To achieve stable holograms, HoloLens has a built-in image stabilization pipeline. The stabilization pipeline works automatically in the background, so there are no extra steps required to enable it. However, developers should exercise techniques that improve hologram stability and avoid scenarios that reduce stability.
The quality of holograms is a result of good environment and good app development. Apps that hit a constant 60 frames-per-second in an environment where HoloLens can track the surroundings will ensure the hologram and the matching coordinate system are in sync. From a user's perspective, holograms that are meant to be stationary will not move relative to the environment.
When environment issues, inconsistent or low rendering rates, or other app problems show up, the following terminology is helpful in identifying the problem.
Frame rate is the first pillar of hologram stability. For holograms to appear stable in the world, each image presented to the user must have the holograms drawn in the correct spot. The displays on HoloLens refresh 240 times a second, showing four separate color fields for each newly rendered image, resulting in a user experience of 60 FPS (frames per second). To provide the best experience possible, application developers must maintain 60 FPS, which translates to consistently providing a new image to the operating system every 16 milliseconds.
To draw holograms to look like they're sitting in the real world, HoloLens needs to render images from the user's position. Since image rendering takes time, HoloLens predicts where a user's head will be when the images are shown in the displays. This prediction algorithm is an approximation. HoloLens has hardware that adjusts the rendered image to account for the discrepancy between the predicted head position and the actual head position. This makes the image the user sees appear as if it was rendered from the correct location, and holograms feel stable. The image updates work best with small changes, and it can't completely fix certain things in the rendered image like motion-parallax.
By rendering at 60 FPS, you are doing three things to help make stable holograms:
Frame rate consistency is as important as a high frames-per-second. Occasionally dropped frames are inevitable for any content-rich application, and the HoloLens implements some sophisticated algorithms to recover from occasional glitches. However, a constantly fluctuating framerate is a lot more noticeable to a user than running consistently at lower frame rates. For example, an application that renders smoothly for 5 frames (60 FPS for the duration of these 5 frames) and then drops every other frame for the next 10 frames (30 FPS for the duration of these 10 frames) will appear more unstable than an application that consistently renders at 30 FPS.
On a related note, the operating system will throttle applications down to 30 FPS when mixed reality capture is running.
There are a variety of tools that can be used to benchmark your application frame rate such as:
The human visual system integrates multiple distance-dependent signals when it fixates and focuses on an object.
Convergence and accommodation are unique because they are extra-retinal cues related to how the eyes change to perceive objects at different distances. In natural viewing, convergence and accommodation are linked. When the eyes view something near (e.g. your nose) the eyes cross and accommodate to a near point. When the eyes view something at infinity the eyes become parallel and the eye accommodates to infinity. Users wearing HoloLens will always accommodate to 2.0m to maintain a clear image because the HoloLens displays are fixed at an optical distance approximately 2.0m away from the user. App developers control where users' eyes converge by placing content and holograms at various depths. When users accommodate and converge to different distances, the natural link between the two cues are broken and this can lead to visual discomfort or fatigue, especially when the magnitude of the conflict is large. Discomfort from the vergence-accommodation conflict can be avoided or minimized by keeping content that users converge to as close to 2.0m as possible (i.e. in a scene with lots of depth place the areas of interest near 2.0m when possible). When content cannot be placed near 2.0m discomfort from the vergence-accomodation conflict is greatest when user’s gaze back and forth between different distances. In other words, it is much more comfortable to look at a stationary hologram that stays 50cm away than to look at a hologram 50cm away that moves toward and away from you over time.
Placing content at 2.0m is also advantageous because the two displays are designed to fully overlap at this distance. For images placed off this plane, as they move off the side of the holographic frame they will disappear from one display while still being visible on the other. This binocular rivalry can be disruptive to the depth perception of the hologram.
For maximum comfort we recommend clipping render distance at 85cm with fadeout of content starting at 1m. In applications where holograms and users are both stationary holograms can be viewed comfortably as near as 50cm. In those cases, applications should place a clip plane no closer than 30cm and fade out should start at least 10cm away from the clip plane. Whenever content is closer than 85cm it is important to ensure that users do not frequently move closer or farther from holograms or that holograms do not frequently move closer to or farther from the user as these situations are most likely to cause discomfort from the vergence-accommodation conflict. Content should be designed to minimize the need for interaction closer than 85cm from the user, but when content must be rendered closer than 85cm a good rule of thumb for developers is to design scenarios where users and/or holograms do not move in depth more than 25% of the time.
When holograms cannot be placed at 2m and conflicts between convergence and accommodation cannot be avoided, the optimal zone for hologram placement is between 1.25m and 5m. In every case, designers should structure content to encourage users to interact 1+ m away (e.g. adjust content size and default placement parameters).
HoloLens performs a sophisticated hardware-assisted holographic stabilization technique. This is largely automatic and has to do with motion and change of the point of view (CameraPose) as the scene animates and the user moves their head. A single plane, called the stabilization plane, is chosen to maximize this stabilization. While all holograms in the scene receive some stabilization, holograms in the stabilization plane receive the maximum hardware stabilization.
The device will automatically attempt to choose this plane, but the application can assist in this process by selecting the focus point in the scene. Unity apps running on a HoloLens should choose the best focus point based on your scene and pass this into SetFocusPoint(). An example of setting the focus point in DirectX is included in the default spinning cube template.
Note that when your Unity app runs on an immersive headset connected to a desktop PC, Unity will submit your depth buffer to Windows to enable per-pixel reprojection, which will usually provide even better image quality without explicit work by the app. If you provide a Focus Point, that will override the per-pixel reprojection, so you should only do so when your app is running on a HoloLens.
// SetFocusPoint informs the system about a specific point in your scene to // prioritize for image stabilization. The focus point is set independently // for each holographic camera. // You should set the focus point near the content that the user is looking at. // In this example, we put the focus point at the center of the sample hologram, // since that is the only hologram available for the user to focus on. // You can also set the relative velocity and facing of that content; the sample // hologram is at a fixed point so we only need to indicate its position. renderingParameters.SetFocusPoint( currentCoordinateSystem, spinningCubeRenderer.Position );
Placement of the focus point largely depends on the hologram is looking at. The app has the gaze vector for reference and the app designer knows what content they want the user to observe.
The single most important thing a developer can do to stabilize holograms is to render at 60 FPS. Dropping below 60 FPS will dramatically reduce hologram stability, regardless of the stabilization plane optimization.
There is no universal way to set up the stabilization plane and it is app-specific, so the main recommendation is to experiment and see what works best for your scenarios. However, try to align the stabilization plane with as much content as possible because all the content on this plane is perfectly stabilized.
Things to Avoid
The stabilization plane is a great tool to achieve stable holograms, but if misused it can result in severe image instability.
Though it's difficult to completely avoid color separation, there are several techniques available to mitigate it.
Color-separation can be seen on:
To attenuate the effects of color-separation:
As before, rendering at 60 FPS and setting the stabilization plane are the most important techniques for hologram stability. If facing noticeable color separation, first make sure the frame rate meets expectations.