High Dynamic Range Display SystemsH. Seetzen, W. Heidrich, W. Stuerzlinger, G. Ward, L. Whitehead
M. Trentacoste, A. Ghosh, A. Vorozcovs
Simultaneously, tone mapping operators have also been developed for compressing the dynamic range so that the images can be displayed on the familiar 8 bit/channel displays with contrast ratios of no more than 300:1 (i.e. any conventional CRT, LCD, and projector-based display). While these tone mapping operators allow for displaying high-dynamic-range (HDR) images in a recognizable and even aesthetically pleasing way, nobody would confuse a photograph rendered in this fashion with, say, watching the same scene through a window. The dynamic range of conventional displays is simply insufficient to create the optical sensation of watching a real sunset or driving a car into oncoming traffic at night. Note that this is not just an issue of top intensity: simply increasing the brightness of a conventional display would wash out the dark tones and turn them into a medium gray. What is needed is a significant expansion of the contrast or dynamic range of the display.
In this project we developed two alternative designs for HDR display systems. We have built prototypes of both, and discuss both the optical design and software issues such as display calibration and rendering HDR images on both displays.
Both display systems are based on the fundamental idea of using an LCD panel as an optical filter of programmable transparency to modulate a high intensity but low resolution image from a second display. For example, assume we have any display with a contrast range of c1:1 between the darkest and the brightest color producible by that display. If we now put an LCD panel with a contrast ratio of c2:1 in front of the first one, then the contrast of the combined system is c1*c2:1. In practice, the first display needs to be able to produce a very high intensity image, because color LCD panels only have a transparency of about 3-8%, even when switched to `white', so that most energy is actually absorbed. Another reason for using a display with a very high base intensity is that a lot of the HDR image we would like to show have, in fact, very bright regions in them.
Based on this principle, we have derived two alternative designs for HDR displays. In the first design (see figure below), a video projector based on digital light projector (DLP) technology serves as the base display. In this version, we directly focus the projector onto the back of the LCD panel. Since the illuminated area is much smaller than during regular use of a projector, the light density is dramatically improved, yielding the high top intensities that we are aiming for. While this design works well in a laboratory setting, it has several drawbacks that restrict its use for a wider class of applications. In particular, these are a large form factor, significant power consumption and heat development, as well as calibration issues.
To overcome these issues, we have devised a second design (see below), in which the projector is replaced with a low-resolution array of ultra-bright LEDs. The intensity of every LED can be programmed individually, yielding a low resolution version of the desired image. High frequency features are introduced by attaching a high-resolution LCD panel to the front of this LED array, and adjusting its transparency accordingly. This design makes use of results from psychophysics, which show that very high contrast, although important on a global scale, cannot be perceived by humans at high spatial frequencies.
The two displays we developed have dynamic ranges well beyond 50,000:1, and a maximum intensity of 2700 cd/m2 and 8500 cd/m2, respectively. This compares to a dynamic range of about 300:1, and a maximum intensity of about 300 cd/m2 for a typical desktop display.
In addition to the hardware setup, the paper also discusses perceptual issues and limitations of the human visual systems that allowed us to design these display systems. Moreover, we describe software and rendering aspects, including a GPU-based rendering algorithm.
The LED HDR displays are now commercially available from BrightSide Technologies Inc.