Jetson TX1 Onboard Camera - cvCaptureFromCAM

gst-launch-1.0 -ev nvcamerasrc ! nvvidconv ! ximagesink

Hello, I used the code above to launch the Jetson TX1 camera.

How can I write this instead of cam_index.

CvCapture *cap = cvCaptureFromCAM(cam_index);


Not sure it works with C API, but with C++ API gstreamer is handled by the constructor for reading from file.

Note that you need an opencv library built with gstreamer support enabled.

Then you have to make a gstreamer pipeline for providing BGR or gray8 to opencv (from opencv-3.3, opencv is able to read more formats, but you may have to convert to BGR anyway for using opencv algorithms).

const char* gst =  "nvcamerasrc  ! video/x-raw(memory:NVMM), format=(string)I420, width=(int)640, height=(int)480, framerate=(fraction)30/1 ! \
		    nvvidconv    ! video/x-raw,              format=(string)BGRx ! \
		    videoconvert ! video/x-raw,              format=(string)BGR  ! \

cv::VideoCapture cap(gst);

Thank you for your answer. Your code works in C ++. But it can not be integrated into the cvCaptureFromCAM function in C language. Is there a way to do that?

I have very few experience with C API of opencv, not sure if I can really help.
I was suggesting to try with cvCaptureFromFile() that expects a string argument. If it doesn’t work you might consider switching to C++ API.

Opencv is developed in C++ since many years now, and these new features such a gstreamer support may have not been ported back to C API that is deprecated.

OpenCV is open source, you can do that backport to C API yourself, but considering the time it would take it is probably better to use C++ API, as a C++ compiler should be able to handle your C code.

Good news…The code below works on my TX2 with L4T R28.1 and opencv-3.3.0, but for using C API it first requires patching opencv-3.3.0 header file opencv2/core/cvdef.h for adding the following lines at the end (before the last #endif seems ok, at least for this example), otherwise compiler will complain about undefined cvRound() :

#ifndef __cplusplus
#include "opencv2/core/fast_math.hpp"

File simple_video_c_api.c:

#include <stdio.h>
#include <opencv2/core/core_c.h>
#include <opencv2/highgui/highgui_c.h>
#include <opencv2/videoio/videoio_c.h>

int main()
    const char* gst =  "nvcamerasrc  ! video/x-raw(memory:NVMM), format=I420, width=640, height=480, framerate=30/1 ! "
		       "nvvidconv    ! video/x-raw,              format=BGRx ! "
		       "videoconvert ! video/x-raw,              format=BGR  ! "
    CvCapture *cap = cvCaptureFromFile(gst);
    if (!cap) {
	fprintf (stderr, "Failed to open camera.\n");
	return (-1);
    cvNamedWindow("MyCameraPreview", CV_WINDOW_AUTOSIZE);
    IplImage* frame_in;
	frame_in = cvQueryFrame(cap);
	if (!frame_in) {
	    fprintf (stderr, "Capture read error\n");
	else {
	    cvShowImage("MyCameraPreview", frame_in);
    return 0;

My opencv-3.3.0 library being installed in /usr/local/opencv-3.3.0, I compiled with:

gcc -o simple_video_c_api simple_video_c_api.c -I/usr/local/opencv-3.3.0/include -L/usr/local/opencv-3.3.0/lib -lopencv_core -lopencv_videoio -lopencv_highgui -lopencv_imgproc -lm

and run with:

export LD_LIBRARY_PATH=/usr/local/opencv-3.3.0/lib

You may however read, especially noting:
C API is not supported anymore and should not be used. Moreover some “C” calls can raise C++ exceptions.
There is no plan to revive direct C API in OpenCV (you should wait for some FFI C bindings). Use OpenCV from C++ programs only.

Thank you very much for your help. Code works very well …