Existing XR APIs support the notion of 2D composition layers “on top” of an existing application’s content. Valve’s IVROverlay is an example.

Overlay applications can add a rich variety of content into other XR Applications:

• Chat (e.g. Pluto VR)
• Desktop OS windows in-world
• In-game HUD
• Virtual keyboard

The XR_EXTX_overlay experimental extension to OpenXR is intended to prove the concept and uncover issues. Some issues to address include:

• Establishing Security and access control requirements
• How to address input focus requirements

We hope OpenXR runtimes will incorporate a future version of the extension after any concerns are addressed and perhaps the extension is promoted to EXT or KHR.

The XR_EXTX_overlay test implementation for Windows and Direct3D 11 consists of an OpenXR API layer and a separate remote overlay test application. The layer and application are implemented against the OpenXR 1.0.9 specification. The layer is named xr_extx_overlay.dll and the overlay test is named OverlaySample.exe. The API layer runs both in the main OpenXR app and the overlay app, and the instantiation of the API layer in the main app receives OpenXR commands over RPC (remote procedure calls using shared memory) from the instantiation of the API layer in the overlay app.

The test implementation demonstrates the remote overlay client application opening an Overlay session within “unaware” host OpenXR applications. At commit time we have tested only the OpenXR-SDK-Source hello_xr as the host application.

The overlay test app can also be compiled as a standalone OpenXR program by changing the variable createOverlaySession in main() to false. (Note that this may be visually disorienting in VR as there will be only the one layer and no other environmental cues.)

Development used CMake 3.14.6, although other versions may be compatible.

Building this test implementation used Visual Studio project files generated using the commands:

mkdir build
cd build
cmake -D OPENXR_SDK_SOURCE_ROOT=openxr-sdk-source-dir -D OPENXR_SDK_BUILD_SUBDIR=sdk-source-relative-build-dir -D OPENXR_LIB_DIR=built_openxr_loader_dir  -D FREEIMAGE_ROOT=freeimage-header-lib-dir -D FREEIMAGEPLUS_ROOT=freeimageplus-header-lib-dir -G visual-studio-generator-name -A x64 ..

As an example, if building using bash in mintty (msysgit "Git Bash"), you have downloaded the FreeImage dist into $HOME/Downloads and unpacked it there, and you're using Visual Studio 2017, your cmake command may look like this:

cmake -D OPENXR_SDK_SOURCE_ROOT=$HOME/trees/OpenXR-SDK-Source -D OPENXR_LIB_DIR=$HOME/trees/OpenXR-SDK-Source/build/win64/src/loader/Debug -D OPENXR_SDK_BUILD_SUBDIR=build/win64 -D FREEIMAGE_ROOT=$HOME/Downloads/FreeImage/dist/x64 -D FREEIMAGEPLUS_ROOT=$HOME/Downloads/FreeImage/Wrapper/FreeImagePlus/dist/x64 -G "Visual Studio 15 2017" -A x64 ..

The layer DLLs and executables were compiled with Visual Studio 2017, version 15.9.12 in “Debug” configuration. Only the 64-bit target is supported at this time.

The implementation has been tested on Microsoft Windows Mixed Reality OpenXR Developer runtime version 100.1910.1004 and on Oculus OpenXR developer channel runtime 1.52.0 with hello_xr from https://github.com/KhronosGroup/OpenXR-SDK-Source/tree/release-1.0.12 .

Replace references to $OVERLAY_PROJECT below with the top-level directory name containing the clone of the overlay project repository. (E.g. if you performed git clone https://github.com/LunarG/OpenXR-OverlayLayer.git from your home directory, this would be $HOME/OpenXR-OverlayLayer.) Replace references to $WORKING_DIRECTORY to a directory name which will contain runtime files relevant to the test implementation. (Perhaps $HOME/Overlay_OpenXR.)

Copy to $WORKING_DIRECTORY the following files:

The openxr_loader.dll built from OpenXR-SDK-Source or other repository
$OVERLAY_PROJECT/build/overlay-sample/Debug/OverlaySample.exe
$OVERLAY_PROJECT/build/overlay-sample/*.png
$OVERLAY_PROJECT/api-layer/xr_extx_overlay.json
$OVERLAY_PROJECT/build/api-layer/Debug/xr_extx_overlay.dll

Build src/tests/hello_xr from the OpenXR repository.

Run hello_xr.exe with the following environment variables:

XR_ENABLE_API_LAYERS=xr_extx_overlay
XR_API_LAYER_PATH=$WORKING_DIRECTORY

You will also need to set the environment variables for finding the runtime Manifest (JSON) if not in the Windows Registry (e.g. for Oculus).

These can be set, for example, in the Visual Studio Preferences for debugging hello_xr in the “Environment” section, or, as another example, at the command-line before running hello_xr. Also provide the command-line option -g D3D11, which can be set in the “Command Arguments” section of Preferences or on the command line.

Run hello_xr. Run OverlaySample.exe. If successful, the console output from OverlaySample.exe will contain various messages and finally should output “First Overlay xrEndFrame was successful! Continuing...”

• The runtime’s xrReleaseSwapchainImage function may return XR_ERROR_VALIDATION_FAILURE, and OverlaySample.exe will break into the debugger if one is running. The reason is unknown.
• OverlaySample.exe does not suggest bindings for the Microsoft or Vive interaction profiles but instead suggests bindings for the “khr/simple_controller” profile. Probably OverlaySample.exe will need to have additional bindings added before being run on Microsoft or Vive runtimes. A workaround is to comment out the calls in openxr_program.cpp that suggest bindings for any profile but “khr/simple_controller”. This would not be an appropriate suggestion for a shipping application.

If the test program or the loader encounter a problem, please open a new issue and attach the contents of the Output pane in Visual Studio for both OverlaySample.exe and hello_xr.exe and the console output from both programs for review.

Additional useful information may be found in the loader output after setting the environment variable XR_LOADER_DEBUG to the value “all” before running hello_xr.exe.