Question

I have been attempting to do some real time video image processing in MonoTouch. I’m using AVCaptureSession to get frames from the camera which works with an AVCaptureVideoPreviewLayer.

I also successfully get the callback method “DidOutputSampleBuffer” in my delegate class. However every way that I have tried to create a UIImage from the resulting CMSampleBuffer fails.

Here is my code setting up the capture session:

captureSession = new AVCaptureSession ();
            captureSession.BeginConfiguration ();
            videoCamera = AVCaptureDevice.DefaultDeviceWithMediaType (AVMediaType.Video);

            if (videoCamera != null)
            {
                captureSession.SessionPreset = AVCaptureSession.Preset1280x720;

                videoInput = AVCaptureDeviceInput.FromDevice (videoCamera);

                if (videoInput != null)
                    captureSession.AddInput (videoInput);

                //DispatchQueue queue = new DispatchQueue ("videoFrameQueue");

                videoCapDelegate = new videoOutputDelegate (this);

                DispatchQueue queue = new DispatchQueue("videoFrameQueue");
                videoOutput = new AVCaptureVideoDataOutput ();

                videoOutput.SetSampleBufferDelegateAndQueue (videoCapDelegate, queue);
                videoOutput.AlwaysDiscardsLateVideoFrames = true;
                videoOutput.VideoSettings.PixelFormat = CVPixelFormatType.CV24RGB;

                captureSession.AddOutput (videoOutput);

                videoOutput.ConnectionFromMediaType(AVMediaType.Video).VideoOrientation = AVCaptureVideoOrientation.Portrait;

                previewLayer = AVCaptureVideoPreviewLayer.FromSession (captureSession);
                previewLayer.Frame = UIScreen.MainScreen.Bounds;
                previewLayer.AffineTransform = CGAffineTransform.MakeRotation (Convert.DegToRad (-90));
                //this.View.Layer.AddSublayer (previewLayer);

                captureSession.CommitConfiguration ();
                captureSession.StartRunning ();
            }

I have tried creating a CGBitmapContext from a CVPixelBuffer casted from the sample buffer’s image buffer like so:

public override void DidOutputSampleBuffer (AVCaptureOutput captureOutput, MonoTouch.CoreMedia.CMSampleBuffer sampleBuffer, AVCaptureConnection connection)
    {

        CVPixelBuffer pixelBuffer = sampleBuffer.GetImageBuffer () as CVPixelBuffer;
        CVReturn flag = pixelBuffer.Lock (0);
        if(flag == CVReturn.Success)
        {
            CGBitmapContext context = new CGBitmapContext
                    (
                        pixelBuffer.BaseAddress,
                        pixelBuffer.Width,
                        pixelBuffer.Height,
                        8,
                        pixelBuffer.BytesPerRow, 
                        CGColorSpace.CreateDeviceRGB (), 
                        CGImageAlphaInfo.PremultipliedFirst
                        );

            UIImage image = new UIImage(context.ToImage());

            ProcessImage (image);

            pixelBuffer.Unlock(0);

        }else
            Debug.Print(flag.ToString()

        sampleBuffer.Dispose();
    }

This results in the following error

<Error>: CGBitmapContextCreate: invalid data bytes/row: should be at least 2880 for 8 integer bits/component, 3 components, kCGImageAlphaPremultipliedFirst.

even with some tweaking of parameters I either get an invalid Handle exception or a segfault in native objective-c.

I have also tried simply creating a CIImage with the CVImageBuffer and creating a UIImage from that like so:

public override void DidOutputSampleBuffer (AVCaptureOutput captureOutput, MonoTouch.CoreMedia.CMSampleBuffer sampleBuffer, AVCaptureConnection connection)
    {

        CIImage cImage = new CIImage(sampleBuffer.GetImageBuffer ());
        UIImage image = new UIImage(cImage);
        ProcessImage (image);

        sampleBuffer.Dispose();
    }

This results in an exception when initializing the CIImage:

NSInvalidArgumentException Reason: -[CIImage initWithCVImageBuffer:]: unrecognized selector sent to instance 0xc821d0

This honestly feels like some sort of bug with MonoTouch but if I’m missing something or just trying to do this in a weird way please let me know of some alternative solutions.

thanks

Answer 1

This error message explains it:

<Error>: CGBitmapContextCreate: invalid data bytes/row: should be at least 2880 for 8 integer bits/component, 3 components, kCGImageAlphaPremultipliedFirst.

With a width of 720 pixels and 1084 bytes per row that is a little bit over 1.5 bytes per pixel – and that’s not RGB24 (which is 3 bytes per pixel), it’s some planar format.

You might want to check AVCaptureVideoDataOutput.AvailableVideoCVPixelFormatTypes for available pixel formats you can use to see if there is any supported format which is easier for you to work with.