Avoiding monocular artifacts in clinical stereotests presented on column-interleaved digital stereoscopic displays

Abstract

New forms of stereoscopic 3-D technology offer vision scientists new opportunities for research, but also come with distinct problems. Here we consider autostereo displays where the two eyes' images are spatially interleaved in alternating columns of pixels and no glasses or special optics are required. Column-interleaved displays produce an excellent stereoscopic effect, but subtle changes in the angle of view can increase cross talk or even interchange the left and right eyes' images. This creates several challenges to the presentation of cyclopean stereograms (containing structure which is only detectable by binocular vision). We discuss the potential artifacts, including one that is unique to column-interleaved displays, whereby scene elements such as dots in a random-dot stereogram appear wider or narrower depending on the sign of their disparity. We derive an algorithm for creating stimuli which are free from this artifact. We show that this and other artifacts can be avoided by (a) using a task which is robust to disparity-sign inversion—for example, a disparity-detection rather than discrimination task—(b) using our proposed algorithm to ensure that parallax is applied symmetrically on the column-interleaved display, and (c) using a dynamic stimulus to avoid monocular artifacts from motion parallax. In order to test our recommendations, we performed two experiments using a stereoacuity task implemented with a parallax-barrier tablet. Our results confirm that these recommendations eliminate the artifacts. We believe that these recommendations will be useful to vision scientists interested in running stereo psychophysics experiments using parallax-barrier and other column-interleaved digital displays.

Overestimation of stereo thresholds by the TNO stereotest is not due to global stereopsis

“…Monocular cues in the other stereo tests can be excluded as a potential explanation, because the use of a dynamic random dot display in the global psychophysical stereotest eliminated any monocular cues . Thus if this had been the explanation, thresholds would have been elevated in our global psychophysical stereotest as well.…”

Stereopsis: are we assessing it in enough depth?

“…The Asteroid test (Accurate STEReotest On a mobIle Device), currently in development, has been designed to address some of the issues identified in current clinical tests. The stimuli are presented on an autostereoscopic 3‐D tablet, using the device's camera to actively monitor test distance and adjust the disparity accordingly, using anti‐aliasing to present subpixel levels of disparity . In addition, an adaptive staircase is utilised for threshold calculation and the whole test is presented in a game format designed to be more engaging.…”

“…Given that adult thresholds are below 18″, this results in a floor effect. However, anti‐aliasing (where shifts in the luminance of the pixels at the edge of the stimuli are augmented) can create smaller disparity shifts than whole pixels allow . The other way to improve the sensitivity and range of an electronic display‐based stereoacuity test is to increase the number of pixels while reducing their size.…”

Efficient estimation of stereo thresholds: What slope should be assumed for the psychometric function?

“…Given that the stereoscopic presentation system uses line interleaving to dissociate left and right images, the dots appeared as ellipses, with a width of 10 pixels and a height of 20 physical pixels (10.07 × 20.14 arcmin). Subpixel disparities were achieved by using an antialiasing technique (see [19]). In Experiment 1, we used a 2AFC task and the target was a random dot stereogram of 8.4 × 8.4 deg with crossed disparities presented on top of a surround composed of random dots with uncrossed disparities (see Fig 2A).…”

“…Target and background had equal and opposite disparity relative to the screen. This procedure reduces monocular cues that could be present for high disparities [19]. The stimulus disparity was defined as the relative disparity between the target and background (see an anaglyph version in Fig 2D).…”