Hololens 2 UWP C# App: Issues with Front-Facing Camera at Higher Resolutions (2272x1278) and NV12

Richard 5 Reputation points
2023-12-04T14:38:04.9133333+00:00

I am developing a UWP C# App for the Hololens2. I am trying to access the front facing camera via MediaCapture and MediaFrameReader. So far all is working fine as long as I stick to the resolution of 896x504. As soon as I try to set the resolution to 2272x1278 the image I'm getting is broken.

Since I am only interested in the gray values, I only copy the luminance values from the SoftwareBitmap. With the NV12 format, these should come right at the beginning of the buffer. When I visualize the copied data, I get an image like the one below (here the camera is facing a white wall). The picture shows strange diagonal stripes and nothing is recognizable.

20231204_150846_HoloLens_crop

I also tried it with the YUV2 format and here it is working fine, but copying the pixel data is more time-consuming.

Am I missing something?

init media capture:

string deviceId = await GetDeviceId();
var mediaInitSettings = new MediaCaptureInitializationSettings { VideoDeviceId = deviceId };
IReadOnlyList<MediaCaptureVideoProfile> profiles = MediaCapture.FindAllVideoProfiles(deviceId);

var match = (from profile in profiles
	from desc in profile.SupportedRecordMediaDescription
	where desc.Subtype == "NV12" && desc.Width == 2272 && desc.Height == 1278 && Math.Abs(desc.FrameRate - 30) < Tolerance
	select new { profile, desc}).FirstOrDefault();

if (match != null)
{
	mediaInitSettings.VideoProfile = match.profile;
	mediaInitSettings.RecordMediaDescription = match.desc;
	mediaInitSettings.SharingMode = MediaCaptureSharingMode.ExclusiveControl;
	mediaInitSettings.MemoryPreference = MediaCaptureMemoryPreference.Cpu;
}
else
{
	mediaInitSettings.VideoProfile = profiles[0];
}

_mediaCapture = new MediaCapture();
await _mediaCapture.InitializeAsync(mediaInitSettings);

init FrameREader:

const MediaStreamType mediaStreamType = MediaStreamType.VideoRecord;
CameraParameters parameters = new CameraParameters(_cameraProfile);

MediaFrameSource source = _mediaCapture.FrameSources.Values.Single(frameSource => frameSource.Info.MediaStreamType == mediaStreamType);
var preferredFormat = source.SupportedFormats.Where(format => 
				format.VideoFormat.Height == parameters.CameraResolutionHeight
				&&
				format.VideoFormat.Width == parameters.CameraResolutionWidth
				&&
				Math.Round(format.FrameRate.Numerator / format.FrameRate.Denominator - parameters.FrameRate) < Tolerance
				&&
				format.Subtype == "NV12");

var selectedFormat = preferredFormat.FirstOrDefault();
await source.SetFormatAsync(selectedFormat);

_frameReader = await _mediaCapture.CreateFrameReaderAsync(source, selectedFormat.Subtype);
_frameReader.FrameArrived += OnFrameArrived;

Copying pixel data:

private async void OnFrameArrived(MediaFrameReader sender, MediaFrameArrivedEventArgs args)
{
	if (sender == null) throw new ArgumentNullException(nameof(sender));
	if (args == null) throw new ArgumentNullException(nameof(args));

	using (MediaFrameReference mediaFrameReference = sender.TryAcquireLatestFrame())
	{
		if (mediaFrameReference == null) return;
		var videoMediaFrame = mediaFrameReference.VideoMediaFrame;
		var softwareBitmap = videoMediaFrame?.SoftwareBitmap;
		
		if (softwareBitmap != null)
		{
			var width = softwareBitmap.PixelWidth;
			var height = softwareBitmap.PixelHeight;

			// copy only luminance plane of NV12
			byte[] rawPixelData = new byte[width * height];
			using (var buffer = softwareBitmap.LockBuffer(BitmapBufferAccessMode.Read))
			using (var reference = buffer.CreateReference())
				unsafe
				{
					byte* pixelData;
					uint capacity;
					((IMemoryBufferByteAccess)reference).GetBuffer(out pixelData, out capacity);
					Marshal.Copy((IntPtr)pixelData, rawPixelData, 0, rawPixelData.Length);
				}
			
			FrameArrived?.Invoke(this, new FrameArrivedEventArgs(rawPixelData, width, height));
			softwareBitmap.Dispose();
		}
	}    
}
HoloLens Development
HoloLens Development
HoloLens: A family of Microsoft self-contained, holographic devices that enable engagement with digital content and interaction with holograms in the surrounding environment.Development: The process of researching, productizing, and refining new or existing technologies.
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