Stereomotion suppression and the perception of speed: accuracy and precision as a function of 3D trajectory
by Kevin Brooks
Brooks, K. R. & Stone L. S. (2006). Stereomotion suppression and the perception of speed: accuracy and precision as a function of 3D trajectory. Journal of Vision, 6, 1214-1223, http://journalofvision.org/6/11/6, doi:10.1167/6.11.6
The precision and accuracy of speed discrimination performance for stereomotion stimuli were assessed for several... more
The precision and accuracy of speed discrimination performance for stereomotion stimuli were assessed for several receding 3D trajectories confined to the horizontal meridian. It has previously been demonstrated in a variety of tasks that detection thresholds are substantially higher when subjects observe a stereomotion stimulus than when simply viewing one of its component monocular half-imagesVa phenomenon known as stereomotion suppression (C. W. Tyler, 1971). Using monocularly visible motion in depth targets, we found mean speed discrimination thresholds to be higher for stereomotion,
compared with monocular lateral speed discrimination thresholds for equivalent stimuli, demonstrating a disadvantage for binocular viewing in the case of speed discrimination as well. Furthermore, speed discrimination thresholds for motion in depth were not systematically affected by trajectory angle; hence, the disadvantage of binocular viewing persists even when there are concurrent changes in binocular visual direction. Lastly, there was a tendency for oblique trajectories of stereomotion to be perceived as faster than equally rapid motion receding directly away from the subject along the midline. Our data, in addition to earlier stereomotion suppression observations, are consistent with a stereomotion system that takes a noisy, weighted difference of the stimulus velocities in the two eyes to compute motion in depth.
Download (.pdf) The swinging doors of perception: Stereomotion without binocular matching
by Kevin Brooks
Brooks, K. R. & Gillam, B. J. (2006). The swinging doors of perception: stereomotion without binocular matching. Journal of Vision, 6, 685-695, http://journalofvision.org/6/7/2, doi:10.1167/6.7.2
Until recently, it was considered necessary for features in the two eyes to be matched before the evaluation of... more Until recently, it was considered necessary for features in the two eyes to be matched before the evaluation of differences in their locations (binocular disparities) could reveal depth information. Motion in depth can also be perceived binocularly from related changes in the locations of matched binocular features. However, unmatched features can arise when a binocular object occludes more distant features in one eye but not the other. The presence and extent of such features can provide quantitative depth information, although perceived depth relative to geometrical predictions may vary from one such arrangement to another. The ability of humans to perceive motion in depth from unmatched stimuli has not previously been explored. Here, we use B. Gillam, S. Blackburn, and K. Nakayama’s (1999) ‘‘monocular gap’’ stimuli to investigate perception of motion in depth simulated by a change in the extent of a monocularly occluded feature in a binocular display. Settings of a motion in depth probe revealed that the magnitude of perceived motion in depth is generally as large as that for a stimulus containing matchable binocular features. We show that our stimuli provide disambiguating information not present in similar static stimuli. We conclude that in the computation of motion in depth, a binocular match is not required. A new cue--dynamic half-occlusion--can be used to reach an accurate percept.
Quantitative perceived depth from sequential monocular decamouflage
by Kevin Brooks
Brooks, K. R. & Gillam, B. J. (2006). Quantitative perceived depth from sequential monocular decamouflage. Vision Research, 46, 605-613. doi:10.1016/j.visres.2005.06.015
We present a novel binocular stimulus without conventional disparity cues whose presence and depth are revealed by... more We present a novel binocular stimulus without conventional disparity cues whose presence and depth are revealed by sequential monocular stimulation (delay P 80 ms). Vertical white lines were occluded as they passed behind an otherwise camouflaged black rectangular target. The location (and instant) of the occlusion event, decamouflaging the targets edges, differed in the two eyes. Probe settings to match the depth of the black rectangular target showed a monotonic increase with simulated depth. Control tests discounted the possibility of subjects integrating retinal disparities over an extended temporal window or using temporal disparity. Sequential monocular decamouflage was found to be as precise and accurate as conventional simultaneous stereopsis with equivalent depths and exposure durations.
Monocular Transparency and unpaired stereopsis
by Kevin Brooks
Grove, P. M., Brooks, K. R., Anderson, B. L. & Gillam, B. J. (2006). Monocular transparency and unpaired stereopsis. Vision Research, 46, 3041-3053. doi:10.1016/j.visres.2006.05.003
Howard and Duke [Howard, I.P., & Duke, P.A. (2003). Monocular transparency generates quantitative depth. Vision... more Howard and Duke [Howard, I.P., & Duke, P.A. (2003). Monocular transparency generates quantitative depth. Vision Research, 43, 2615–2621] recently proposed a new source of binocular information they claim is used to recover depth in stereoscopic displays. They argued that these displays lack conventional disparity and that the metrical depth experienced results from transparency rather than occlusion relations. Using a variety of modified versions of their stimuli, we show here that the conditions for transparency are not required to elicit the depth experienced in their stereograms. We demonstrated that quantitative and precise depth depended not on the presence of transparency but on the presence of horizontal contours of the same contrast polarity. Depth was attenuated, particularly at larger target offsets, when horizontal contours had opposite contrast polarity for at least a portion of their length. We also show that a demonstration Howard and Duke used to control for the role of horizontal contours can be understood as an example of Gillam et al.’s Gillam, B.J., Blackburn, S., & Nakayama, K. (1999). Stereopsis based on monocular gaps: metrical coding of depth and slant without matching contours. Vision Research, 39, 493–502 monocular gap stereopsis; a form of binocular occlusion. In summary the findings reported by Howard and Duke can be understood by known processes for the computation of binocular disparity and binocular occlusion.
Monocular motion adaptation affects the perceived trajectory of stereomotion
by Kevin Brooks
Brooks, K. R. (2002b). Monocular motion adaptation affects the perceived trajectory of stereomotion. Journal of Experimental Psychology: Human Perception and Performance, 28, 1470-1482.
Perceived stereomotion trajectory was measured before and after adaptation to lateral motion in the dominant or... more Perceived stereomotion trajectory was measured before and after adaptation to lateral motion in the dominant or nondominant eye to assess the relative contributions of 2 cues: changing disparity and interocular velocity difference. Perceived speed for monocular lateral motion and perceived binocular visual direction (BVD) was also assessed. Unlike stereomotion trajectory perception, the BVD of static targets showed an ocular dominance bias, even without adaptation. Adaptation caused equivalent biases in perceived trajectory and monocular motion speed, without significantly affecting perceived BVD. Predictions from monocular motion data closely match trajectory perception data, unlike those from BVD sources. The results suggest that the interocular velocity differences make a significant contribution to stereomotion trajectory perception.
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Hinge versus twist: the effects of “reference surface” and discontinuities on stereoscopic slant perception
by Kevin Brooks
Gillam, B. J., Blackburn, S. & Brooks, K. R. (2007). Hinge versus twist: the effects of “reference surface” and discontinuities on stereoscopic slant perception. Perception, 36, 596-616, doi: 10.1068/p5535
Stereoscopic slant perception around a vertical axis (horizontal slant) is often found to be strongly attenuated... more
Stereoscopic slant perception around a vertical axis (horizontal slant) is often found to be strongly attenuated relative to geometric prediction. Stereo slant is much greater, however,
when an adjacent surface, stereoscopically in the frontal plane, is added. This slant enhancement is often attributed to the presence of a `reference surface' or to a spatial change in the disparity gradient (introducing second and higher derivatives of disparity). Gillam, Chambers, and Russo (1988 Journal of Experimental Psychology: Human Perception and Performance 14 163-175) questioned the role of these factors in that placement of the frontal-plane surface in a direction collinear with the slant axis (twist configuration) sharply reduced latency for perceiving slant whereas placing the same surface in a direction orthogonal to the slant axis (hinge configuration) had little effect.We here confirm these findings for slant magnitude, showing a striking advantage for twist over hinge configurations.We also examined contrast slant measured on the frontal-plane surface in the hinge and twist configurations. Under conditions where test and inducer surfaces have centres at the same depth for twist and hinge, we found that twist configurations produced strong negative slant contrast, while hinge configurations produced significant positive contrast or slant assimilation.We conclude that stereo slant and contrast effects for neighbouring surfaces can only be understood from the patterns and gradients of step disparities present. It is not adequate to consider the second surface merely as a reference slant for the first or as having its effect via a spatial change in the disparity gradient.
Stereomotion perception for a monocularly camouflaged stimulus
by Kevin Brooks
Brooks, K. R. & Gillam, B. J. (2007). Stereomotion perception for a monocularly camouflaged stimulus. Journal of Vision, 7, 1-14, http://journalofvision.org/7/13/1, doi:10.1167/7.13.1
Under usual circumstances, motion in depth is associated with conventional stereomotion cues: a change in disparity... more Under usual circumstances, motion in depth is associated with conventional stereomotion cues: a change in disparity and differences between object velocities in each monocular image. However, occasionally these cues are unavailable due to the fact that in one eye the object may be occluded by, or camouflaged against appropriately positioned binocular objects. We report two experiments concerned with stereomotion perception under conditions of monocular camouflage. In Experiment 1, the visible half-image of a monocularly camouflaged object translated laterally. In this binocular context, percepts of lateral motion and motion in depth were equally consistent with the stimulus. Subjects perceived an oblique trajectory of 3D motion, compared to the more direct 3D trajectory experienced for binocularly matched stimuli. In Experiment 2, the perceived velocity of stereomotion was assessed. Again, for the stimulus used in Experiment 1, perceived stereomotion speed was lower than that for matched stimuli. However, when additional background objects were present, tightening the ecological constraints, perceived stereomotion velocity was often equivalent to that for matched stimuli. These results demonstrate for the first time that the motion of a monocularly camouflaged object can result in the perception of stereomotion, and that the perceived trajectory and speed are influenced by the ecological constraints of binocular geometry.
Breaking camouflage: Binocular disparity reduces contrast masking in natural images
by Kevin Brooks
Wardle, S., Cass, J., Brooks, K.R. & Alais, D. (2010). Breaking camouflage: Binocular disparity reduces contrast masking in natural images. Journal of Vision, 10, 38, 1-12, http://www.journalofvision.org/content/10/14/38, doi:10.1167/10.14.38.
Visual overlay masking is typically studied with a mask and target located at the same depth plane. Masking is reduced... more Visual overlay masking is typically studied with a mask and target located at the same depth plane. Masking is reduced when binocular disparity separates the target from the mask (G. Moraglia & B. Schneider, 1990). We replicate this finding for a broadband target masked by natural images and find the greatest masking (threshold elevation) when target and mask occupy the same depth plane. Masking was reduced equally whether the target appeared at a crossed or an uncrossed disparity. We measure the tuning of masking and determine the extent of the benefit afforded by disparity. Threshold elevation decreases monotonically with increasing disparity until +/-8 arcmin. Two underlying components to the masking are evident; one accounts for around two-thirds of the masking and is independent of disparity. The second component is disparity-dependent and results in additional masking when there is zero disparity. Importantly, the reduction in masking with disparity cannot be explained by interocular decorrelation; we use a single-interval orientation discrimination task to exclude this possibility. We conclude that when the target and mask are presented at different depths they activate distinct populations of disparity-tuned neurons, resulting in less masking of the target.
Pushing the limits of transparent-motion detection with binocular disparity
Greenwood, J.A., and Edwards, M. (2006). Pushing the limits of transparent-motion detection with binocular disparity. Vision Research 46, 2615-2624.
When transparent motion is defined purely by direction differences, observers fail to detect more than two signal... more When transparent motion is defined purely by direction differences, observers fail to detect more than two signal directions simultaneously [Edwards, M., & Greenwood, J.A. (2005). The perception of motion transparency: A signal-to-noise limit. Vision Research, 45, 1877–1884]. This limit is strongly related to signal-detection thresholds for transparent motion, which are several times higher than uni-directional thresholds. When the effective signal intensities are elevated by speed differences that drive independent global-motion systems, the transparent-motion limit can be extended to allow detection of three signals [Greenwood, J.A., & Edwards, M. (2006). An extension of transparent-motion detection limit using speed-tuned global-motion systems. Vision Research, 46, 1440–1449]. Because there are independent disparity-tuned global-motion systems, distributing transparent-motion signals across distinct depth planes also allows an increase in their effective signal intensity. In the present study, the addition of depth differences enabled the simultaneous detection of three signals. However, as with the addition of speed differences, observers were not able to detect four signals, which would be predicted if signal intensity were the sole constraint on transparent-motion detection. The combination of depth and speed produced similar results, suggesting that there is a strict higher-order limit, possibly related to attention, restricting the maximum number of signals that can be detected simultaneously to three.
Improving Perceptual Quality of 3DTV Systems
This is a draft of the thesis I submitted for my PhD viva on Sept. 20th.
3D video has become a buzzword in the recent years within the entertainment industry. It has the capability to enhance... more
3D video has become a buzzword in the recent years within the entertainment industry. It has the capability to enhance the multimedia experience of viewers by providing an added sensation of depth. Stereoscopic 3D video has already made its mark in the cinema industry in the recent past with wide consumer acceptance of Hollywood blockbusters such as 'Avatar'. Driven by the developments in capturing techniques, display technologies, storage and fibre optic broadband technologies, the entertainment industry is now focused on delivering 3D media to the home entertainment systems and mobile devices. In the light of these developments, this thesis presents several models and techniques to perceptually optimize production, compression, transmission and quality evaluation stages of the 3D video distribution chain.
The thesis begins by deriving a mathematical model that quantifies the sensitivity of the human visual system for depth cues provided by a stereoscopic 3D display, such as binocular disparity, retinal blur and relative size cue. The applications of the derived sensitivity models are illustrated by developing a novel 3D video pre-processing technique and a 3D video assessment method.
The depth maps are an aid for virtual view generation process in DIBR applications. However, depth map estimation techniques are yet to mature. The quality of virtual views, which are required for multiview displays, generated with DIBR algorithms depends on the quality/accuracy of the depth maps. Therefore, the depth maps need to be processed and compressed in a way that is suitable for virtual view generation. However, depth map estimation techniques are yet to mature. Therefore, this thesis presents a pre-processing technique based on the sensitivity models derive, which eliminates optical noise in depth maps due to production imperfections. The next part of this thesis presents novel techniques to compress depth maps used in DIBR applications. To compress depth maps in a way that is suitable for virtual view generation process, a novel object based coding technique, which exploits the correlations that exist between the color image and its corresponding depth maps, is proposed. In an attempt to adapt the existing video codecs to encode depth maps in such a way that the rendering artefacts are minimised, two techniques are proposed in this thesis to optimize the encoding mode selection strategy in a H.264 video codec. The first technique is based on Genetic Algorithms and the other is based on an analytical model that approximate rendering errors caused by depth pixel errors. Furthermore, an adaptive bilateral filter that could be implemented as a decoder based post-processing filter is proposed to reduce the effects of quantization of depth maps, to improve the quality of rendered views.
Finally, this thesis addresses two issues related to the transmission of 3D video over error-prone channels. Firstly, the error propagation characteristics of interview predicted stereoscopic video streams is compared against independent encoding of stereoscopic views at various packet loss rates. Secondly, transmission of color plus depth represented 3D video is compared against the stereoscopic 3D video under detrimental channel conditions.
Significant improvements have been made by the proposed techniques over the existing methods. It is expected that the proposed models and techniques will have important use cases in advanced 3D video distribution systems.
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Optical Idealism and the Languages of Depth in Descartes and Berkeley
by David Morris
Southern Journal of Philosophy (1997), 35 (3). pp. 363-392.