Learning OpenCV, 2nd Edition

void onTrackbarSlide( int pos, void *) {

g_cap.set( cv::CAP_PROP_POS_FRAMES, pos );

if( !g_dontset )

g_run = 1;

g_dontset = 0;

}

int main( int argc, char** argv ) {

cv::namedWindow( "Example2_4", cv::WINDOW_AUTOSIZE );

g_cap.open( string(argv[1]) );

int frames = (int) g_cap.get(cv::CAP_PROP_FRAME_COUNT);

int tmpw = (int) g_cap.get(cv::CAP_PROP_FRAME_WIDTH);

int tmph = (int) g_cap.get(cv::CAP_PROP_FRAME_HEIGHT);

cout << "Video has " << frames << " frames of dimensions("

<< tmpw << ", " << tmph << ")." << endl;

cv::createTrackbar("Position", "Example2_4", &g_slider_position, frames,

onTrackbarSlide);

cv::Mat frame;

while(1) {

if( g_run != 0 ) {

g_cap >> frame; if(!frame.data) break;

int current_pos = (int)g_cap.get(cv::CAP_PROP_POS_FRAMES);

g_dontset = 1;

cv::setTrackbarPos("Position", "Example2_4", current_pos);

cv::imshow( "Example2_4", frame );

g_run-=1;

}

char c = (char) cv::waitKey(10);

if(c == 's') // single step

{g_run = 1; cout << "Single step, run = " << g_run << endl;}

if(c == 'r') // run mode

{g_run = -1; cout << "Run mode, run = " << g_run <<endl;}

if( c == 27 )

break;

}

return(0);

}

In essence, the strategy is to add a global variable to represent the trackbar position and then add a callback

that updates this variable and relocates the read position in the video. One call creates the trackbar and

attaches