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Stepping Up: The Floor Is Yours with promotion of the XR_EXT_local_floor extension to OpenXR 1.1 Core Banner

Stepping Up: The Floor Is Yours with promotion of the XR_EXT_local_floor extension to OpenXR 1.1 Core

by openxr

The XR_EXT_local_floor extension recently made its way into the core specification with the release of OpenXR 1.1. In this blog post, we will delve into the technical aspects of the LOCAL_FLOOR[1] reference space. While STAGE space is still available to developers for defining playspace bounds, we will show how LOCAL_FLOOR offers a convenient alternative for obtaining a recenterable floor space that does not require user calibration. Additionally, we will explore how this extension includes an estimated floor height, adding further convenience to XR development workflows.

Initial Problem

Originally, before the XR_EXT_local_floor extension, OpenXR defined two main reference spaces to be used by applications, STAGE space and LOCAL space. STAGE space is typically set up and calibrated by the user once corresponding to their real world room and then typically stays fixed until the user manually starts a new calibration procedure. LOCAL space on the other hand is dynamically placed by the OpenXR runtime with no relation to the real world and has no specific relation to the floor or floor height. Typically LOCAL space can also be "recentered", meaning that the user's position and forward direction in the XR world gets aligned to the user's current position and forward direction in their physical space. With the rise of standalone headsets, users often want to put on the headset where they stand and start playing without starting a calibration procedure for stage space, but still have a virtual floor at a height that matches their actual physical floor.

How does XR_EXT_local_floor solve the problem?

This is where LOCAL_FLOOR space was born: A space that can typically be recentered to the current position at the press of a button without a whole calibration procedure, and has an estimated floor height built in. LOCAL_FLOOR is also required to be available on all runtimes, including those that do not provide a STAGE space.

LOCAL_FLOOR space is not a complete novelty: before the XR_EXT_local_floor extension, developers could already locate STAGE space relative to LOCAL space and then use LOCAL space, but at stage space height. LOCAL_FLOOR space eliminates that one step and so, as an explicitly supported space now, it is more likely that developers make use of this functionality.

What are the immediate consequences for developers?

In OpenXR 1.1, applications do not need to enable the XR_EXT_local_floor extension anymore to use the XR_REFERENCE_SPACE_TYPE_LOCAL_FLOOR reference space. Runtimes may still make the extension available, but because the XR_REFERENCE_SPACE_TYPE_LOCAL_FLOOR value in OpenXR 1.1 is aliased to the XR_REFERENCE_SPACE_TYPE_LOCAL_FLOOR_EXT value in the XR_EXT_local_floor extension, enabling the extension in OpenXR 1.1 is redundant.

As a developer, be aware that LOCAL_FLOOR’s horizontal coordinates (X and Z) and orientation always match LOCAL space ones. If the runtime you are targeting provides a STAGE space, and LOCAL_FLOOR location is within that STAGE’s horizontal bounds, then LOCAL_FLOOR’s Y coordinate will be set to 0. However, if there’s no STAGE (or if that space is not currently tracked), your LOCAL_FLOOR’s Y axis origin set by the OpenXR runtime using its best estimate of the floor level under the origin of the LOCAL space.

In addition, even if the user has not calibrated the STAGE space, the LOCAL_FLOOR space will still have the best estimate of the floor height that the OpenXR runtime can determine. LOCAL_FLOOR space defines clear conditions when its height (Y coordinate) must match STAGE's height.

That’s nice, but what happens when LOCAL space changes? The LOCAL_FLOOR space will then change accordingly, and the runtime will emit an XrEventDataReferenceSpaceChangePending event. xrLocateSpace(s) or xrLocateViews will only take the updated LOCAL_FLOOR space location into account when using an XrTime greater than the changedTime parameter returned in the event.

Finally, while OpenXR runtimes will make a best effort to estimate a proper floor level that’s coherent with the user's physical floor, the LOCAL_FLOOR space does not have any requirements about the floor level being walkable, or being an established play area. In some environments, it might just be a static offset from the LOCAL space: the OpenXR 1.1 specification requires this space to be provided on all systems, and it is not requiring environment-understanding capabilities to do so. Applications that want to support runtimes both with and without an established floor are still encouraged to use STAGE and LOCAL space respectively for two different modes of interaction.

Conclusion

All that being said, applications that need more information about floor levels can look at the STAGE space or use an environment-understanding extension, if available. However, developers that do not want to implement a separate interaction mode for devices without a STAGE space will find the LOCAL_FLOOR space handy with its ability to easily locate objects relative to a floor-based, recenterable space across all systems.

With the promotion of the XR_EXT_local_floor extension in OpenXR 1.1 core specification, we sincerely look forward to seeing increased adoption of the LOCAL_FLOOR space by OpenXR developers.

Read more about everything new in OpenXR 1.1 and please share your feedback via the OpenXR Discord channel, OpenXR Forums, or the GitHub Issue Tracker. 

[1] To make the text lighter, we have used the terms STAGE, LOCAL and LOCAL_FLOOR in place of their actual XR_REFERENCE_SPACE_TYPE_STAGE, XR_REFERENCE_SPACE_TYPE_LOCAL and XR_REFERENCE_SPACE_TYPE_LOCAL_FLOOR counterparts from the specification

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