Subjective reactions to misconvergence on a colour display

P.J. ROBERTSON, M.R. JONES

Subjective ratings of various degrees of misconvergenco were obtained from subjects viewing text and graphics pictures on a 388 nun high-resolution shadow mask colour CRT display. The pictures were displayed in four secondary colours: white, yellow, turquoise (cyan), and pink (magenta). In commercial applications, user acceptance and satisfaction often present more stringent require- merits for front-of-screen quality than do objective performance criteria such as legibility. In this case, it is expected that the picture quality would become unacceptable at lower levels of miscon- vergenco than would cause it to be considered illegible. The results show a marked degradation in apparent quality of convergence as misconvergenco increases from about 0.2 nun to about 0.8 nun. To obtain subjectively acceptable picture quality, misconvergence should be no more than about 0.3 nun and should be better than 0.2 mr, over the majority of the screen. (The spot size of the dis- play used was about 0.8 ram.) Textual and graphical pictures did not produce significantly different results.

Keywords: display demces (compmers); colom7 ergononucs; cathode ray tubes; misconw~genc'e.

Misconvergenee of the primary colours in a shadow mask CRT is one of its inherent disadvantages relative to the monochrome CRT. It is more of a problem on a data display, where relatively fine detail in characters is viewed from relatively close, than it is for normal television applications. The degree of misconvergence varies with position on the screen, typically being worst in the comers.

The worst-case misconvergenee on a 355 mm screen (a typical size for computer visual display units) could be anywhere between 0.1 mm and 1 ram, depending on the tube technology and the amount of effort that the designer has put into convergence correction. Some modem displays employ digital convergence correction in order to achieve the best possible convergence.

The visual effects of misconvergenee are: first, an apparent 'fringing' of colours that are not fully in regis- tration; second, a blurred appearance that can be dis- turbing to the observer; and in the extreme case the character appears as two or three separate characters in the primary colours. These effects reduce the legibility of characters on the screen and are also displeasing or annoying subjectively. If the screen were displaying graphics, the equivalent of legibility might be that the various lines were difficult to discriminate, or that their colours became difficult to identify. For example, a yel- low line might appear to be a red line and a green line close together.

P.J. Robertson is at the Human Factors Laboratory, IBM United Kingdom Laboratories Ltd, Hursley Park, Winchester, Hants S021 2JN, UK; M.R. Jones is at the Department of Human Sciences, Loughborough Uni- versity of Technology, Loughborough, Leics LEl l 3TU, UK. This paper was given at a NATO workshop on colour-coded versus monochrome elec- tronic displays (RAE, Farnborough, Hants, UK, 28 February-1 March 1984).

The cost of improving misconvergence increases with the quality that the designer is trying to achieve. It is therefore useful to establish both the lower limit beyond which there is no effective improvement even if the misconvergence is reduced, and the relationship between degree of misconvergence and perceived quality. That is what this study attempted to investi- gate. The relationship between legibility (or its graphics equivalents) and misconvergence is also of interest, but is probably less directly useful to commercial display design for reasons discussed below.

Previous work Little previous work was found on this topic in the human factors literature - - most of the effort in this area has been technological in nature, rather than the user requirements approach that was adopted here. Snadowsky et al 1 in a colour display simulation found that misregistration of primary colours of 100 per cent of the stroke width was needed to produce a significant performance change in a symbol identification task.

2 Booth and Farell , in an experiment using a high reso- lution line graphics display, found that misconvergence of 25 per cent of stroke width was detectable and 50 per cent of stroke width objectionable.

Survey work by the current authors indicated that some users of commercially available terminals were dissatis- fied with the levels of misconvergence present on their displays. They tend to consider the misconvergence as an annoyance or a potential cause of fatigue, rather than as a factor which limits the productive usefulness of the display. A quick test in the laboratory showed that subjects were able to detect misconvergence and identify correctly the direction of one colour relative to the other, down to misconvergence levels as low as 0.I mm.

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Subjective versus objective measures The work mentioned above, and experience with front- of-screen characteristics of displays in general, led to the conclusion that the subjective acceptability of mis- convergence was likely to be more critical in a commer- cial application than any performance degradation caused by normal levels of misconvergence. That is, an operator will object to or reject a display because of misconvergence at levels where it does not measurably interfere with performance at visual tasks. The approach chosen for the investigation therefore con- centrates on the subjective reactions of viewers of a colour display.

METHOD Two basic conditions were investigated. In the first experiment, subjects gave opinions based on viewing a smaU-area picture in which the degree of miscon- vergence could be controlled and measured accurately. In the second experiment, subjects gave opinions based on viewing full-screen pictures within which the mis- convergence varied significantly from point to point but where the maximum misconvergence on view was con- trolled. The second experiment simulated practical dis- plays, where misconvergence in the corners of the screen is the problem. The method, of the second experiment in particular, is described more fully elsewhere 3.

Aptmratus Both experiments used an IBM 3279 Color Display Station, Model 3B, to display the stimulus pictures. This display has a 355 mm tube, 720 x 404 pixels (80 x 32 characters), and seven colours. It is equipped with a keyboard-controlled digital convergence correc- tion system that enabled us to vary the misconvergence systematically. The subjects' responses were captured and filed by the program that displayed the stimulus pictures.

Procedure In both cases, the subjects were introduced to the con- cept of misconvergence and given practice trials and opportunities to ask questions before the experimental trials began. The explanations and instructions appeared as panels of text on the display used for the experiment. Normal office lighting and furniture were used for the tests. Subjects were asked to give ratings that they felt would be appropriate if they had to use the display regularly every day.

Sman-area ~ t The subject sat at an uncontrolled distance from the screen. A block of text was displayed in a single colour in an area of one sixteenth of the screen. The subject rated the misconvergence in the block on a seven-point scale from 'excellent' to 'very bad', by positioning the display cursor on a scale that was included as the bot- tom row of the block. The scale showed abbreviated forms of the seven points of the scale as follows (upper case shows the abbreviation in each case):

Excellent; Very Good; Good; Adequate; Poor; Bad; Very Bad.

Iatrge-area experiment In this case, the whole screen showed a text or graphics picture repeated sixteen times. The graphics version had continuous lines between the sections and re- sembled a circuit diagram. The text version resembled four columns of text. All the sections were in the same secondary colour, except where the maximum miscon- vergence required by the experimental design was exceeded by that present in a section, in which case the section was shown in the primary blue. The subject gave a rating for the misconvergence on the picture, on the same scale as above, by pressing an appropriately labelled key.

Measuring misconvergence The picture area of the display was divided into sec- tions, each 20 characters (180 pixels) wide by eight characters (96 pixels) deep. The misconvergence was measured section by section for each colour by viewing short lines on the screen through a low-power micro- scope. The positions of the brightness centres of the primary-colour lines making up the lines were esti- mated visually, and the distance between these bright- ness centres was counted in phosphor dot pitches, to the nearest quarter of a dot pitch. The known dot pitch was then used to calculate the misconvergence in milli- metres. Horizontal and vertical misconvergence were measured separately, and the resultant calculated.

This procedure was repeated for each of the three secondary colours at five points in each section of the screen - - four corners and centre. White miscon- vergence was taken as the worst of the other three secondaries at each point. For each section, a worst- case figure and a centre-weighted typical figure were calculated. The typical figure took account of variation in direction of misconvergence within the section.

Experimental design The independent variables were colour (four secondary colours); degree of misconvergence; and text or graphics. In the small-area experiment, miscon- vergence was related to position on the screen for any given subject, but the misconvergence present in the different areas was varied four times during the experi- ment to reduce any bias due to screen position. In the large-area experiment, misconvergence was deliber- ately set to be good at the centre and poor in the corners, in order to simulate the most common real case.

In the small-area case, the misconvergence was set up to vary with screen position as described above and the actual misconvergence present in each of four areas measured. The subject was then presented with a pic- ture in a randomly-selected colour in one of the four areas, also selected at random. Subjects were asked to complete 80 trials.

The results were classified according to the miscon- vergence present in the stimulus picture, into the fol-

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lowing categories: less than 0.15 ram, 0.15 mm to 0.25 mm, 0.25 mm to 0.35 mm, 0.35 mm to 0.45 mm, 0.45 mm to 0.55 ram, 0.55 mm to 0.65 ram, and above 0.65 ram. The experiment was run twice, with text the first time and with graphics about a week later.

In the large-area case, subjects saw both text and graphics, half starting with graphics and half with text. Each subject saw 60 graphics and 60 text pictures. The colour of each was chosen randomly. Six conditions of maximum misconvergence were employed: 0.15 mm, 0.25 mm, 0.35 mm, 0.45 mm, 0.55 mm and 0.65 mm. The program chose one of these conditions at random. It then used a table of misconvergence measurements to check which sections of the picture in the chosen colour exceeded the maximum misconvergence for that condition. Any section where the maximum was exceeded was switched into a primary colour (pale blue) and hence was excluded from the judgement of misconvergence. After half the subjects had completed the experiment, the 0.15 mm condition was dropped and a 0.75 mm condition added.

Subjects All the subjects were volunteers from the IBM Hursley Laboratory. They were users of the 3279 display, but were not involved in CRT design. The small-area experiment used 14 subjects, all male. The large-area experiment used 24 males and 2 females. The age range was 18 to 53.

Smal l - a rea ratings

Tables 1 and 2 show rating results from this experi- ment. The text and graphics data are combined in the

Tables, as their ratings were not significantly different. (Inspection of the data, however, showed that at mis- convergence levels of 0.55 mm and 0.65 mm the text tended to be rated somewhat worse than the graphics, but this may be an effect of the presentation order of the two runs.)

Table 1 shows the data categorized by the typical mis- convergence of the stimulus area, calculated as a centre-weighted average for the area taking changes of misconvergence direction within the area into account. The misconvergence in the 'greater than 0.65 mm' cate- gory was between 0.7 mm and 0.85 mm.

The data in Table 1 were analysed using the chi-square test, and are significant, p < 0.001. A row-by-row analysis was then conducted, which showed that all the misconvergence conditions were significantly different at p < 0.001, except the 0.15 mm against the 0.25 mm conditions, wherep < 0.05.

Table 2 shows the same data, categorized by the worst misconvergence measurement from the stimulus area. The 0.15 mm category is not present because all the areas contained at least one point that had miscon- vergence worse than 0.15 mm. The 'greater than 0.65 mm' category includes worst-case miscon- vergences between 0.7 mm and 1.0 mm.

These data are also significant atp < 0.001 overall, and all the pairs of misconvergence conditions are signifi- cantly different at p < 0.001, except the 0.65 mm versus >0.65 mm conditions, where p < 0.01. The results, therefore, show a steady reduction in the sub- jective rating of quality as misconvergence worsens from about 0.25 mm worst case or 0.15 mm typical.

Table I. Ratings given to typical misconvergence levels

Rating (%)

Misconvergence (ram) Excellent Very good Good Adequate Poor Bad Very bad

0.15 10.1 25.0 31.4 26.0 5.3 1.6 0.5 0.25 5.9 21.1 28.5 29.3 13.0 2.2 0 0.35 0.8 4.5 11.4 25.4 40.2 13 A 4.2 0.45 0.3 3.0 7.1 16.9 34.6 31.1 7.1 0.55 0 0 0 2.9 25.1 48.0 24.0 0.65 0 0 0.5 4.5 15.3 38.7 41.0

>0.65 0 0 0 1.9 2.5 9.3 86.3

Table 2. Ratings givea to worn cue miaconverllem~ levels

Ratings

Misconvergence (mm) Excellent Very good Good Adequate Poor Bad Very bad

0.25 13.7 29.7 27.8 22.4 4.2 1.9 0.4 0.35 3.1 16.2 32.1 30.0 16.9 1.7 0 0.45 1.4 10.1 17.6 33.0 32.1 5.3 0.6 0.55 0.2 1.9 6.4 18.1 33.3 29.9 10.2 0.65 0.5 0.9 2.3 8.7 28.2 34.3 25.0

>0.65 0 1.6 3.4 6.6 21.5 26.5 40.5

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Table 3. Ratinm given to maximum mimconvergenee leveb

Rating (%)

Misconvergence (ram) Excellent Very good Good Adequate Poor Bad Very bad

0.15 37.5 31.6 27.5 3.3 0 0 0 0.25 13.1 17.7 33.0 30.8 4.9 0.4 0 0.35 2.1 3.1 13.0 45.0 32.2 4.4 0.2 0.45 0.2 2.5 9.6 28.0 46.2 12.1 1.3 0.55 0 1.5 1.7 12.6 40.0 29.9 14.4 0.65 0.2 1.6 3.7 14.4 39.8 28.7 11.5 0.75 0 0 1.4 9.6 39.0 30.7 19.3

Full screen ratings

The results from the two experiments were in broad agreement. There was a marked fall-off in subjective quality from about 0.25 mm misconvergence, where over 90 per cent of ratings were 'adequate' or better, to about 0.55 mm misconvergence, where over 80 per cent of ratings were 'poor' or worse. The miscon- vergence measurements referred to here are the mean misconvergence in the worst visible area of 1.5 per cent of the screen.

No difference in ratings between text and graphics pic- tures was found, and the results below are therefore a combination of the two. Table 3 shows the rating results from this experiment.

The results for the various levels of misconvergence were compared using the chi-square test. An overall test on the seven conditions was significant, p < 0.001. A pair-by-pair comparison was then conducted, and all the pair-by-pair comparisons were significant at p < 0.001, except 0.55 mm versus 0.65 mm, which was not significant. Comparison of the ratings of the various colours at the different levels of misconvergence did not reveal significant systematic differences.

DISCUSSION

The results show that over the range of interest, the subjective quality of misconvergence changes dramati- cally. At levels around 0.2 mm it is predominantly rated as good or better, but tends to be rated poor or bad at about 0.5 mm or worse.

The interaction between misconvergence and spot size requires mention: the papers referred to above quoted misconvergence as a percentage of stroke width, and this seems reasonable intuitively. The spot size on the display used for this experiment varies, of course, with colour and screen position (brightness was held con- stant during the test). We have not conducted an analysis of misconvergence against spot size, but our measurements indicate a typical spot size on this par- ticular display of about 0.8 mm, measured at 5 per cent of the peak luminance.

In the small-area results, there was an indication that subjects were less sensitive to misconvergenee in the turquoise secondary colour (green plus desaturated

blue) than in the other secondaries. This was not borne out in the large-area results.

The conclusions to be drawn about misconvergence requirements in real life situations depend on where in the scale between 'excellent' and 'very bad' one chooses to fix the lower limit of acceptability, and also on the relationship between subjects' responses in the artificial setting of the laboratory and users' attitudes to display quality in real applications.

The 'adequate' rating provides an apparently reason- able break-point: one might require that all or nearly all the users of a display would consider it at least adequate. This would be equivalent to setting a limit on the number of users who found the display actually objectionable. We have not scientifically validated the scale used for the ratings in these experiments. In a number of unpublished studies on other front-of-screen variables, however, we have found a degree of agree- ment between the laboratory ratings and subsequent reports from real-life users: where a quality was rated poor to bad by a majority in the laboratory, corrective action was required; ratings of adequate or good in the laboratory predicted satisfaction in practice.

The rating results can be used relative to each other without external validation. The data is examined for a misconvergence level beyond which the ratings fall sud- denly: this level is therefore a good misconvergence/ subjective quality breakpoint for practical use.

lOO

5( o ,O

b

0 I ! I I I I C 0 0 .15 0 . 2 5 0 . 3 5 0 4 5 0 . 5 5 0 . 6 5 0 . 7 5

Moximum misconvergence (ram)

Fig. 1 Ful l screen ratings: a ~ p o o r or better; b - - adequate or better; c - - g o o d or better

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A study of users of a colour display 4 showed that many of them were prepared to tolerate areas of miscon- vergence on a colour display, even though they had the means available to adjust it easily, in cases where the usage of the misconverged colour was small. Where secondary colours were regularly used for large areas of the picture however, a number of users did object to misconvergence in the range of 0.4 mm or 0.5 mm. They nonetheless considered the picture quality of the display to be generally superior to a monochrome equivalent.

In order to estimate the required misconvergence, therefore, we selected values where our data showed that 90 per cent or more subjects rated the typical mis- convergence as adequate or better, and where the worst case was also rated adequate or better in the small area test. We then compared these conclusions with the results from the large-area experiment, to check that the ratings of the full-screen picture with similar small-area worst-case misconvergence were also acceptable.

From Table 1, we conclude that typical misconvergence of 0.15 mm is acceptable but that 0.35 mm is not. A level of 0.25 mm satisfied about 85 per cent of the sub- jects. Table 2 shows a small-area worst case of 0.25 mm to be acceptable, 0.45 mm to be not acceptable, and 0.35 mm to satisfy about 80 per cent of subjects. This gives a tentative result that misconvergence should be typically about 0.2 mm or better with small areas of up to about 0.3 mm acceptable.

Comparing this result with data in Table 3, we find that 95 per cent of subjects were satisfied with miscon- vergence up to 0.25 mm, but that misconvergence of 0.35 mm is not satisfactory, even in a small area. We therefore conclude that for this type of display, miscon- vergence should be 0.3 mm maximum, and should be

0.2 mm or better over the majority of the display. (38 per cent and 25 per cent of the spot width at 5 per cent of maximum luminance, respectively.)

CONCLUSION

It was found that the subjective quality of a colour dis- play reduces significantly as misconvergence increases above about 0.2 mm or 25 per cent of spot width. Areas of greater misconvergence are tolerable if small, however.

For this type of display, in order to satisfy the users' subjective requirements, the misconvergence should be not more than 0.3 mm or 38 per cent of spot width and should typically be not more than approximately 0.2 mm or 25 per cent of spot width.

R eferences

1 Snadowsky, A.M., Rizy, E.E., Elias, M.F. 'Symbol identification as a function of misregistration in color additive displays' Perceptual and Motor Skills 22 (1966) 951

2 Booth, J.M., Farell, R.J. 'Overview of human engineering considerations for electro-opgk~l displays' Proc SPIE 199 (1979) 89

3 Jones, M.R. 'Subjective response to misconvergence on a colour display' Part A of report for Diploma in Professional Studies (Department of Human Sciences, University of Loughborough, Lough- borough, Leics, UK, 1983)

4 Robertson, P.J. 'A survey of users of the IBM 3279 Colour Display Station' Tech Rep TR12.193 (IBM United Kingdom Laboratories Ltd, Hursley Park, Hants, UK, 1981)

Controller, Her Majesty's Stationery Office, London, 1984.

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