Learning OpenCV

Moving Around

OK, that was great. Now it's time to tinker around, enhance our toy programs, and explore a little more of the available functionality. The first thing we might notice about the AVI player of Example 2-2 is that it has no way to move around quickly within the video. Our next task will be to add a slider bar, which will give us this ability.

The HighGUI toolkit provides a number of simple instruments for working with images and video beyond the simple display functions we have just demonstrated. One especially useful mechanism is the slider, which enables us to jump easily from one part of a video to another. To create a slider, we call cvCreateTrackbar() and indicate which window we would like the trackbar to appear in. In order to obtain the desired functionality, we need only supply a callback that will perform the relocation. Example 2-3 gives the details.

Example 2-3. Program to add a trackbar slider to the basic viewer window: when the slider is moved, the function onTrackbarSlide() is called and then passed to the slider's new value

#include "cv.h" #include "highgui.h" int g_slider_position = 0; CvCapture* g_capture = NULL; void onTrackbarSlide(int pos) { cvSetCaptureProperty( g_capture, CV_CAP_PROP_POS_FRAMES, pos ); } int main( int argc, char** argv ) { cvNamedWindow( "Example3", CV_WINDOW_AUTOSIZE ); g_capture = cvCreateFileCapture( argv[1] ); int frames = (int) cvGetCaptureProperty( g_capture, CV_CAP_PROP_FRAME_COUNT ); if( frames!= 0 ) { cvCreateTrackbar( "Position", ...