This repository contains a few OpenXR code samples tailored for developers who are familiar with and using Visual Studio tool chain, e.g. HoloLens 2 developers.
These OpenXR samples are using C++17 and D3D11. The same source code works across UWP applications running on HoloLens 2 and Win32 applications running on Windows Desktop with the Mixed Reality headset.
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Understand what is OpenXR and why OpenXR. Read latest OpenXR 1.0 spec (HTML) and latest openxr.h header file (Khronos GitHub).
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Prepare Windows 10 May 2019 Update (1903) and Visual Studio 2019 latest version.
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Prepare HoloLens 2 device or Windows Mixed Reality device.
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Clone this sample repo:
git clone https://github.com/microsoft/OpenXR-SDK-VisualStudio.git -
Open the sample_uwp.sln or sample_win32.sln file in Visual Studio. F5 to build and run the sample. You typically choose ARM64 platform when running on HoloLens 2 devices, or choose x64 platform when running on a Windows Desktop PC with Mixed Reality headset or the HoloLens 2 Emulator.
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Most OpenXR API usage patterns can be found in OpenXRProgram.cpp file. The Run() function at the beginning captures a typical OpenXR app code flow from initialization to the event and rendering loop.
Always enumerate supported pixel formats using xrEnumerateSwapchainFormats, and choose the first color and depth pixel format from the runtime that the app supports, because that's what the runtime prefers. Note, on HoloLens 2, DXGI_FORMAT_B8G8R8A8_UNORM_SRGB and DXGI_FORMAT_D16_UNORM is typically the first choice to achieve better rendering performance. This preference can be different on VR headsets running on a Desktop PC.
Performance Warning: Using a format other than the primary swapchain color format will result in runtime post-processing which comes at a significant performance penalty.
Although this applies to all OpenXR runtimes, care must be taken to ensure the rendering pipeline is gamma-correct. When rendering to a swapchain, the render-target view format should match the swapchain format (e.g. DXGI_FORMAT_B8G8R8A8_UNORM_SRGB for both the swapchain format and the render-target view). The exception is if the app's rendering pipeline does a manual sRGB conversion in shader code, in which case the app should request an sRGB swapchain format but use the linear format for the render-target view (e.g. request DXGI_FORMAT_B8G8R8A8_UNORM_SRGB as the swapchain format but use DXGI_FORMAT_B8G8R8A8_UNORM as the render-target view) to prevent content from being double-gamma corrected.
HoloLens 2 has limited GPU power for applications to render content. Always using a single projection layer can help the application's framerate, hologram stability and visual quality.
Performance Warning: Submitting anything but a single protection layer will result in runtime post-processing which comes at a significant performance penalty.
Create one xrSwapchain for both left and right eye using arraySize=2 for color swapchain, and one for depth.
Render the left eye into slice 0 and the right eye into slice 1.
Use a shader with VPRT and instanced draw calls for stereoscopic rendering to minimize GPU load.
This also enables the runtime's optimization to achieve the best performance on HoloLens 2.
Alternatives to using a texture array, such as double-wide rendering or a separate swapchain per eye, will result in runtime post-processing which comes at a significant performance penalty.
Always render with the recommended view configuration width/height (recommendedImageRectWidth and recommendedImageRectHeight from XrViewConfigurationView), and always use xrLocateViews API to query for the recommended view pose, fov, and other rendering parameters before rendering.
Always use the XrFrameEndInfo.predictedDisplayTime from the latest xrWaitFrame call when querying for poses and views.
This allow HoloLens to adjust rendering and optimize visual quality for the person who is wearing the HoloLens.
Always use XR_KHR_composition_layer_depth_extension and submit the depth buffer together with the projection layer when submitting a frame to xrEndFrame.
This can help hologram stability by enabling the hardware depth reprojection on HoloLens 2.
Prefer a narrower depth range to scope the virtual content to help hologram stability on HoloLens.
For example, the OpenXrProgram.cpp sample is using 0.1 to 20 meters.
Use reversed-Z for a more uniformed depth resolution.
Note, on HoloLens 2, using the preferred DXGI_FORMAT_D16_UNORM depth format can help achieve better frame rate and performance.
Therefore above practice for depth range is more important.
Always enumerate supported environment blend mode using xrEnumerateEnvironmentBlendModes API, and prepare rendering content accordingly.
For example, for a system with XR_ENVIRONMENT_BLEND_MODE_ADDITIVE such as the HoloLens, the app should use transparent as clear color, but for a system with XR_ENVIRONMENT_BLEND_MODE_OPAQUE, the app should use some opaque color as background.
An application typically uses an intermediate space to connect views, actions and holograms together.
Use XR_REFERENCE_SPACE_TYPE_UNBOUNDED_MSFT when the extension is supported to avoid undesired hologram drift when the user moves afar (e.g. 5 meters away) from where the app starts.
Use XR_REFERENCE_SPACE_TYPE_LOCAL as a fallback if the unbounded space extension doesn't exist.
Always use a distinct spatial anchor through xrCreateSpatialAnchorSpaceMSFT extension for any hologram that's locked into the environment.
This allows the hologram stay put even if user walked away to other rooms and comes back.
The spatial anchor can remember and locate the hologram where it was created.
Although HoloLens 2's primary display uses additive environment blending, when the user initiates mixed reality capture, the app's rendering content might be alpha blended with the environment video stream.
To achieve the best visual quality in mixed reality capture videos, it's better to set the XR_COMPOSITION_LAYER_BLEND_TEXTURE_SOURCE_ALPHA_BIT in the projection layer's layerFlags.
Performance Warning: Omitting the XR_COMPOSITION_LAYER_BLEND_TEXTURE_SOURCE_ALPHA_BIT flag on the single projection layer will result in runtime post-processing which comes at a significant performance penalty.
Rather than submitting quad layers as composition layers with XrCompositionLayerQuad, render the quad content directly into the projection swapchain.
Performance Warning: Providing additional layers beyond a single projection layer, such as quad layers, will result in runtime post-processing which comes at a significant performance penalty.
On the HoloLens 2, there are a number of ways to submit composition data through xrEndFrame which will result in post-processing that will have a noticeable performance penalty.
To avoid performance penalities, submit a single XrCompositionProjectionLayer with the following characteristics:
- Use a texture array swapchain
- Use the primary color swapchain format
- Set the alpha blending flag
- Use the recommended view dimensions
- Do not set the
XR_COMPOSITION_LAYER_UNPREMULTIPLIED_ALPHA_BITflag - Set the
XrCompositionLayerDepthInfoKHRminDepthto 0.0f andmaxDepthto 1.0f
Additional considerations will result in better performance:
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