Getting image from CSI and sending it to OpenCV does not give correct image

I want to read synchronized images (stereo) from two CSI-Cameras connected to my Jetson Nano B01 and then process them in OpenCV. It kind of works, but I do not get correct images but something like this:


I think there could be a problem with the image format (YUV420 instead of BGR), the mmap-operation or something else, but I can’t figure it out.

My code to get the images and send them to OpenCV (only for left image for clarity reasons):

bool StereoCameraInterface::getSyncedImages(cv::Mat *left, cv::Mat *right)
	const uint64_t FIVE_SECONDS_IN_NANOSECONDS = 5000000000;
	// Initialize EGL.

	// Initialize the Argus camera provider.
	UniqueObj<CameraProvider> cameraProvider(CameraProvider::create());

	// Get the ICameraProvider interface from the global CameraProvider.
	ICameraProvider *iCameraProvider = interface_cast<ICameraProvider>(cameraProvider);

	// Get the camera devices.
	std::vector<CameraDevice*> cameraDevices;
	if (cameraDevices.size() < 2)
		ORIGINATE_ERROR("Must have at least 2 sensors available");

	std::vector <CameraDevice*> lrCameras;
	lrCameras.push_back(cameraDevices[0]); // Left Camera (the 1st camera will be used for AC)
	lrCameras.push_back(cameraDevices[1]); // Right Camera

	// Create the capture session, AutoControl will be based on what the 1st device sees.
	UniqueObj<CaptureSession> captureSession(iCameraProvider->createCaptureSession(lrCameras));
	ICaptureSession *iCaptureSession = interface_cast<ICaptureSession>(captureSession);

	// Create stream settings object and set settings common to both streams.
	UniqueObj<OutputStreamSettings> streamSettings(
	IOutputStreamSettings *iStreamSettings = interface_cast<IOutputStreamSettings>(streamSettings);
	IEGLOutputStreamSettings *iEGLStreamSettings =

	// Create egl streams
	PRODUCER_PRINT("Creating left stream.\n");
	UniqueObj<OutputStream> streamLeft(iCaptureSession->createOutputStream(streamSettings.get()));
	IEGLOutputStream *iStreamLeft = interface_cast<IEGLOutputStream>(streamLeft);

	// Create consumer for left frame
	Argus::UniqueObj<EGLStream::FrameConsumer> consumerLeft(

	EGLStream::IFrameConsumer *iFrameConsumerLeft =
	// Create a request
	UniqueObj<Request> request(iCaptureSession->createRequest());
	IRequest *iRequest = interface_cast<IRequest>(request);

	Argus::Status statusLeft;

	// Enable both streams in the request.
	statusLeft = iRequest->enableOutputStream(streamLeft.get());

	uint32_t requestId = iCaptureSession->capture(request.get());
	EXIT_IF_NULL(requestId, "Failed to submit capture request");
	 * Acquire a frame generated by the capture request, get the image from the frame
	 * and create OpenCV Mat
	Argus::UniqueObj<EGLStream::Frame> frameLeft(
		iFrameConsumerLeft->acquireFrame(FIVE_SECONDS_IN_NANOSECONDS, &statusLeft));    
	EXIT_IF_NULL(frameLeft, "Failed to acquire left frame");

	EGLStream::IFrame *iFrameLeft = Argus::interface_cast<EGLStream::IFrame>(frameLeft);
	EXIT_IF_NULL(iFrameLeft, "Failed to get IFrame interface left");

	EGLStream::Image *imageLeft = iFrameLeft->getImage();
	EXIT_IF_NULL(imageLeft, "Failed to get Image from iFrame->getImage()");

	// Shut down Argus.
	// create OpenCV Images

	EGLStream::NV::IImageNativeBuffer *iImageNativeBufferLeft
		  = interface_cast<EGLStream::NV::IImageNativeBuffer>(imageLeft);
	int fd_left = iImageNativeBufferLeft->createNvBuffer(STREAM_SIZE,
		   NvBufferColorFormat_YUV420, NvBufferLayout_Pitch, EGLStream::NV::Rotation::ROTATION_0, &statusLeft);

    NvBufferParams params;
    NvBufferGetParams(fd_left, &params);

    int fsizeLeft = params.height[0] * params.pitch[0];
	char* data_mem_left;

    data_mem_left = (char *)mmap(NULL, 1.5*fsizeLeft, PROT_READ | PROT_WRITE, MAP_SHARED, fd_left, params.offset[0]);
    printf("%dx%d, pitch=%d offset=%d data_mem_left=%p\n",  params.width[0],  params.height[0],
                     params.pitch[0],  params.offset[0], data_mem_left);
	*left = cv::Mat(480, 640, CV_8UC3, *data_mem_left);

	std::cout << "*left finished" << "\n" ;


And to display the image:

StereoCameraInterface interface;

interface.getSyncedImages(&left, &right);

cv::Mat left_bgr;
cv::cvtColor(left, left_bgr, cv::COLOR_YUV420p2RGBA  );

cv::namedWindow("CSI Camera", cv::WINDOW_AUTOSIZE);
cv::imshow("CSI Camera", left_bgr);

If I don’t do the cvtColor, the whole image is just blue.
Any ideas what could be the problem?

You may try to convert to BGR rather than to RGBA for imshow.

Hi @Honey_Patouceul,

thanks for your suggestion, you’re obviously right.
But even if I use cv::cvtColor(left, left_bgr, cv::COLOR_YUV420p2BGR);, it does not work and gives me the same wrong image as a result.

Any idea, how I can fix this?
The pixel format in the stream is PIXEL_FMT_YCbCr_420_888, which I think is just YUV 420 with 8 bits per channel. The NVBuffer then uses NvBufferColorFormat_YUV420, which I then try to convert to BGR by using the aforementioned OpenCV-method.
Is there an incompatibility there maybe?

Another thing:
When I create the NvBuffer, I save it as int fd_left, which is the file descriptor I then use in mmap.
I just copied these lines from samples without really understanding them (as there doesn’t seem to be a proper API documentation, at least I could not find one for createNvBuffer in EGLStream).
How can an int describe the data we want to use in mmap? This is all really not clear to me.
When I use cv::COLOR_YUV2BGR in the colorspace-conversion, I get an all green image, maybe indicating that the YUV-image is all black. So maybe the problem is not (only) the conversion…

Not sure, but you may try cv::COLOR_YUV2BGR_I420 or cv::COLOR_YUV2BGR_NV12.

You may also try 09_camera_jpeg_capture example with this patch using ARGB nvBuffer.

Thanks again for your suggestions.
Unfortunately, I cannot get it working.
My “hacky” solution is now to save the images as JPEG to /tmp and then read them using OpenCV.

It should be possible to repeat in my code exactly what the writeJPEG-method of EGLStream::IImageJPEG does, but I can’t seem to find the file where this method is implemented.
Possibly any hints on that issue? :)

You may also refer to this:

To create NvBuffer in RGBA and map to CV_8UC4.