# AR Displays: Next-Generation Technologies to Solve the Vergence-Accommodation Conflict
## 5 Multifocal Approach by LightSpace Technologies
### Computational Aspects for Multi-Plane 3D Rendering  
When computer-generated 3D scenes are to be rendered for a multi-plane display architecture, it is quite similar to conventional stereoscopic rendering. In the first step a stereo render target is issued and a basic stereoscopic 2D texture is rendered to the intermediate buffer. Alongside with this 2D texture, a depth buffer is also output. In the following step the division into image depth planes is carried out. This procedure essentially is similar to any other post-processing shader operation, such as the application of the bloom effect or antialiasing. For the \"slicing\" operation, a larger output texture is created in the back buffer, the size of which typically is matched to the image resolution of the target spatial light modulator multiplied by the number of used image depth planes--thus representing full uncompressed data array for all available depth planes. Further a custom post-processing shader based on the stored depth map data remaps pixels from the rendered 2D texture to the final output texture, which when finished is output to the front buffer. With shader operations, a very efficient parallel computation can be achieved, thus making this post-processing operation brief in comparison to the initial rendering. Consequently, multi-plane stereoscopic rendering is computationally undemanding and, from practical benchmarks, we observe only up to 3% decrease in generated frames per second in comparison to basic stereoscopic rendering. It must be noted that the differences are more pronounced for simpler 3D scenes, for which the initial rendering takes less time, while for more complex scenes  
Figure 6: A photograph of a multifocal virtual reality headset being evaluated for the first time. Figure 7: A view through one of the oculars of the proof-of concept multi-focal stereoscopic display. (**a**) Focus on a near-image plane; (**b**) focus on the second furthest image plane. Photographs taken with an APS-C CMOS sensor and 35 mm lens with aperture setting at F 2.8.  
the differences can be virtually negligible. A more through comparison of rendering approaches and expected performance is analyzed in our previous work [32]. Another important aspect of the \"slicing\" procedure, which directly impacts the end-result appearance of the visual information, is the methodology of how the content is distributed among available image depth planes. Typically, when rendering for stereoscopic displays, only the top surface is output, meaning that there is no possibility for pixels with multiple depth values. This is convenient and also ensures occlusion, though might cause some inconsistencies in very special and rare cases. Thus, the rendered pixels can have a semi-continuous range of depth values, while the display device based on its optical design has a limited discrete number of depth planes. The simplest way data can be distributed among the discrete number of image depth planes is shown in Figure 8a. Here, a reciprocal depth space or optical space, the axis of which is measured in diopters, is shown. The vertical purple lines represent positions of focal planes, grey dotted lines represent midpoints between adjacent focal planes, whereas blue dotted lines mark absolute boundaries for the content, which can be surely displayed on a device with given arrangement of focal planes. The far boundary at 0 D corresponds to infinity is a self-explanatory boundary, whereas the near boundary can coincide with the focal plane or typically it can be displaced closer towards the eye. The amount of displacement depends on the allowable VAC that would inherently arise, and typically it is set up to 0.3 D in front of the closest focal plane, though it is suggested that at close accommodation distances, even up to 0.6 D, VAC could be entirely tolerable [3]. When a 3D scene is considered (corresponds to the blue figure in Figure 8a,b), it can span over all depth space.