Understanding Crosstalk in Stereoscopic Displays” (Keynote Presentation) at 3DSA (Three-Dimensional Systems and Applications) Conference, Tokyo, Japan, 19-21 May 2010
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Woods, A.J. (2010) “Understanding Crosstalk in Stereoscopic Displays” (Keynote Presentation) at 3DSA (Three-Dimensional Systems and Applications) conference, Tokyo, Japan, 19-21 May 2010. www.AndrewWoods3D.com Understanding Crosstalk in Stereoscopic Displays Andrew Woods Centre for Marine Science & Technology Curtin University of Technology Perth, Australia A.Woods @ curtin.edu.au Abstract— Crosstalk is a critical factor determining the image Looks like a double exposure. Crosstalk is a physical entity quality of stereoscopic displays. Also known as ghosting or and can be objectively measured, whereas ghosting is a leakage, high levels of crosstalk can make stereoscopic images subjective term.” hard to fuse and lack fidelity; hence it is important to achieve low levels of crosstalk in the development of high-quality stereoscopic Ghosting: “The perception of crosstalk is called ghosting.” displays. In the wider academic literature, the terms crosstalk, Huang, et al [2] provide the following definitions: ghosting and leakage are often used interchangeably but it would be helpful to have unambiguous descriptive and mathematical System Crosstalk: “for the left-eye case … can be defined definitions of these terms. The paper reviews a wide range of as: mechanisms by which crosstalk occurs in various stereoscopic displays, including: time-sequential on PDPs and CRTs System crosstalk (left eye) = β2 / α1 (1) (phosphor afterglow, shutter timing, shutter efficiency), MicroPol describing the degree of the unexpected leaking image from the LCDs (polarization quality, viewing angle), time-sequential on other eye.” Where: “α1 describes the percentage part of the LCDs (pixel response rate, update method, shutter timing & left-eye image observed at the left eye position”, and “β efficiency), autostereoscopic (inter-zone crosstalk), polarised 2 projection (quality of polarisers and screens), anaglyph (spectral describes the percentage part of the right-eye image leaked to quality of glasses and displays). Crosstalk reduction and the left-eye position.” crosstalk cancellation are also discussed along with methods of Viewer Crosstalk: “means the crosstalk perceived by the measuring and characterising crosstalk. viewer [7].” It is “defined as the ratio of the luminance of unwanted ghost image, which leaks from the image for the Keywords - crosstalk; ghosting; leakage; LCD; PDP; CRT; other eye, to the luminance of the correct information received DLP; polarised; anaglyph, stereoscopic, autostereoscopic. by the viewer’s eyes. The viewer crosstalk for the viewer’s left eye can be defined as: I. INTRODUCTION Viewer crosstalk = B β / A α ” (2) This paper provides an overview of the concepts of 2 1 crosstalk (sometimes ‘cross talk’, or ‘cross-talk’ [37]) in Where: A is the luminance of a particular point in the left eye stereoscopic 3D displays, including a review of the image, and B is the luminance of the same corresponding point terminology relating to stereoscopic crosstalk as it is in (same x,y location) in the right-eye image. common usage, a review of the sources of ghosting in a Therefore, System Crosstalk is independent of the content, selection of commonly used stereoscopic displays, and a whereas Viewer Crosstalk varies depending upon the content. discussion of methods of measuring, characterising and reducing stereoscopic crosstalk. There is some similarity between Lipton’s “Crosstalk and Ghosting” vs. Huang’s “System Crosstalk and Viewer In its simplest form crosstalk in stereoscopic displays is the Crosstalk” definitions but there are still some notable leakage of the left image channel into the right eye view and differences, specifically “ghosting” includes any perception vice versa. issues, whereas Huang’s “viewer crosstalk” only considers co- location image contrast [7] and not the amount of disparity [6]. II. TERMINOLOGY AND DEFINITIONS Stevens [34] provides a slightly different definition for The terms crosstalk and ghosting are often used viewer crosstalk: “the ratio of luminance of the ‘wrong’ image interchangeably in some of the published literature and to the luminance of the ‘correct’ image as seen by the observer. unfortunately very few publications actually provide a It will be a function of the image contrast and the disparity.” descriptive or mathematical definition of these terms. Unfortunately when definitions are provided they are This paper will not provide recommendations on sometimes contradictory: terminology usage, but perhaps this can happen after further literature search and industry-wide debate. Lipton [1] provides the following definitions: Crosstalk: “Incomplete isolation of the left and right image channels so that one leaks (leakage) or bleeds into the other. Three other terms which have been used when referring to III. PERCEPTION OF CROSSTALK crosstalk are: The perception of crosstalk in stereoscopic displays has Leakage: in very general terms this refers to the raw been studied widely [6][7][8][9][10][11]. It is widely amount of light which leaks from one channel to another – but acknowledged that the presence of high levels of crosstalk in a it is usually undefined in the literature. stereoscopic display are detrimental. The effects of crosstalk in an image include: ghosting and loss of contrast, loss of 3D Extinction and extinction ratio: usually refer to the ratio effect and depth resolution, and viewer discomfort [34]. between the intensity of the intended image as compared with the intensity of the unintended image – but it is usually not The visibility of crosstalk (ghosting) increases with specifically defined in the stereoscopic literature. increasing contrast and increasing binocular parallax of the image [6][9][21] (see Fig. 1). For example, a stereoscopic 3D contrast: the inverse of 3D crosstalk [32]. image with high-contrast will exhibit more ghosting on a particular stereoscopic display than will an image with low A. Mathematical Definitions contrast. In addition to equations (1) and (2) provided above, a The literature provides various advice on the amounts of selection of other mathematical definitions appear in the crosstalk that are acceptable and unacceptable – but literature: unfortunately not all the advice agree. Some examples include Liou, et al [22] provides the following equations: [36]: “crosstalk between 2 and 6% significantly affected image quality and visual comfort”, “crosstalk should be as low as CL = (BW – BB) / (WB – BB), and (3) 0.3%” [6], “crosstalk. … visibility threshold of about 1 to 2%”, CR = (WB – BB) / (BW – BB) (4) “crosstalk level of about 5% is sufficient to induce visual Where: WB = a video stream with all white as left-eye discomfort in half of the population” [8]. images, and all-black as right-eye images, BW = a video stream with all-black as left-eye images and all-black as right IV. MEASUREMENT OF CROSSTALK eye images, BB = a video stream with all-black for both left Two methods exist for the measurement of crosstalk: and right eyes (i.e. the black level of the display), and CL and CR = the crosstalk experienced by the left eye and the right A. Optical Sensors eye. An optical measurement device (such as a photometer or a Pala, et al [10] and Boher, et al [32] use something very radiometer) can be used to measure crosstalk. Examples of similar to the above but with different variable names. sensors that have been used to measure crosstalk include: Integrated Photomatrix Inc. IPL10530 DAL photo-diode [12], Woods and Harris [40] and Hong, et al [31] used a simpler Ocean Optics USB2000 spectroradiometer [39], Minolta equation: CS1000 spectroradiometer [31][34], Minolta CS-100 spot Crosstalk (%) = leakage / signal 100 (5) chroma meter [7][22], and Eldim EZContrastMS [32]. Many other devices can also be used for this purpose. It is important Shestak, et al [21] provide equations for dark crosstalk, to note that for any optical sensors that are used, consideration light crosstalk, and grey-to-grey crosstalk specifically relating needs to be given to how well the spectral sensitivity of the to crosstalk in time-sequential 3D on LCDs: sensors match the spectral sensitivity of the human eye (Fig. 2). dark Dark crosstalk C = (W’2 – W2) / (W1 – W2) (6) Traditionally, crosstalk is measured by displaying full- Light crosstalk Clight = (W’ – W ) / (W – W ) (7) black and full-white in opposing eye-channels of the display 1 1 2 1 and using an optical sensor to measure the amount of leakage Where: W1 and W2 are the original desired luminance for between channels. For example, the optical sensor is placed at points in the left and right eye view (W1 is the lower of the two the left eye position (either behind the left eye of 3D glasses, or luminances), W’1 is the displayed luminance affected by in the left eye viewing zone for an autostereoscopic display) crosstalk which brightens the image, and W’2 is the displayed luminance affected by crosstalk which darkens the image. Grey-to-grey crosstalk is the matrix of values of C for all grey level combinations for W1 and W2. Additionally, Boher, et al [32] define 3D contrast as: 3D 0.5 CL = 1 / χL , CR = 1 / χR and C = (CR × CL) (8,9,10) Where: CL and CR are 3D contrast for the left and right eyes as viewed through the left and right filters, χL and χR is the 3D crosstalk for left and right eyes, and C3D is the combined 3D contrast for both eyes. Note that the variable name C is used for contrast in [32] whereas C is used for Crosstalk in most other stereoscopic papers. Figure 1. The threshold of visibility of crosstalk for different amounts of disparity and image contrast ratio. [6] Due to the limitations of the visual measurement charts, appropriate electro-optic tools should be used when high accuracy crosstalk data is needed. V. CROSSTALK MECHANISMS There are various mechanisms by which crosstalk occurs in stereoscopic displays and these mechanisms vary between different stereoscopic display technologies. The sections below summarise the important performance attributes for various stereoscopic display technologies and the mechanisms by which crosstalk occurs in each.