VARIABLES E HIPÓTESIS
4.2. Diseño de la investigación.
3.2.1High Dynamic Range Displays
HDR is generally understood to denote devices whose luminance and/or color depth exceeds that of standard 24-bit RGB (red, green, and blue). One industry definition of HDR is the UHD-10
specification (UHD Alliance Premium, Myszkowski, Mantiuk and Krawczyk 2008, Vandervell 2016), which specifies that a conformant display’s luminance must have a dynamic range of either 86 dB referenced to 5·10-2 cd/m2 for LED-based displays or 120 dB referenced to 5·10-4 cd/m2 for OLED-based displays32. The same specification requires that the display support 10 bits-per-color and must be able to display 90% of the P3 color space (gamut) (Masaoka and Nishida 2015).
Methods for increasing the dynamic range of displays have been pursued widely. Larson, Rushmeier and Piatko (1997) reported tone reproduction operator that uses aspects of human visual perception to create the appearance of an HDR scene with a standard dynamic range output device or medium. Myszkowski, Mantiuk and Krawczyk (2008) present a survey of HDR video, including image capture, representation, and reproduction (display). Whitehead, et al. (2007) disclose a method for producing HDR display devices using two spatial light modulators (SLMs) in series; the first SLM produces the full-resolution image at a reference luminosity and the second SLM, whose resolution is lower than that of the first SLM, adjusts the luminosity of regions of the image. In a similar manner, LCD displays with over 100 dB of dynamic range in luminance have been demonstrated. These LCD displays use an array of variable-intensity white backlights (as opposed to a monolithic backlight); the resolution of the backlight array is significantly less than that of the LCD itself, so each “pixel” in the backlight array sets the luminance for a region of the display (Torres 2005 and Brightside Technologies 2006). Methods for increasing color depth/gamut in DMD-based displays have also been proposed; Gibbon, et al. (2006) disclose a technique for increasing the dynamic range of DMD-based projection systems wherein two DMDs are used in series. The first DMD pre-modulates a constant-intensity light source,
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producing variable intensities globally, regionally, or on a per-pixel basis; the second DMD spatially- modulates the (now variable-intensity) light in like manner to a traditional DMD-based display (see section 3.2.4).
3.2.2Reducing Integrated Frame Times
Chang, Kumar, and Sankaranarayanan (2016) demonstrate up to 16-bit color depth DMD-based video projectors using an illumination method they call Hybrid Light Modulation (HLM). HLM combines PTM and variable-intensity illumination such that HDR video can be displayed without a reduction in integrated frame rate and with minimal impact to maximum intensity. HLM, as proposed and
implemented in [idem], cannot achieve the temporal performance of our method; among other factors, their variable-intensity, pulse-width modulated (PWM) light source has about 8 to 9 bits of precision; in comparison, our DBI implementation, described in section 3.3, has 16-bit precision.
Both HLM and DBI leverage variable-intensity light sources for the same basic purpose: to reduce the number of binary frames per integrated frame. DBI and HLM were independently-conceived for different purposes (OST AR versus video projectors, respectively) and optimize for different factors (low-latency versus maximum intensity, respectively).
Some DMD-based displays reduce the number of binary frames per integrated frame by using bivalent light sources (Texas Instruments 2018). A bivalent light source, when on, emits light at one of two intensities; the resulting illuminator is perhaps tantamount to 2-bit DBI. The reduction in binary frames per integrated frame is proportional to the intensity of the lower-intensity light level compared to the full-scale light level. Even with this technique, the number of binary frames per integrated frame is still exponential in bits per pixel. As a simple example, consider an n bit-per-color bivalent-illuminated display where the lower intensity is 2k and the higher intensity is 2n, (k<n). If only the higher-intensity level is PTM-modulated (as in a conventional display), the number of binary frames per integrated frame would be O(2n). If both intensities are used, i.e., the higher-intensity level is modulated for 2n-k binary frames (for the high-order n-k bits) and the lower-intensity level is modulated for 2k binary frames (for the
low-order k bits), then the total number of binary frames per integrated frame drops to O(2n-k + 2k) – which is still exponential in n. In fairness, if we consider the best case, where k=n/2, only O(2∙2n/2
) binary frames are required per integrated frame; this is a significant savings.
3.2.3The Human Element
The human visual system operates effectively over a large range of luminance – from
10-6 to 108 cd/m2—a total range of 280 dB (Stockman and Sharpe 2006, IDS Imaging 2009, and Lagunas, et al. 2017). Figure 3 depicts a subset of this range with contextual examples for several lighting levels. There does not appear to be general agreement on the dynamic range of “typical” scene lighting, but estimates of 100-120 dB are common [idem]. If we declare our reference luminance level to be moonlight (10-1 cd/m2), then, assuming a dynamic range of 120 dB, the maximum illumination level is sunlight (105 cd/m2). Due to the rate of adaptation of rod cells, particularly during desaturation, the full range of visual sensitivity is not available simultaneously (Pattanaik, et al. 2000). For example, if a scene’s
luminosity extends from 10-4 to 106 cd/m2, rod saturation will render the dimmer content invisible. Even if we could build a display with this dynamic range (200 dB), only a scene-dependent subset of this range would be perceptually-useful; that is, under any given lighting conditions, at most about 120 dB of the display’s dynamic range would be visible to the user and the remainder of the display’s dynamic range would be unused.
3.2.4Binary Frame Displays and Digital Micromirror Device-Based Projectors
A binary frame is a one-bit monochrome image – each pixel is either fully-on or fully-off. In this dissertation, we assume that the binary frames are generated by reflective spatial light modulators
(SLMs), such as DMDs. A DMD comprises a grid of tiny mirrors, as shown in Figure 4. Under electronic control, each mirror can be pivoted such that incident light reflects in one of two directions: When a mirror is in the “on” position, light is reflected into the display’s exit optics; when a mirror is in the “off” position, light is reflected into an absorber or back into the light source. In LCoS BSLMs, each pixel
either reflects light (“on” state) or absorbs light (“off” state). Optically, the principle of operation of the two technologies is otherwise identical.
The source of illumination in a BSLM-based display is typically a white light passed through a color wheel, as illustrated in Figure 5, or constant-intensity color LEDs, as illustrated in Figure 6. Pulse train modulation (PTM) is used to produce continuous-tone images. Depending on the device, a DMD’s mirrors can be switched at speeds up to 32 kHz; this switching rate is the DMD’s (maximum) binary frame rate. Each input image frame is output as a set of binary frames. The time-domain sum of the set of binary frames comprising one input frame is an integrated frame. The time required to output an
integrated frame is the integrated frame time (or period), the reciprocal of which is the integrated frame rate. The integrated frame time is equal to the binary frame time multiplied by the number of binary frames per integrated frame. Likewise, the integrated frame rate is equal to the binary frame rate divided by the number of binary frames per integrated frame.
Figure 7: OST AR HMD apparatus of (Lincoln, Blate, et al. 2017).
The DBI illuminator is on the far side of the DMD optics package (occluded by mounting hardware). The DMD device itself is located at the top of the optics package, indicated by the orange line (center). The display’s “exit optics,” indicated by the red brace, comprise the lens assembly at the bottom of the optics package, ending at the silver ring. The “eye box”/“exit pupil” is located to the left of the optical combiner; in this figure, a small video camera is located in this position. The (prototype) position-sensitive light sensor is oriented such that its field-of-view is coincident with that of the user. The display controller, comprising several boards, is located at the top-center and is fed video via a DisplayPort interface. The video is generated by a computer/GPU (not shown). The HMD can rotate about two axes: yaw (center- left) and pitch/nod (bottom-center). Each axis is tracked by a rotary shaft encoder.