HDR DISPLAY USING MULTI-PROJECTORS
Rei Miyake
∗, Yuki Uranishi
∗,
Hiroshi Sasaki
∗∗, Yoshitsugu Manabe
∗and Kunihiro Chihara
∗∗
Graduate School of Information Science，Nara Institute of Science and Technology 8916-5 Takayama, Ikoma, Nara, 630-0192 Japan
∗∗
Information Science and Technology Center，Kobe University 1-1 Rokkodai, Nada, Kobe, Hyogo, 657-8501 Japan
E-mail:
∗{rei-m, uranishi, manabe, chihara}@is.naist.jp,
∗∗sasaki@kobe-u.ac.jp
ABSTRACT
This paper proposes an HDR display system using multi- projectors for presentation of HDR contents to multi-users.
An HDR image is resolved by luminance and the resolved images are assigned to several projectors. The proposed system projects the HDR contents onto a large screen, and the system can display the HDR contents to multi-users.
The proposed system realized to output the broad luminance by emitting the light of multi-projectors onto the same screen.
In addition, tonal steps of the proposed system increase so that other projectors cover the tonal steps in the region where the dynamic range is expanded. In this paper, we indicate an effectiveness of the proposed system.
Keywords: High Dynamic Range image, display system, multi-projectors
1. INTRODUCTION
Recently, displaying more real image is needed for an area of entertainments, such as a movie, a game, and a presen- tation. However, when displaying the images which have larger dynamic range than the range of the display device, the existing display system displays their images as the over- exposed or underexposed images. To resolve the issue of the limited dynamic range of the display device, capturing HDR (High Dynamic Range) image [1][2] and tone map- ping operators [3][4] have been developed. Tone mapping operators are able to display the HDR image on LDR (Low Dynamic Range) display device such as Cathode Ray Tube (CRT) and Liquid Crystal Display (LCD).
Additionally, to display HDR images appropriately, it has been developed that the dynamic range is extended by increasing an output luminance of the display device. Sev- eral techniques have been proposed for expanding a dy- namic range. The display device increases tonal steps and expands a dynamic range by using DMD (Digital Mirror Device) and LED (Light Emitting Diode) in accordance with a luminance distribution of the image [5]. However the sys- tem is for a LCD. It is difficult for the system to present HDR contents to multi users, because a screen size and a view angle are limited.
Fig. 1: System outline
In this study, we propose an HDR display system us- ing multi-projectors to present HDR contents to multi users.
The proposed system can display HDR contents onto a large screen by using projector, and it can display HDR contents to multi-users because a view angle of a projector is wider than a LCD. In this paper, an HDR image with broad dy- namic range is resolved plural images by luminance and the images are assigned to several projectors.
2. PROPOSED HDR DISPLAY SYSTEM
Figure 1 shows an outline of the proposed HDR display system. The proposed system is to increase output lumi- nance and to expand a dynamic range by arranging multi- projectors and overlapping images onto the same screen.
The important points in this system are that mapping and overlapping image in this system is detailed in the follow.
Additionally, output luminance values are non-linear to in- put luminance values so that the proposed system is used multi-projectors. To present the input image accurately, it is needed to correct property (e.g. gamma correction) of the proposed system (Section 3.2).
Fig. 2: Arrangement of projectors of proposed system. Both projectors are same performance.
2.1 Mapping and overlapping the images
To display an HDR image using multi-projector, it is need to divide an image has a broad dynamic range and map the dividing image to each projector. It is important that how to divide the image has a broad dynamic range. We use the mapping approach was adapted from [7]. This approach is that an image has a broad dynamic range is divided by spectrum. We map diffuse and specular reflection to each projector in order to present HDR image with Computer Graphics using proposed system (Section3.3.1). In addition, we use the approach that takes the plural photographs are taken with different exposure [1] to display an HDR pho- tographed image. To keep the tonal steps and to present the HDR displayed image without having the underexpo- sure and overexposure, the images which are changed an exposure is mapped to each projector (Section 3.3.2).
An image has a broad dynamic range is resolved by these approaches, and a display system which had a broad luminance range is realized by assigning the resolved im- age to each projector. A region where dynamic range is ex- panded is expected increasing tonal steps since serves gra- dation.
3. EXPERIMENT OF AN HDR DISPLAY SYSTEM In order to confirm the effectiveness of a proposed system, we measured a dynamic range compared an image which displayed by the proposed system to the image displayed by the conventional 1-projector. Figure 2 shows an arrange- ment of projectors of the proposed system.
• projector A, B：CHRISTILE LW40
(luminance：4000lm，resolution：1366×768 pixels)
• Chromemeter：KONICA MINOLTA CS-200
3.1 Measurement of a dynamic range
Input images are shown in horizontal axis of Figure 3 in this experiment. The images are displayed from a left image to a right image in order by a conventional display and the pro- posed system. At that time, the luminance was measured by chromameter. The maximum luminance of the HDR dis- play using 2-projectors and the conventional display system
(a) projector B
(b) projector B ＋ projector A
Fig. 3: Measured luminance values. (a) : the conventional system using the 1-projector. (b) : the proposed system us- ing 2-projectors
are 449.73cd/m2and 207.64cd/m2, respectively as shown in Table 1. Then, it is expanded to approximately twice that the luminance range of the HDR display system compared with the one of the display system using 1-projector.
Figure 3 shows the result of the measurement. It is con- firmed that the HDR display system can present the lumi- nance onto the screen appropriately as the luminance steps has increased.
Table 1: Maximum and minimum luminance projector A projector A + B maximum luminance 0.27cd/m2 0.53cd/m2 minimum luminance 207.64cd/m2 449.73cd/m2
3.2 Gamma correction of the proposed system
The HDR display system need to have a linear response in order to display an input image appropriately, but the pro- posed system dose not have a linear response (See Figure 3). The system need the gamma correction of a displayed image. In this experiment, to correct the displayed image by the proposed system, each displayed image should be corrected because the proposed system is consisted of two projectors. Then, the linearly of the system is evaluated by a determination coefficient. The determination coefficient
Fig. 4: Measured luminance values after the gamma correc- tion
is given by a regression expression and the measured lumi- nance values, and it is defined as
R2≡ 1 − P
i(yi− fi) P
i(yi− ¯y) (1)
yiis sample value, ¯y is mean value of the sample value, and fiis an estimate by the regression equation. The measured luminance values after the gamma correction are shown in Figure 4. The determination coefficients of the regression line of R, G, B and RGB values are shown in table2.
Table 2: Determination coefficient of the regression line
R G B RGB
Determination
coefficient 0.9994 0.9990 0.9987 0.9956
3.3 Representation of an HDR projector image 3.3.1 HDR indication with the Computer Graphics It is confirmed that a displayed image by the HDR dis- play system is expanded a dynamic range than the one of displayed image by the conventional display system. In this section, an HDR image is generated by CG (Computer graphics). An objects surface is separated specular and dif- fuse reflection [6] and the separated image is displayed by the each projector of the proposed system. It is assumed that a dynamic range of the HDR displayed image is ex- panded because of tonal steps of the HDR display system increase so that the other projectors cover the tonal steps.
Figure 5 shows the result of the HDR displayed image and conventional displayed image. The saturated area in (a) is smaller than the area in (b), and (a) can display gradation smoothly. Additionally, the conventional displayed image is dark whereas the HDR displayed image is brighter than con- ventional projection since the dynamic range of the HDR displayed image is higher than the conventional displayed image. Furthermore, some of the overexposure area where is smaller on the conventional projection after the gamma correction are shown in Figure 6.
(a) HDR displayed image
(b) conventional displayed image
Fig. 5: HDR displayed image and conventional displayed image.
(a) HDR displayed image
(b) conventional displayed image
Fig. 6: HDR displayed image and conventional displayed image after the gamma correction
3.3.2 HDR indication with the photograph
In this experiment, the images with high exposure and low exposure are mapped to each projector. Additionally, the
displayed image by conventional display is used the HDR tone mapping approach which synthesizes the image with high exposure and the image with low exposure [7].
The input images are to present HDR displayed image using the HDR display system and the conventional system are shown in Figure 7 and Figure 8. Figure 7 shows the map- ping image which has different exposure to each projector.
Figure 8 shows the result of the generated image by Geral- dine’s approach. Figure 9 shows an HDR displayed image using the HDR display system, and Figure 10 shows an HDR displayed image using the conventional display sys- tem.
It is confirmed that the image which displayed HDR display system has little overexposure and underexposure.
The measurement result of luminance value in the HDR dis- played images which displayed by the HDR display system and conventional display system is shown in Table 3. As for the HDR displayed image by the HDR display system, the luminance value measured compares with conventional display system is about twice.
(a) projectorA
(b) projectorB
Fig. 7: Input image by the HDR display system. The map- ping image which has different exposure to each projector
Fig. 8: Input image by the conventional display system. the displayed image is used the HDR tone mapping approach which genarated the image with high exposure and the im- age with low exposure.
Fig. 9: Output image by the HDR display system. the image has little overexposure and underexposure.
Fig. 10: Output image by the conventional display system.
Table 3: Measured luminance values projector A + B projector B the sky 61.57cd/m2 38.49cd/m2 the river 128.32cd/m2 57.85cd/m2 a boat 14.82cd/m2 1.94cd/m2
3.3.3 HDR indication using high-resolution projector We confirmed the effectiveness of HDR projection when the proposed system was constituted using the high resolution
(a) HDR displayed image (b) conventional displayed image
Fig. 11: HDR projector image and conventional projector image. The HDR displayed image can displayed graduation smoothly.
projector.
• projector A：SONY SRX-S110 (luminance：10,000lm±10%，
resolution：4096×2160pixels)
• projector B：Panasonic PT-DW5100L
(luminance：5,500lm，resolution：1024×768pixels)
• Chromemeter：KONICA MINOLTA CS-200 The maximum luminance of the HDR display using projec- tor A and Projector B is 333.86cd/m2, and the luminance of the conventional display using projector A and projec- tor B are 185.78cd/m2 and 161.46cd/m2, respectively as shown in Table 4. Figure 11 shows the result of the outputs of the HDR displayed image and the conventional displayed image.
Table 4: maximum and minimum luminance minimum luminance maximum luminance projector A 0.52cd/m2 185.78cd/m2 projector B 0.48cd/m2 161.46cd/m2 projector A + B 0.62cd/m2 333.86cd/m2
4. DISSCUSION
The HDR displayed image by the proposed HDR display system was keeping gradation in comparison with conven- tional display system, and the proposed system was able to project the high luminance range. However, even if the im- age which have been kept the low luminance step on the area of high luminance values was displayed, steps on the gradation of the low luminance values was indiscernible, and the HDR displayed image with photographed image by proposed system might be unnatural in the high luminance area. It is necessary to correct the input image to each pro- jector for the HDR projection.
5. CONCLUSIONS AND FUTURE WORK In this paper, we proposed that the HDR display system which increased tonal steps and expanded a dynamic range using multi-projector. Additionally, we confirm that a high dynamic range image can be displayed actually by testing luminance value measurement, gamma correction, compar- ison of the HDR projection and conventional projection. In the future, in order to project the HDR image in effectively using the proposal system, it is entertained that the approach which the tone mapping and that the image projection [8]
and the evaluation approach in mind visual feature of hu- man [9][10].
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