PV camera frame's origin

[emaschino]

Hello,

As we all know, OpenCV image origin [0,0] is the top, left corner of the image, +Y pointing down [1].

When retrieving HoloLens 2 PV camera frames using Media Foundation, the CameraIntrinsics.PrincipalPoint API documentation clearly states that "The origin [0,0] is the bottom, left corner of the image (+Y pointing up)" [2]. So the Y axis is reversed w.r.t. the OpenCV coordinate frame.

In the StreamRecorder sample [3], PrincipalPoint.y data is however passed as-is, whereas I was expecting CameraIntrinsics.ImageHeight() - PrincipalPoint.y. This has no effect in the context of the StreamRecorder sample, as the data are simply logged to a file. But a similar discrepancy can also be observed in the older HoloLensForCV repository sample when initializing cy component in the camera matrix later used for image undistortion [4].

What am I missing here? I've experimentally determined the HoloLens[2]ForCV samples being correct, i.e. taking PrincipalPoint.y as-is for OpenCV camera matrix cy component gives the correct visual result (with a sample app of my own), but I would also be very surprised that the CameraIntrinsics.PrincipalPoint API documentation could be so wrong, with a remark section even emphasizing the frame origin peculiarity from other image frameworks.

Thanks for any input/suggestion/clarification.

[1] https://docs.opencv.org/4.0.1/d9/d0c/group__calib3d.html
[2] https://learn.microsoft.com/en-us/uwp/api/windows.media.devices.core.cameraintrinsics.principalpoint?view=winrt-17763#windows-media-devices-core-cameraintrinsics-principalpoint
[3]

HoloLens2ForCV/Samples/StreamRecorder/StreamRecorderApp/VideoFrameProcessor.cpp

Line 177 in 2072055

file << m_latestFrame.VideoMediaFrame().CameraIntrinsics().PrincipalPoint().x << "," << m_latestFrame.VideoMediaFrame().CameraIntrinsics().PrincipalPoint().y << ","

[4] https://github.com/microsoft/HoloLensForCV/blob/master/Samples/ComputeOnDevice/AppMain.cpp#L138

[emaschino]

The documentation counterpart [1].

[1] MicrosoftDocs/winrt-api#2312