Although a typical display consists of red-green-blue (RGB) subpixels, displays with various subpixel layouts have been used owing to their various advantages such as luminous efficiency. The purpose of this study is to verify the effects of display MTF on subjective spatial resolution by creating new subpixel layouts. We designed BRGRB and BRGRB525 subpixel layouts with slightly higher and much higher MTF than RGB subpixel layouts, and conducted visual evaluation experiments with RGB, PenTile RGBG and the two new subpixel layouts at 20 and 30 cycles per degree. It was verified that subjective spatial resolution generally follows the large and small relationship of display MTF. Additional experiments showed that the integral of the product of the contrast sensitivity function and MTF was highly correlated with the subjective spatial resolution.
count with one pixel as the minimum element. Conventional displays consist of colored red-green-blue (RGB) subpixels. Recently, various subpixel layouts such as RGB and white (RGBW) and RGB and yellow (RGBY) have been used. Be-cause such subpixel layouts employ various subpixel rendering technique, a new method for evaluating resolution is required.
(IDMS) by the International Committee for Display Metrology (ICDM) proposed a method to evaluate
the resolution capability of a display, with respect to its addressability, based on threshold contrast
modulation (Michelson contrast) associated with grille patterns. Several studies on subjective spatial
resolution have also been reported [
In our previous work, we conducted evaluation experiments to examine the subjective spatial
resolution for RGB, RGBW, PenTile RGBG (hereafter abbreviated as PenTile) subpixel layouts and
confirmed that differences in subjective spatial resolution occur even at a viewing distance equivalent
to the ITU-R [
there was no significant difference in subjective spatial resolution between the RGB and PenTile subpixel layouts which had the next highest MTF, we designed new subpixel layouts with a higher MTF than the RGB. Several new candidate subpixel layouts were created in reference to conventional RGB and PenTile subpixel layouts. By manually calculating the MTF of these structures and selecting subpixel layouts with higher MTF than RGB, two new types of subpixel layouts were designed—
was added to BRGRB525 because the area ratio of R, G, and B subpixels is 5: 2: 5.
Masaoka [
!"# !"# 4
5 + 6 1 : + 3
!"# !"# 4
3 + 6 1 : 3
(1) 5 !"# + 6 + !"# 4 1 : ; 3 where is a rectangular function. !"# , !"# , and !"# represent the luminance of the RGB subpixels, where !"# = !"# + !"# + !"#. Then, the () is obtained as () = >sinc C1 D C !"# $&%'( + !"# $'( + !"# $&)'(D> (2) 3 !"# !"# !"# where is the spatial frequency in cycles per display pixel. Using this method, the MTF for each of the
are shown in Fig. 2. The MTF values are in the order BRGRB525 ≫ BRGRB > RGB ≫ PenTile.
(a)BRGRB Figure 1: Designed subpixel layouts (b)BRGRB525
In addition to the conventional RGB and PenTile, and the newly designed BRGRB and BRGRB525,
four types of vertical black and white grille patterns were used as experimental stimuli. Figure 3 shows
the black and white grille patterns of each subpixel layout. In this experiment, the grille patterns were
presented on a liquid crystal display (ColorEdge CG248-4K, Eizo Corp., Japan). Table 1 shows the
display specification. Because this display has the RGB subpixel layout, it is not possible to display the
grille patterns with other subpixel layouts. Therefore, as in Ref. [
created as the experimental stimuli. The area ratio of the R, G, and B subpixels were different and the difference in brightness and chromaticity of the experimental stimuli affected the evaluation only for
subpixel layouts using a spectroradiometer (CS-2000, KONICA MINOLTA, INC., Japan). For instance, the G subpixel in BRGRB525 is half the area of the G subpixel in the other subpixel layouts; hence, adjustments were made to double the luminance value. The luminance values of the black and white lines in the grille pattern were set to be 0.4 and 49.5 /*. The background color of the experimental stimuli patterns was supposed as the average of the black and white luminance in the grille pattern.
When the experimental stimuli were displayed, Moire fringes appeared at the boundary
between the left and right edges of the stimuli and background. To prevent the stripes at the edge
of the stimulus from becoming a response cue, gradient processing was applied to the boundary
between the stimulus and background using the following procedure [
Resolution
Luminance Color temperature
Gamma value Color gamut
better (three with naked eyes and one with glasses). A total of 16 stimuli pairs were used—4 (RGB,
right stripe). The experiments were conducted in a dark room at 20 cpd (3.77 m) and 30 cpd (5.66 m),
based on the ITU-R [
To ensure that the observers had binocular vision of at least 20/20, their visual acuity was tested each time before the experiment. After the visual acuity test, a gray image consisting of only the background color was presented on the display for 3 min to adjust to the brightness of the display. Then, the stimuli were shown on the left and right sides of the display, and the participants were asked to respond by the two-alternative forced-choice task method—which stimulus they perceived more clearly—thereby confirming subjective spatial resolution. To account for the non-uniformity of luminance of the monitor, the experiment was also conducted with the display rotated by 180 degrees.
2.4.
and (b), respectively. The numbers in the pie charts in Fig. 5 represent the percentages of stimuli on the left and right side that were clearly perceived. For example, the 2-line, 1-column pie chart in Fig. 5(a) shows that 68% of the participants perceived RGB stripe clearly and 32% perceived BRGRB clearly when stimuli with RGB and BRGRB subpixel layouts were presented on the left and right, respectively. A response rate (in favor of one pattern) exceeding 75% for each experiment was defined as a significant difference. This is because the chance level of the two-alternative forced-choice is 50% and its psychological discrimination level is between 50–100%. The experimental results showed no significant differences at all viewing distances and for all stimulus pairs. The inter-observer standard deviation was 18.21% at 20 cpd and 11.53% at 30 cpd. The average of intra-observer standard deviation was 22.78% at 20 cpd and 21.00% at 30 cpd. Because no significant differences were observed in the stimulus pairs with the same subpixel layouts on the left and right sides, it appeared that the left and right responses are stable. (b)30 cpd Figure 5: Results of the experiment at (a) 20 and (b) 30 cpd as percentages
large difference in their MTF. Therefore, we created a new subpixel structure called GBWR with MTF
value between that of PenTile and RGBW, for which a clear perceptual difference was confirmed in a
previous study [
Two participants with normal color vision and visual acuity of 20/20 or better participated in the additional experiment. The results of the experiment at 20 and 30 cpd are shown in Figs. 8(a) and (b), respectively, as percentage of responses. Pairs for which significant differences were confirmed are circled in gray. The experimental results showed that subjective spatial resolution generally depended on the display MTF because significant differences were observed for the PenTile-RGBW pair, which has the largest MTF difference, at all viewing distances. At 20 cpd, however, only the GBWR-RGBW pair showed significant differences among the PenTile-GBWR and GBWR-RGBW pairs. The interobserver standard deviation was 9.375% at 20 cpd and 3.819% at 30 cpd. The average of intra-observer standard deviation was 22.64% at 20 cpd and 23.84% at 30 cpd. To discuss the experimental results, the MTF values at the Nyquist frequency (0.5 cpd) for each subpixel layout are shown in Table 2. The difference in modulation at the Nyquist frequency between PenTile and GBWR and between GBWR and RGBW are shown in Eqs. (4) and (5), respectively.
0.612 0.704 0.654 0.704 0.612 0.654 ≃ 0.869
by Mannos and Sakrison [
0.321 0.332 0.327 0.332 0.321 0.327 0.331 0.357 ≃ 0.969 ≃ 0.985 ≃ 0.984 ≃ 0.972 (9) (10) (11) (12) (b)30 cpd Figure 8: The results of the additional experiment
To further analyze the effect of display MTF on subjective spatial resolution, we designed two new types of subpixel layouts and experimentally verified the effects of these subpixel layouts on subjective spatial resolution. Our results confirmed that the subjective spatial resolution generally follows the magnitude relationship of display MTF. Additional experiments showed that the magnitude of the difference of subjective spatial resolution cannot be explained by the difference and ratio in modulation at Nyquist frequency. Furthermore, we found that the integral of the product of the contrast sensitivity function and MTF was highly correlated with subjective spatial resolution; however, they did not explain all the results. In the future, we will conduct more experiments with more participants and analyze metrics that can explain subjective spatial resolution.