Sensitivity of the HVS for Binocular disparity Cue in 3D Displays under Different Ambient Illumination Conditions G. Nur, V. De Silva, H. Kodikara Arachchi, A. Kondoz, W. Fernando, M.O. Martínez-Rach and S. Dogan Abstract —In this paper, the sensitivity of the HVS towards binocular disparity depth cue, which is the most important cue in modern stereoscopic 3D displays, is investigated under different ambient illumination conditions. The experimental results indicate that as ambient illumination increases the sensitivity of the HVS for depth details increases or the Just Noticeable Difference in Depth (JNDD) decreases. It is expected that the investigation results will have important use cases in designing 3D display setups and 3D content production. I. INTRODUCTION The depth perception ability of humans with the aid of two eyes is exploited by the stereoscopic 3-Dimensional (3D) displays to provide an additional sensation of depth in 3D video. As 3D video is a technology that exploits the properties of the Human Visual System (HVS), it is important to consider relevant human factors in designing 3D display systems and setups. One such factor to consider is the sensitivity of the HVS for perceiving depth cues in 3D displays. This factor enables the optimization of 3D content creation, capture, and display technologies. Furthermore, this information is also useful in assessing quality of 3D video. Another factor that affects the enjoyment of 3D video is contextual information about the users and environment surrounding them such as ambient illumination conditions and personal preferences. In [1], the sensitivity for three depth cues, namely, binocular disparity, retinal blur and relative size are modeled and experimentally verified. The models proposed in [1], are used for display dependant 3D video processing in [2]. In [3], the authors suggest that the subjective 3D visual experience is affected by the ambient illumination conditions. Specifically, the overall perceived depth quality in 3D video sequences increases as the background illumination decreases. To explain the phenomenon reported in [3], in this paper, we extend the conclusions of [1] to include the effects of ambient illumination conditions. In other words, the sensitivity of the HVS for binocular disparity is measured at different ambient illumination conditions. II. PERCEPTION OF DEPTH AND SENSITIVITY TO BINOCULAR DISPARITY Depth can be perceived using physiological (e.g., accommodation, binocular disparity, etc) and psychological (e.g., aerial perspective, lateral motion, etc) cues available in real world [4]. Physiological cues exploit binocular vision whereas psychological cues are based on monocular vision. Binocular vision represents the vision in which the left and right eyes are utilized together. In the near field of vision (<10m) Binocular disparity is the most important depth cue that enables depth perception [4]. Furthermore, binocular disparity is the only additional depth cue provided by stereoscopic 3D displays. In [1], it is derived that the sensitivity to binocular disparity or the Just Noticeable Difference in Depth (JNDD), S d Δ , depends on two factors, as given in (1). d s v s s d d d , , Δ + Δ = Δ (1) where v S d , Δ is JNDD that depends upon the viewing distance (v) and d S d , Δ is JNDD that depends upon the initial disparity (d).             d s v s d w d s d K v d , , 25 . 2 ) ( log 94 . 0 10 Δ Δ ⋅ + - ⋅ = Δ (2) where, K w refers to the Weber Constant for stimulation of binocular disparity. According to (2), the sensitivity to binocular disparity changes linearly with the viewing distance. Thus, at a greater viewing distance, the JNDD is larger, or the sensitivity is lower. Furthermore, according to (2), as the initial disparity increases the JNDD also increases. This is due to the fact that binocular disparity acts as the stimulation for depth perception, and as the initial disparity increases the magnitude of stimulation increases. Thus, increasing the depth simulated by the stereoscopic display. According to the Weber’s law [4], as the magnitude of stimulation increases, the difference in stimulation required to perceive a change of stimulation also increases. Note that this is independent of whether the initial disparity is positive (objects constituted in front of screen level) or negative. The slope of d S d , Δ depends on the K w . Fig. 1 illustrates the relationship between the JNDD and the simulated depth. In the next section the behavior of the JNDD is investigated under different illumination conditions. Fig. 1. JNDD for binocular disparity on stereoscopic displays III. EXPERIMENTS In this section, the experiments that are performed to measure the sensitivity of the HVS towards binocular disparity cue under different ambient illumination conditions are discussed. The test stimuli used in [1], which contains two images of a car, are used in the experiments (see Fig. 2). At the beginning of the experiments, both of the cars (i.e., left and right cars) are placed at the same depth level relative to the screen by initially giving the same horizontal disparity to both of them. Then, the disparity of the right object is gradually changed to reflect a movement towards or away from the user. The