Attenuation-Based 3D Display

Built with Neil Movva

COMPUTATIONAL IMAGING; DISPLAY HARDWARE FALL '17

Introduction

Unlike traditional 2D displays, attenuation-based 3D displays enable the accurate, high-resolution depiction of motion parallax, occlusion, translucency, and specularity. We have implemented iterative tomographic reconstruction for image synthesis on a stack of spatial light modulators (multiple low-cost iPad LCDs). We illuminate these volumetric attenuators with a backlight to recreate a 4D target light field. Although five-layer decomposition generates the optimal tomographic reconstruction, our two-layer display costs less than $100 and requires less computation. More Detailed Write-Up

Background

Engineers have promulgated designs for 3D displays, and even automultiscopic displays, as early as the turn of the 19th century. In particular, we consider four types of 3D display technologies that stand in contrast to what we have produced: parallax barriers, integral imaging, volumetric displays, and holograms. What relates these technologies is their shared ability to replicate disparity, motion parallax, and binocular depth cues without the need for special eyewear. Multi-layer displays present a fifth class of displays. Differing from volumetric displays with light-emitting layers, overlaid attenuation patterns allow objects to appear beyond the display enclosure. We compute these patterns using tomographic techniques to create a 4D light-field illuminated by a uniform backlight (see below). The benefit of a multi-layered display is that it possess high resolution and contrast with only moderate trade-offs in brightness and complexity. Since our display relies only on two layers and the reconstructed light-fields are precomputed, we mitigate these limitations, although the produced image is merely static. Additionally, the stacked LCD configuration relies on multiplicative light absorption, rather than additive absorption; the benefit of this is that the display can construct occlusion, specularity, and depth without the need for any moving components.

Assembly

We first procured two 2048x1536, 9.7”, IPS 60Hz, iPad 3 LCD displays. This display model was chosen for its abundance online, which has kept the price low and created a wealth of readily available related-resources such as display drivers. We then carefully disassembled the front LCD, removing its backlight. The enclosure was then designed such that the stack of LCDs would be well-fastened and close together in approximate alignment. The enclosure also leaves space on the back panel to protect the driver circuitry from obstructing the view. To fabricate the enclosure, we purchased two sheets of Duron, laser cut the sheets to our design specification, and constructed the case. In this initial prototype, we screwed the back LCD into place and then attached the front LCD with adhesive so that we could manually adjust the display for approximate pixel alignment. Since the two LCDs were linearly polarized, we inserted a quarter-wave polarizing sheet between the LCDs to circularly polarize the light and greatly increase the illumination of the front panel.

Methods

We implemented the computed tomography techniques decsribed in “Layered 3D” by G. Wetzstein, D. Lanman, W. Heidrich, R. Raskar (SIGGRAPH 2011) to produce two 2048x1536x7x7 reconstructed images from many precomputed views of the light field, spanning a 20-degree FOV. We solve these layer decompositions ahead of time, and paint them as static images to the LCDs. The light field images were collected from the Stanford Light Field Archive and The Stanford 3D Scanning Repository. For example, the dice scene was created in POV-Ray and then released under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Conclusion

Our goals for this project were modest - we wanted to recreate G. Wetzstein et. al's work from 2012 using low-cost, commodity components, as a rough proof-of-concept for what could eventually become a compelling implementation of 3D display. As in the original paper, we find that the theory is generally sound, but significant challenges remain in engineering a performant system. However, we believe that the resources available to even amateurs in 2017 place us in a significantly better position to iterate and refine our prototype. With time, we see a promising roadmap to a well-engineered, perceptually pleasing product, and the authors intend to develop the work shown here much further.