are the selection of characters and overprints to produce the best gray level representation, the avoidance of unwanted texture particularly of a directional nature, contrast smoothing tech- niques in the range of print densities, and the general optimization, efficiency, and convenience of the technique.

In spite of these efforts line printer pictures are still relatively crude. Also, because the better gray level representations require overprinting of characters (up to eight levels) there is room for improvement in the efficiency of ~he tech- nique. It is suggested that the problems with gray level output on a line printer are inherent in the mechanics and attributes which are necessary for the device to fu]fill its prime reason of being, i.e., to print clear and legible alphanumeric

485

Copyright ~ 1977 by Academic Press, Inc. All rights of reproduction in any form reserved.

4S6 PHILIP HAMILL

text. The requirements for good gray level picture output are different and it would a.ppear that little further improvement is possible unless the standard line printer is mechanically modified toward this end. Fortunately, the required modifications are neither difficult or expensive and do not conflict with the prime function of the device.

2. LIMITATIONS OF THE LINE PRINTER

A machine whose prime purpose is to produce good legible alphanumeric text has an inherent disadvantage as a gray level picture device. Text is an arrange- ment of discrete, recognizable, and separate elements. Ideally a picture is a continuous global ensemble. Distinct elements or clusters of the same element in a regular array are distracting and intrude on the eye.

On a sLandard line printer the character spacing is 10 per inch horizontal and 6 per inch vertical. Maximum character size is 0.067 in. wide by 0.10 in. deep. Line printer pictures therefore have a redundant texture of vertical white lines 0.033 in. wide at a spacing of 0.067 in. and horiT, ontal white lines 0.067 in. wide at a spacing of 0.10 in. This texture, being uniform throughout the picture, is acceptable to the eye as part of the medium in much the same way as a tapestry weaving pattern. I t is nevertheless a considerable imposition on %he ideal con- tinuous pictm'e and it is little wonder that line printer pictures start to look really good only at viewing distances of about 10 feet.

More serious is the loss of contrast since the largest overprint of characters occupies only 40% of an eIemental area so that the darkest element possible is, in fact, 60% white.

The difference in horizontal and vertical character spacing produces serious distortion if all elements from the usual square picture scan are printed. This may be overcome either by interpolating and printing, in the standard line printer setup, only six output lines for every ten data rows or, alternately, a suitable picture scan spacing may be employed. In either case the net result is a loss of information and some added complication.

The need for overprinting characters to produce the required gray levels is simply inefficient. While not perhaps too serious in the relatively fast output time of a line printer picture the technique would clearly be better without overprints.

Finally, the alphanumeric characters are themselves a disadvantage. Dis- crete elements in what ought to be a continuous ensemble are disturbing enough. Lack of rotational symmetry in such elements is an additional distraction. But most disconcerting is the recognition of familiar characters and there are surely few markings as recognizable as alphanumeric text.

3. LINE PRINTER MODIFICATIONS

In view of the above limitations two modifications suggest themselves to im- prove gray level output on the standard line printer.

The first of these is straightforward : A. switch option to allow the same character spacing vertically and horizontally. Since many existing machines already allow

LINE PRINTER MODIFICATION 487

a choice of six or eight rows per inch it should not be difficult to include an option giving ten per inch.

The second modification would require a special gray level character set. While the details of such a set are open to discussion and experiment there is no question about what is the required range, which is from clear white to solid black.

Given these two modifications the line printer would be a serious contender as a gray level device. The existing problems of redundant texture, lack of con- trast, information loss, and inefficiency due to overprinting would be solved while retaining the advantages of convenience, high speed~ and low cost.

~i. GRAY LEVEL CI-IAI-£ACTEI~ SET

The ch:~racter set design problem may be stated as follows :

(i) In an element~l area of 0.10 in. square the set should imprint a series of markings from clear white to solid black.

(ii) The maximum number of gray levels possible are required and, as far as possible, the steps should conform to the f'mt that the eye perceives a doubling of intensity as a unit increase.

(iii) Characters should not be recognizable, should not be directional, and should preferably have rotational symmetry.

(iv) Overprinting should be minimized and, if possible, eliminated. (v) The characters should be mechanically acceptable and sufficiently rugged

for the job.

The ide:tl solution wouhl produce gray tones uniformly distributed over the whole elemental area with increasing density steps between clear white and black. It is natural to look first at the best available approximations to this ideal. A pattern of lines covering the elemental area with the line thickness doubling for each successive gray level, or its complement, an arrangement of dots of increasing size, would seem to offer the best approximation to continuous gray tones. Un- fortunately the practical limitations on the minimum line thickness and dot size which can be printed, accepting that existing line printer characters fulfill the criterion for ruggedness, would seem to exclude this approach. The elemental 0.1-in.-square area may be usefully considered as a 10 X 10 area in unit lengths of 0.01 in. The minimum line thickness with standard character designs is about 1} units. A very simple symmetric pattern with some pretensions to uniformity across the whole area would be two horizontal and two vertical lines of a length of 10 units, l~'or this character the area covered by tl~e minimum imprint would thus be 51 units (4 X 10 X 1½ - 4 intersections of 1½ X 1}) allowing only one doubling for a total of three gray levels including clear. Even the rudimentary cross which is far from ideal as an area distributed pattern has a minimum imprint of 2 X 10 X 1} - 1½ X 1} = 27} units and would permit only four gray levels. Similar limitations are found when a uniform dot pattern is con- sidered. The minimum dot imprint is 2 X 2 units. It is, in fact, more realistieMly 2 units deep X 3 units wide on a chain-train-type machine since the fast-moving character loop imparts a horizontal smear to the dot imprint. Ignoring this

4S8 PHILIP IIA3'IILL

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2 3 4

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l~m. 1. Gray Level Character Set.

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complication the minimum symmetrical dot distribution in a square element would require four dots with the ]east possible imprint area of 16 units so that the available number of gr~y levels, including clear white, is a somewhat un- satisfactory five. It would appear that meehanica~ design criteria [or existing line printer characters produce a minimum imprint area that is too large for even the most elementary ~rea-distributed patterns and the required doubling of intensity per unit gray step will not allow enough levels.

:From this point the practical mechanics of the situation, as is so often the case, tend to dominate the design. Starting at level 1 which is clear white it is obvious that level 2 must be the smallest possible black imprint. In practice this is the

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STANDARD LINE PRINTER CHARACTERS

2 3 4 5

S~MULATION OF PROPOSED NEW CHARACTERS

FIe. 2. Gray Level B~r Char~.

6 7 8

LINE PRINTER MODIFICATION 489

single dot character which prints as a 2 X 3 unit rectangle on a chain-train-type machine. I t is considered highly desirable that this basic character, which in subsequent gray level steps will grow to fill the elemental square area, should be itself a square. While a 2 >< 2 unit imprint might well be possible it seems prefer- able, at this stage, to go along with the more robust and certainly practical 3 X 3 unit imprint. The choice of the 3 X 3 square character as the gray level step above clear white is also justifiable on the basis of symmetry when the dark end of the range is considered. Here the concern is with the gray level below solid black and it is doubtful that a white square on a black background any smaller than 3 X 3 units would print consistently and visibly. With both ends of the range determined the intermediate levels follow by ~he doubling bIack area up- ward and the white area downward to give the following distribution of black and white areas :

Level nmnber Black units White units

1 0 100 2 9 91 3 IS 82 4: 36 64 5 64 36 6 82 18 7 91 9 8 100 0

The proposed character set imprints are shown in Fig. 1. This may be recognized as the familiar newspaper photograph halftone treatment. I t seems likely that one additional gray level could be realized by designing a character that would imprint a (2 X 2)-unit square instead of the (3 X 3)-unit square proposed for gray level 2.

In passing it should be noted that Fig. 1 shows the imprints required and not the actual character set. These should be developed by experiment on the particu- lar machine to counter inherent distortions such as the horizontal smearing of the dot character already mentioned.

A compromise in the requirement for no overprinting would allow the eight gray levels to be realized with only six new characters. Since levels (2, 7), (3, 6), and (4, 5) all complement to solid black the level eight character could be realized with one overprint.

5. DIGITAL PLOTTER SIMULATION

In order to get a realistic assessment of the proposed gray level character set a simulation was made using a digital XY plotter. The resultant'characters are as close to the proposed set in Fig. 1 as the pen mechanics and resolution of the digital plotter would allow. Figure 2 shows the gray level ba.r charts for the standard line printer characters with six overprints ~nd for the proposed gray level character set. A test picture has been processed using both character sets. Figure 3a shows the standard set and Fig. 3b the new gray level character set. Clearly the proposed line printer modifications will make much better pictures.

490 PHILIP HAMILL

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Fro. 3a. Tes~ Picture wi[h S~and~rd Character Set;.

6. CONCLUSION

A comparison has been made between the gray level picture output of a standard line printer and that of a proposed line printer modification using a realistic simulation on an X Y digital plotter. I t has been shown tha~ a significant im- provemen~ in gray level output can be realized. The proposed modifications are practicable and would improve picture quality and output speed. The 1-to-1 correspondence between picture elements and picture output allows reproduc[,ion of a square raster scan without interpolation so that programming is easier and there is no loss of information. The proposed charaet, er set would give eight distinct gray levels without any overprinting and nine would be possible if, as seems ]ikely~ a small enough single dot character can be realized. With minimal overprinting there should be little d i~eul ty ia increasing the number of gray levels. It is suggested that the proposed modification would be an extremely useful

option on a line printer.

LINE PRINTER MODIFICATION 491

Fro. 3b. Tes~ Picture with New Ch~raeter Set.

REFERENCES

1. B. Perry and M. L. Mendelsohn, Picture generation with a s~andald line pHnLer, Comm. ACM 7, 1964, 311-313.

2. P. Stucki, Generation of gray tones by computer for simtd~tion of visual information systems, IEEE Trans. Computers ¢-18, 1969, 642-643.

3. D. G. MacLeod, Pictorial output with a line printer, TREE Tram, Computers C-19, 1970, 160-162.

4. P. Henderson and S. T~nimoto, Considerations for efficient picture output~ vi~ line printer, Computer Graphics fmage Processing 3, 1974, 327-335.