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I have the same problem described here but with Ubuntu 18.04.

The program runs and does not stop but it does not catch the keyboard input and as soon as it starts I get the error

(process:11477): GStreamer-CRITICAL **: 18:44:25.567: gst_element_get_state: assertion 'GST_IS_ELEMENT (element)' failed
OpenCV | GStreamer warning: GStreamer: unable to query pipeline state (/home/fra/Documents/openCV/openCV/modules/videoio/src/cap_gstreamer.cpp:420)

I have installed the gstreamer libraries because they were required by opencv and I am including and linking them in the Makefile like this (see the complete Makefile later):

Have anyone had the same problem? What is generating the exception?

I am thinking the error is related to Opencv deendencies but I am not sure is not a problem related to SDL2.0.

SOURCE CODE OF THE C++ PROGRAM

Here is a minimal working example: clicking the left and right buttons of the mouse works (catched directly with Opencv callbacks) while pressing and releasing the keyboard buttons does not call the proper SDL callback. The script can be launched with ./executable camID anyNameYouWant where camID is an integer (typically 0 or 1) that identifies the cam you want to use.

#ifndef __OPENCV__
#define __OPENCV__
#include "opencv2/opencv.hpp"
#endif
#include<iostream>
//#include "utils.hpp"
//#include "constants.hpp"
#include<unistd.h>
#include <vector>
#include<SDL.h>
#include <SDL_events.h>

using namespace cv;
using namespace std;
static const int delay = 2;
#define WINDOW_SCALE 1.7

void onTrackbar_changed(int, void* data);
void onMouse(int evt, int x, int y, int flags, void* param);
void PrintKeyInfo( SDL_KeyboardEvent *key );
int keyboardCallback(SDL_KeyboardEvent ev);

//InputStateContext context;

int main(int argc, char* argv[])
{
    /* Initialise SDL */
    if( SDL_Init( SDL_INIT_VIDEO ) < 0)
    {
        fprintf( stderr, "Could not initialise SDL: %s\n", SDL_GetError() );
        exit( -1 );
    }

  string host;
  unsigned int port;
  const String sourceReference = argv[1];
  int camNum;
  string sensorName;
  try
  {
    camNum = stoi(sourceReference); // throws std::length_error
  }
  catch (const std::exception& e)// reference to the base of a polymorphic object
    {
    std::cout<<"Exception: " << e.what()<<endl; // information from length_error printed
    return -1;
  }
  if (argc>4)
  {
    try
    {
      host = argv[2];
      port = atoi(argv[3]);
      sensorName = argv[4];
    }
    catch (const std::exception& e)
    {
      cout<<"impossible to convert host or port"<<endl;
      return -1;
    }
  }
  else if(argc>2)
  {
    cout<<"argumetns less than 4"<<endl;
    host = "http://localhost";
    port = 3456;

    sensorName = argv[2];
    cout<<argc<<endl;
    cout<<"sensor name set from arguments: "<< sensorName<<endl;

  }
  else
  {
    cout<<"stopping execution: too few arguments."<<endl;
    return -1;
  }
  VideoCapture cam(camNum);
  /* or
  VideoCapture captUndTst;
  captUndTst.open(sourceCompareWith);*/
  if ( !cam.isOpened())
  {
    cout << "Could not open reference " << sourceReference << endl;
   return -1;
  }

  namedWindow("Camera", WINDOW_NORMAL);
  Mat frame;

  SDL_Event keyboardEvent;

  cam>>frame;
  resize(frame, frame, cv::Size(frame.cols/WINDOW_SCALE, frame.rows/WINDOW_SCALE));
  resizeWindow("Camera", cv::Size(frame.cols/WINDOW_SCALE, frame.rows/WINDOW_SCALE));
  setMouseCallback("Camera", onMouse, &frame);
  while(1)
  {
      while( SDL_PollEvent( &keyboardEvent) )
      {
          switch( keyboardEvent.type )
          {
              /* Keyboard event */
              /* Pass the event data onto PrintKeyInfo() */

              case SDL_KEYDOWN:
              break;
              case SDL_KEYUP:
              keyboardCallback(keyboardEvent.key);
              break;

              /* SDL_QUIT event (window close) */
              case SDL_QUIT:
              return 0;
              break;

              default:
              break;
          }

      }
     Mat frame_out;
     frame_out = frame.clone();
    cam>>frame;
    resize(frame_out, frame_out, cv::Size(frame.cols/WINDOW_SCALE, frame.rows/WINDOW_SCALE));

    imshow("Camera", frame_out);

    /* A delay is needed to show (it actually wait for an input)*/
    if(waitKey(delay)>delay){;}

  }
    return 0;
}

void onMouse(int evt, int x, int y, int flags, void* param)
{

    if(evt == EVENT_LBUTTONDOWN)
    {
      cout<<"Left button pressed"<<endl;

    }
    else if(evt == EVENT_RBUTTONDOWN)
    {
      cout<<"Right button pressed"<<endl;
    }
}

int keyboardCallback(SDL_KeyboardEvent ev)
{
    switch(ev.keysym.sym)
    {
        case(SDLK_a):
        cout<<"calling context keyboardA"<<endl;
        break;
        case(SDLK_e):
        cout<<"calling context keyboardE"<<endl;
        break;
        case(SDLK_m):
        cout<<"calling context keyboardM"<<endl;
        break;
        case SDLK_UP:
        case SDLK_RIGHT:
        cout<<"calling context RIGHT ARROW"<<endl;
        break;
        case SDLK_DOWN:
        case SDLK_LEFT:
        cout<<"calling context RIGHT ARROW"<<endl;
        break;
        case (SDLK_RETURN):
        cout<<"calling context RIGHT ARROW"<<endl;
        break;
        default:
        break;

    }
    return 0;
}

Furthermore I am also using SDL library to catch keyboard events but while mouse events are catched by setting callbacks with opencv, keyboard events are not catched by SDL2 callback.

Did anyone find any solution to this problem?

[Here] (https://stackoverflow.com/questions/52595657/gstreamer-exception-in-gst-element-get-state-runnig-opencv-program) is the source code and other information.