Perceptual Evaluation of Sub-pixel Rendering in a Four-primary Display System

Li Chen, Yan Tu, Lili Wang, Fudong Li School of Electronic Science and Engineering,

Southeast University Nanjing, China

Abstract—Large color gamut can be achieved using multi-primary system. Perceived color varies with different sub-pixel arrangement in multi-primary system. Paired comparison experiment has been carried out to study the influence of sub-pixel arrangement of four-primary system. A novel color conversion method has been put forward to achieve unique control signals and fast color conversion in the multi-primary system. With this novel color conversion method, the displayed images in four-primary system for the experiment were obtained. Results show that difference is not statistically significant between different sub-pixel arrangements and structures. With different additional primary, sub-pixel preference is similar. Influence of image content on the results is also not statistically significant.

Keywords-four-primary; color conversion; sub-pixel

rendering; perception experiment

I INTRODCTUION

Displays with large color gamut are required to present more colorful and vivid image. Multi-primary display system has been developed to enlarge color gamut [1, 2]. This technology faces several challenges. Color conversion method is non-unique due to that various stimulus are feasible to obtain same color. To look for an optimal color conversion method is the essential problem to implement multi-primary system. What’s more, optimization of sub-pixel design is required to achieve best image quality.

A Color conversion method Several color conversion method has been presented in

the published works. Most widely used methods are MS (Matrix Switching) [3], LIQUID (linear interpolation on equal luminance plane method) [4-5] and 3D-LUT (look-up table) [6].

For MS method, the color gamut in CIE-XYZ color space, which is a polyhedron, is divided into several square pyramids. The device control signals can be calculated by the linear transform using three by three matrices which defined differently from pyramid to pyramid. As each pyramid is corresponding to an area in the xy chromaticity plane (fig.1), the matrix can be specified using a two-dimensional LUT. The color conversion is high efficient using this operation. Signal discontinuities arise at the boundaries between pyramids.

LIQUID has been introduced to solve the discontinuity. Linear interpolation is utilized in the equal luminance plane (fig.2) to acquire the control signal. This conversion method is complexity due to the uncertainty and numerousness of the vertex in the plane.

Figure.1 Color gamut (a) Color solid in the CIE-XYZ color space (b) The corresponding xy chromaticity plane. Two shadow areas are corresponding parts.

Figure.2 (a) Color solid in the CIE-XYZ color space (b) The plane with equal luminance as the target color D. Each vertex is the intersection of the equal luminance plane and the polyhedron edge.

Color conversion using 3D-LUT is limited because large amount of measured data is needed to build up a look-up table.

B Pixel arrangement Arrangement of sub-pixels, which is the elementary

components of pixel, will influence the visual perceptual of the image quality. There is existing literature on the evaluation of sub-pixel arrangement, but conclusions about the preference of sub-pixel arrangement of multi-primary system are unambiguous. Michael E. Miller [8] investigated the relationship between just-noticeable differences (JNDs) and sub-pixel arrangement. Results indicated that JNDs of interlaced arrangements (fig.3 a) is larger than that of aligned arrangements (fig.3 b) with the same resolution.

Figure 3: Different sub-pixel arrangement (a) interlaced arrangements (b) aligned arrangements

This work was under the 973 project (2003CB314706).

(a) (b)

(a) (b)

(a) (b)

978-1-4244-4131-0/09/$25.00 ©2009 IEEE

Lili wang [9] evaluated the image quality with different sub-pixel arrangement for RGBW system. Results showed that sub-pixel arrangement has no influence on image quality for RGBW system.

The aim of this study is to evaluate the preference of different sub-pixel arrangement for four-primary system based on visual perception. An improved color conversion method was used to implement color conversion. Influence of sub-pixels arrangement and structure were investigated. Influence of image content was also observed. Comparison of the four-primary system with additional chromatic primary and special multi-primary system, RGBW system, is carried out.

II NOVEL COLOR CONVERSION METHOD

Uncertainty and numerousness of the vertex in the equal luminance plane result in the difficulty of LIQUID. The number of vertex is uncertain. In a 4-primary system, number of vertex is only 4 at low luminance, which is equal to the number of primaries. With the increasing luminance, number of vertex can reach to 7. So judgment is needed to confirm the vertex. For the assured vertex, judgment is also needed to determine which triangle the target color belongs to. An improved method has been put forward to overcome the shortcoming of this method. Equal luminance line is used here to replace the equal luminance plane (fig.4). This method is more fast than the method LIQUID and avoid signal discontinue at the boundaries between pyramids using the method Matrix Switching.

In the CIE-XYZ color space, each color can be described using tristimulu values (X, Y, Z). Y stands for the luminance. Coordinates of the polyhedron vertex is fixed for a given display. Color D is the target color. Color Gray is grayness in the White-Black line, luminance of which is equal to the target color. Liner interpolation between color White (W) and Black (K) is adopted to get color Gray according to formula (1).

( ) ( )

( ) ( )+

−−×−

=

=

+−

−×−=

KKW

KGrayKWGray

DGray

KKW

KGrayKWGray

ZYY

YYZZ

Y

XYY

YYXX

Z

Y

X (1)

Here subscript stands for different color and (X, Y, Z) are the color coordinates. The other color P is the intersection of the equal luminance line and the surface of the polyhedron. Equal luminance line can be obtained using formula (2).

−×−×

+×−−

=

=

GrayD

GrayDDGray

GrayD

GrayD

D

XXXZXZ

XXXZZ

Z

YY (2)

Equations of the 12 quadrangles, surface of the polyhedron described in figure4, are fixed. The target quadrangle is determined using judgment. For a given quadrangle, the description of the plane is fixed. Color P can be achieved according to formula (3).

=++

−×−×

+×−−

=

=

MCZBYAX

XXXZXZ

XXXZZ

Z

YY

PPP

GrayD

GrayDDGrayP

GrayD

GrayDP

DP

(3)

Where A, B, C, M are decided by judgment to confirm the target quadrangle. Liner interpolation between color P and color Gray is utilized to acquire the control signal according to formula (4) and (5) [7]. (S1, S2, S3, S4) is the control signal for each primary.

[ ] [ ]1−

=GrayGray

PPDD ZX

ZXZXnm (4)

[ ] T

GrayP

GrayP

GrayP

GrayP

D

D

D

D

nm

SSSSSSSS

SSSS

=

44332211

4321

(5)

Compared with LIQUID, judgment only needs to determine which quadrangles intersected with the equal luminance line.

Figure 4: Improve method using equal luminance line to replace the equal luminance plane.

III EXPERIMENT METHOD

Since the display panel with different sub-pixel arrangement was not available for the perception experiment, the different sub-pixel arrangements were simulated by software and a 32 inch LCD with a resolution of 1280*768 was used to display the simulation images. The additional primary ‘Yellow’ or ‘Cyan’ was used together with the traditional three primaries R, G and B to enlarge the color gamut to a four-primary gamut. Improved color conversion method was adopted. Each pixel included four sub-pixels in the new four-primary system. As JNDs of interlaced arrangements is larger than that of aligned arrangements, aligned arrangements will not been considered [8]. Two different experiments were carried out to investigate the preference of sub-pixel arrangement and structure respectively.

A Evaluation of sub-pixel arrangement Two natural images referred as “Cloth” and “Portrait”

(fig.5) were selected to evaluate the influence of image content. Luminance difference of these two images is not significant, while chroma difference is significant. Image “Cloth” is more saturated than image ‘Portrait’. Table 1 lists the average luminance and chroma of the two images.

TABLE I. AVERAGE LUMINANCE AND CHROMA IN LCH SPACE OF

THE TWO IMAGES

Image Luminance Chroma

Cloth 71.0 38.1

Portrait 63.6 11.4

In order to evaluate the influence of sub-pixel arrangement, different arrangements were included. For a four primary system, there were 4!=24 possible way to arrange these primaries. Considering the similar situations for different start sub-pixel, the first primary was fixed to be red and there were 6 different arrangements (fig.6). To find the influence of color gamut, primary ‘Yellow’ and ‘Cyan’ were added respectively to form different color gamut. Color gamut with additional primary ‘Yellow’ is about 1.7 times the gamut with additional primary ‘Cyan’. White point was always kept same. 15 compared pairs composing of 2 different sub-pixel arrangements from 6 were used. 2 images in a compared pair were located side by side and in the middle of the panel. In order to find the influence of nonuniformity of LCD, two different positions of the image, left and right, were included.

20 subjects participated in this experiment and were asked to select the preferred one from each compared pairs.

Totally, there were 120 stimuli (26C compared pairs × 2

positions × 2 color gamut × 2 images). The observation distance was set to be about 2m. Viewing angle of one simulated pixel was about 1.5’. People almost can not distinguish the sub-pixels at this viewing distance.

Figure.5: Two images used in the experiment.

Figure.6: Six different sub-pixel arrangements used in the experiment

B Evaluation of sub-pixel structure Two (fig.7) different sub-pixel structures were used to

study the preference of structure. 2 color gamut and 2 different images as the previous experiment were also included. Results of sub-pixel arrangement evaluation indicated that influence of position is not significant. Images in a pair displayed in the left or right is random. The experimental environment and procedure were the same as described in the previous experiment.

Totally, there are 4 stimuli (2 color gamut × 2images).

Figure.7: Two structures of sub-pixels used in the experiment

IV RESULTS AND DISCUSSION

The data of the paired comparison experiment were analyzed with the statistical package S-Plus, using the methodology described in R.Rajae-Joordens’s paper [10]. Data from the paired-comparison experiment were first combined in a count matrix. Each cell gives the number of times that the image corresponding to the percentage of sub-pixel arrangement mentioned in the column head is less preference than the image corresponding to the percentage of sub-pixel arrangement mentioned in the row head. Then the preference matrix can be got from the count matrix. Each cell of the visible matrix gives the fraction of times that the image corresponding to the percentage of sub-pixel arrangement mentioned in the column head which is less preferred than the image corresponding to sub-pixel arrangement mentioned in the row head. This fraction is calculated by dividing the count matrix for a given image over the subjects. The cells at the diagonal of the preference matrix have not been measured (in order to limit experimental time for a subject), but are assumed to have a value of 0.5, which would result from randomly picking one image with less preference sub-pixel arrangement out of a pair of equal sub-pixel arrangement. From the preference matrix, the Quality Scale can be deduced by first transforming all values into Z-Scores (assuming a normal distribution of the data), and then averaging all Z-Scores within a column.

A Sub-pixel arrangement preference First of all, influence of position, color gamut and image

content is not statistically significant (P>0.05). The nonuniformity of LCD doesn’t influence the preference significantly. For different color gamut, the preference is always similar. Fig.8 gives the simulation results for the image ‘Portrait’ with an additional primary ‘Yellow’.

Face in images with sub-pixel arrangement 1, 2, 3 and 5 look more reddish. However, difference is not statistically significant because some subjects prefer more reddish image and some prefer less reddish. Fig.9 gives out the quality scale for the arrangement preference for different image content. Sub-pixel arrangements with the same underline are in the same group, which the difference is not statistically significant.

Results indicated that there is no statistically significant difference between different sub-pixel arrangements used in the experiment with different factors.

Compared with RGBW system [9], results indicated that chromatic aberration is observed in both four-primary system and RGBW system. But preference of different arrangement is not obvious. Results are coincident.

Arr_1 Arr_2 Arr_3 Arr_4 Arr_5 Arr_6

Cloth Portrait

Str_1 Str_2

Figure 8: Simulation of different sub-pixel arrangements for the image ‘Portrait’ with an additional primary ‘Yellow’.

Figure.9 The quality scale of the arrangement preference as an average result of different image content and color gamut

B Sub-pixel structure preference Image content and color gamut has no statistically

significant influence on the structure preference (P>0.05). It means that the structure preference is similar for different image content and different color gamut RGBY and RGBC. The following fig.10 gives the quality scale for the structure preference. Structures with the same underline are in the same group, which the difference is not statistically significant.

Figure.10 The quality scale of the structure preference as an average result of different image content and color gamut

Results indicated that there is no statistically significant difference between different pixel structures used in the experiment.

Fig.11shows the simulation images corresponding to the different sub-pixel structures. There is no significant difference between different structures at the viewing distance 2m.

Figure 11: Simulation of different sub-pixel structures for the image ‘Portrait’ with an additional primary ‘Yellow’.

V CONCLUSIONS

A novel color conversion method was put forward to achieve fast color conversion. Using this algorithm, preference of different sub-pixel arrangement was investigated in a perception experiment for different image content and different additional primary. Results show that influence of color gamut on the sub-pixel rendering is not statistically significant. Sub-pixel arrangement preference is similar for the four-primary system with the additional primary Yellow and Cyan. Influence of image content is not statistically significant on the arrangement preference and the structure preference. Similar as RGBW system [9], influence of sub-pixel arrangement is not significant for four-primary system. Preference of different structure used in this experiment has no significant difference. Considering manufacture, Structure 1 is easier and better.

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Str 1 Str 2

Arr_1 Arr_2 Arr_3

Arr_4 Arr_5 Arr_6

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