Why is Yp Cb Cr image buffer all shuffled in iOS 13?

Question

I have a computer vision app that takes grayscale images from sensor and processes them. The image acquisition for iOS is written in Obj-C and the image processing is performed in C++ using OpenCV. As I only need the luminance data, I acquire the image in YUV (or Yp Cb Cr) 420 bi-planar full range format and just assign the buffer's data to an OpenCV Mat object (see aquisition code below). This worked great so far, until the brand new iOS 13 came out... For some reason, on iOS 13 the image I obtain is misaligned, resulting in diagonal stripes. By looking at the image I obtain, I suspect this is the consequence of a change in ordering of the buffer's Y Cb an Cr components or a change in the buffer's stride. Does anyone know if iOS 13 introduces this kind of changes and how I could update my code to avoid this, preferably in a backward-compatible manner?

Here is my image acquisition code:

//capture config
- (void)initialize {
    AVCaptureDevice *frontCameraDevice;
    NSArray *devices = [AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];
    for (AVCaptureDevice *device in devices) {
        if (device.position == AVCaptureDevicePositionFront) {
            frontCameraDevice = device;
        }
    }
    if (frontCameraDevice == nil) {
        NSLog(@"Front camera device not found");
        return;
    }

    _session = [[AVCaptureSession alloc] init];
    _session.sessionPreset = AVCaptureSessionPreset640x480;

    NSError *error = nil;
    AVCaptureDeviceInput *input = [AVCaptureDeviceInput deviceInputWithDevice:frontCameraDevice error: &error];
    if (error != nil) {
        NSLog(@"Error getting front camera device input: %@", error);
    }
    if ([_session canAddInput:input]) {
        [_session addInput:input];
    } else {
        NSLog(@"Could not add front camera device input to session");
    }

    AVCaptureVideoDataOutput *videoOutput = [[AVCaptureVideoDataOutput alloc] init];
    // This is the default, but making it explicit
    videoOutput.alwaysDiscardsLateVideoFrames = YES;

    if ([videoOutput.availableVideoCVPixelFormatTypes containsObject:
                      [NSNumber numberWithInt:kCVPixelFormatType_420YpCbCr8BiPlanarFullRange]]) {
        OSType format = kCVPixelFormatType_420YpCbCr8BiPlanarFullRange;
        videoOutput.videoSettings = [NSDictionary dictionaryWithObject:[NSNumber numberWithUnsignedInt:format]
                                                                 forKey:(id)kCVPixelBufferPixelFormatTypeKey];
    } else {
        NSLog(@"YUV format not available");
    }

    [videoOutput setSampleBufferDelegate:self queue:dispatch_queue_create("extrapage.camera.capture.sample.buffer.delegate", DISPATCH_QUEUE_SERIAL)];
    if ([_session canAddOutput:videoOutput]) {
        [_session addOutput:videoOutput];
    } else {
        NSLog(@"Could not add video output to session");
    }

    AVCaptureConnection *captureConnection = [videoOutput connectionWithMediaType:AVMediaTypeVideo];
    captureConnection.videoOrientation = AVCaptureVideoOrientationPortrait;
}

//acquisition code 
- (void)captureOutput:(AVCaptureOutput *)output didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection {

    if (_listener != nil) {
        CVPixelBufferRef pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
        OSType format = CVPixelBufferGetPixelFormatType(pixelBuffer);

        NSAssert(format == kCVPixelFormatType_420YpCbCr8BiPlanarFullRange, @"Only YUV is supported");

        // The first plane / channel (at index 0) is the grayscale plane
        // See more infomation about the YUV format
        // http://en.wikipedia.org/wiki/YUV
        CVPixelBufferLockBaseAddress(pixelBuffer, 0);
        void *baseaddress = CVPixelBufferGetBaseAddressOfPlane(pixelBuffer, 0);

        CGFloat width = CVPixelBufferGetWidth(pixelBuffer);
        CGFloat height = CVPixelBufferGetHeight(pixelBuffer);

        cv::Mat frame(height, width, CV_8UC1, baseaddress, 0);

        [_listener onNewFrame:frame];

        CVPixelBufferUnlockBaseAddress(pixelBuffer, 0);
    }
}

Answer 1

I found the solution to this problem. It was a row stride issue: appearently, in iOS 13, the row stride of the Yp Cb Cr 4:2:0 8 bit bi-planar buffer was changed. Maybe for it to always be a power of 2. Therefore in some cases, the row stride is no longer the same as the width. It was the case for me. The fix is easy, just get the row stride from the buffer's info and pass it to the OpenCV Mat's constructor as shown below.

void *baseaddress = CVPixelBufferGetBaseAddressOfPlane(pixelBuffer, 0);         
size_t width = CVPixelBufferGetWidthOfPlane(pixelBuffer, 0);
size_t height = CVPixelBufferGetHeightOfPlane(pixelBuffer, 0);
size_t bytesPerRow = CVPixelBufferGetBytesPerRowOfPlane(pixelBuffer, 0);

cv::Mat frame(height, width, CV_8UC1, baseaddress, bytesPerRow);

Note that I also changed how I get the width and height by using the dimensions of the plane instead of the ones of the buffer. For the Y plane, it should always be the same. I am not sure that this makes a difference.

Also be careful: after the Xcode update to support the iOS 13 SDK, I had to uninstall my app from the test device because otherwise, Xcode kept running the old version instead of the newly compiled one.