SENTENCIAS INFUNDADAS.

The current way to display HDR content relies on technology termed local-

dimming displays or dual-modulation displays. The idea behind this approach is to optically combine the two displaying devices so that their intensities multi- ply. For instance these could be two projectors or a projector and an LCD screen. If aligned properly the brightest value which can be displayed is the multiple of the maximum value of each display. In such a setup, the dynamic range is signifi- cantly increased and may be controlled on a pixel level. However, in practice it is difficult to achieve this theoretical increase in dynamic range because of optical

imperfections caused by light scatter.

Stereoscopic High Dynamic Range Viewer

Ward (2002) implemented the first device which utilised the dual-modulating technology. Not only did it allow native visualisation of HDR images but it was also a stereoscope (see Figure 3.14). The device consisted of the three main pairs of parts: two wide-field lenses, two bright uniform backlights - 50 watt lamps, and a pair of layered transparencies. The optics used was the Large Expanse Extra Perspective (LEEP) ARV-1 (Howlett, 1990). The two main challenges were mapping the view for the optics and designing a technique for layering transparencies in order to increase dynamic range.

(a) Prototype of SHDR viewer

50 W Lamps C oo lin g F an Heat Absorbing Glass Diffuser ARV-1 Optics Reflector for Uniformity

(b) Schematic of the Viewer

Figure 3.14: The stereoscopic viewer constitutes of three main parts: lenses, lamps and image transparencies.

The ARV-1 optics exhibited major chromatic aberrations resulting in coloured fringes at the view edges. This was offset by a camera with aberration in the opposite direction. To avoid the image appearing blurred, a high resolution was required. The images were printed using 800 dots per inch (dpi) and totaled 2048

× 2048 pixels.

Film transparencies were able to encode only 8 bit images so an extra image was needed to increase the dynamic range utilising the light source modulation (dual-modulation technique). The process of the pair creation is illustrated in Figure 3.15. The first image hit by light modulated it and encoded the global

Square Root Luminance

Input HDR Image Gaussian Filter _{(32 x 32)} Modulation Image

Divide (÷ ) Reduce Chromatic _Abberations Colour and Details _Image

Figure 3.15: Pipeline showing how transparency images were generated for the stereoscopic HDR viewer. The input image is separated into a low frequency monochromatic modulation film and a coloured film.

luminance distribution. The image was generated by calculating the square root of the original luminance and filtering the result with a 32 × 32 Gaussian. The second image encoded details and colours and accounted for chromatic abbera- tion. It was generated by dividing the input HDR image by the modulated one. Chromatic aberration was counteracted by scaling the red channel up by 1.5% compared to the blue one and positioning the green one halfway between the two. The viewer provided around 120◦ field of view and a dynamic range of over 10,000 : 1 with the maximum and minimum luminance of 5,000 cd/m2 _{and 0.5} cd/m2_{. In a user study, Ledda et al. (2003) compared the device output with} a real scene and the image on a CRT monitor tone mapped with the histogram adjustment operator (Ward et al., 1997). The results suggested that the image presented on the SHDR viewer was closer to reality than to the TM reproduction. While the system reproduced the scene appearance well it is only limited to static images which can be viewed by a single observer at the time. Moreover, the cost of printing one such image (consisting of four transparencies) was estimated to be approximately $200 US.

Projector Based HDR Displays

Seetzen et al. (2004) developed the first display which was viewed in a con- ventional way like a TV screen. Similar to the SHDR viewer it was based on dual-modulation technology (as shown in Figure 3.16) and consisted of two de- vices: a digital light processing (DLP) projector and a transmissive liquid crystal display (LCD). The projector’s modulated image increased the dynamic range image while the front LCD encoded colour and details.

While the images were processed in a similar manner to the stereoscopic HDR viewer there were a number of differences. A lack of optics meant no chromatic aberrations hence no counteracting was required. The square root of luminance

(a) Prototype of HDR Display

Projector

LCD Diffuser Fresnel Lens

Dual VGA Graphic

Card (PC) LCD Controller

(b) Schematic of the Display

Figure 3.16: The HDR display by Seetzen et al. (2004) consists of: a projector providing strong modulated backlighting and an LCD panel providing colour and details.

was filtered based on the point spread function of the projector instead of a Gaussian. Lastly, the signal was linearised by measuring the response functions of both the LCD and the projector and applying their inverse to the modulation image.

The hardware used to build the prototype consisted of a Sharp LQ150X1DG0

15.100 LCD panel (300 : 1 DR) and an Optoma DLP EzPro737 (800 : 1 DR). A

Fresnel lens was put immediately behind the LCD to collimate the light into a sharp angle. This helped to achieve a maximum brightness and avoid colour distortion. A standard LCD diffuser, put between LCD and Fresnel lens, redis- tributed the collimated light and achieved an acceptable viewing angle. The final prototype achieved a dynamic range of 50,000 : 1 with a peak luminance of 2,700 cd/m2 and a minimum measured luminance of 0.054 cd/m2.

One drawback of the design was the large optical path (approximately 1 m) that rendered the device impractical for everyday use. While the diffuser helped to increase the viewing angle it was still rather limited and the device had a strong falloff at wide viewing angles. Lastly, the projector generated very bright light, which resulted in significant power consumption and heat generation.

LED Based HDR Displays

To overcome practicality issues, Seetzenet al. (2004) built a second HDR display which resembled classic TV sets. The display was based on the concept of veiling glare in which local contrast on the retina is reduced because of light scattering in

the eye. This phenomenon impacts visibility of dark detail next to bright regions and meant that the HVS processes local contrast poorly, around edges, compared to the global contrast, which is present over a wider spatial range. Hence, it was not required to generate very dark pixels next to very bright ones as long as those intensities could be presented in different parts of the image.

LCD Panel

LED LED

(a) HDR Display Schematic (b) Brightside’s HDR Screen

Figure 3.17: HDR LED displays use a low resolution LED array as backlighting and a standard LCD screen in the front. (a) A schematic shows the design of a display from the side and front. A single LED provides backlighting for multiple pixels. (b) One of the first HDR LED display prototypes.

The authors utilised this insight and used a low resolution LED panel to mod- ulate the light and the LCD panel for details and colour (see Figure 3.17a). The way in which an input image was processed was similar to the projector-based displays, except for the step in which the luminance for a single LED was cal- culated. The square root of the luminance of the input image was downsampled to the resolution of the LED panel and approximately deconvolved by the LED point spread function.

The DR 37P (see Figure 3.17b) consisted of 1380 LEDs and a 3700 LCD panel (300 : 1 DR). It was capable of a 200,000 : 1 dynamic range with a peak lu- minance of 3,000 cd/m2 and a minimum luminance of 0.015 cd/m2. While the dynamic range was improved, the image quality suffered due to the low resolution of LEDs. In addition it consumed a considerable amount of power (1680 W) and required an extensive and noisy cooling system (both fans and liquid cooling).

Dolby Labs acquired Brightside in 2007 and licenced it to SIM2 who in 2009 released the first commercial HDR display named the Solar 47 The display size was 4700 with a full HD resolution of 1920 × 1080 and 2,206 LEDs. It was also

capable of processing 16 bpc images.