The XR_EXTX_overlay extension.

Application developers may desire to implement an OpenXR application that renders content on top of another OpenXR application. These additional applications will execute in a separate process, create a separate session, generate separate content, but want the OpenXR runtime to composite their content on top of the main OpenXR application. Examples of these applications might include:

• A debug environment outputting additional content
• A Store application that hovers to one side of the user’s view
• A interactive HUD designed to expose additional chat features

This extension introduces the concept of "Overlay Sessions" in order to expose this usage model.

This extension allows:

• An application to identify when the current sessions composition layers will be applied during composition
• The ability for an overlay session to get information about what is going on with the main application

To enable the functionality of this extension, an application must pass the name of the extension into CreateInstance via the XrInstanceCreateInfo::enabledExtensionNames parameter as indicated in the extensions section.

To create an overlay session, an application must pass an XrSessionCreateInfoOverlayEXTX structure to CreateSession via the XrSessionCreateInfo structure’s next parameter.

An overlay application should not assume that the values returned to it by WaitFrame in predictedDisplayTime in XrFrameState will be the same as the values returned to the main application or even correlated.