Dynamically changing video in OpenCV - c++

Question

I have 3 video source connected to my PC and I want to show them on one screen.
I initially started to put the video sources next to each other and that works fine, but I want to be able to enable/disable each video source at run time.
So I am want to use keyboard keys (r(right) and l(left)) to change what cameras are being shown at the moment.
I want to move the declaration of the following 3 variables to the outside of the while loop so I can access them in the if-cases and change them.
cv::Mat3b combinedFrame(camRightSize.height, camMiddleSize.width + camRightSize.width);
cv::Mat3b leftSideOfScreen(combinedFrame, Rect(0, 0, camMiddleSize.width, camMiddleSize.height));
cameraMiddleFrameMirroredResize.copyTo(leftSideOfScreen);
cv::Mat3b rightSideOfScreen(combinedFrame, Rect(camMiddleSize.width, 0, camRightSize.width, camRightSize.height));
Below is my whole code:
int main(int argc, char **argv) {
int combinedScreenWidth = 1440;
int combinedScreenHeight = 540;
int rearCameraBiggerByThis = 200;
int combinedScreenWidthHalv = combinedScreenWidth / 2;
bool showRight = true;
bool showLeft = false;
//initialize and allocate memory to load the video stream from camera
cv::VideoCapture cameraRight(0); // RIGHT
cv::VideoCapture cameraMiddle(3); // REAR
cv::VideoCapture cameraLeft(3); // LEFT
if (!cameraRight.isOpened()) return 1;
if (!cameraMiddle.isOpened()) return 1;
if (!cameraLeft.isOpened()) return 1;
cv::Mat3b cameraRightFrame;
cv::Mat3b cameraMiddleFrame;
cv::Mat3b cameraLeftFrame;
cv::Mat3b cameraRightFrameMirrored;
cv::Mat3b cameraMiddleFrameMirrored;
cv::Mat3b cameraLeftFrameMirrored;
Size camRightSize;
Size camMiddleSize;
Size camLeftSize;
cv::Mat3b cameraRightFrameMirroredResize;
cv::Mat3b cameraMiddleFrameMirroredResize;
cv::Mat3b cameraLeftFrameMirroredResize;
while (true) {
// Grab and retrieve each frames of the video sequentially
cameraRight >> cameraRightFrame;
cameraMiddle >> cameraMiddleFrame;
cameraLeft >> cameraLeftFrame;
// Mirror
cv::flip(cameraRightFrame, cameraRightFrameMirrored, 1);
cv::flip(cameraMiddleFrame, cameraMiddleFrameMirrored, 1);
cv::flip(cameraLeftFrame, cameraMiddleFrameMirrored, 1);
// Resize
camRightSize = cameraRightFrame.size();
camMiddleSize = cameraMiddleFrame.size();
camLeftSize = cameraLeftFrame.size();
resize(cameraMiddleFrameMirrored, cameraMiddleFrameMirroredResize, Size(combinedScreenWidthHalv + rearCameraBiggerByThis, combinedScreenHeight));
resize(cameraRightFrameMirrored, cameraRightFrameMirroredResize, Size(combinedScreenWidthHalv - rearCameraBiggerByThis, combinedScreenHeight));
// Compilation
camRightSize = cameraRightFrameMirroredResize.size();
camMiddleSize = cameraMiddleFrameMirroredResize.size();
camLeftSize = cameraLeftFrameMirroredResize.size();
if (showRight && showLeft) { // LEFT + REAR + RIGHT
} else if (showRight) { // REAR + RIGHT
} else if (showLeft) { // LEFT + REAR
} else { // REAR
}
cv::Mat3b combinedFrame(camRightSize.height, camMiddleSize.width + camRightSize.width);
cv::Mat3b leftSideOfScreen(combinedFrame, Rect(0, 0, camMiddleSize.width, camMiddleSize.height));
cameraMiddleFrameMirroredResize.copyTo(leftSideOfScreen);
cv::Mat3b rightSideOfScreen(combinedFrame, Rect(camMiddleSize.width, 0, camRightSize.width, camRightSize.height));
cameraRightFrameMirroredResize.copyTo(rightSideOfScreen);
// declare windows
cv:namedWindow("Combined", CV_WINDOW_NORMAL);
cv::setWindowProperty("Combined", CV_WND_PROP_FULLSCREEN, CV_WINDOW_FULLSCREEN);
cv::putText(combinedFrame, "REAR", cv::Point(500, 50), cv::FONT_HERSHEY_PLAIN, 2.0, cv::Scalar(255, 255, 255), 2 );
cv::putText(combinedFrame, "RIGHT", cv::Point(950, 50), cv::FONT_HERSHEY_PLAIN, 2.0, cv::Scalar(255, 255, 255), 2 );
cv::imshow("Combined", combinedFrame); // 1440 x 540 Screen size
//cv::imshow("Right Cam", cameraRightFrame);
//cv::imshow("Middle Cam", cameraMiddleFrame);
//cv::imshow("Left Cam", cameraLeftFrame);
//wait for 40 milliseconds
int c = cvWaitKey(1);
//exit the loop if user press "Esc" key (ASCII value of "Esc" is 27)
if (27 == char(c)) {
break;
}
else if (114 == char(c)) {
showRight = !showRight;
}
else if (108 == char(c)) {
showLeft = !showLeft;
}
}
return 0;
}

Answer 1

According to your description, I think what you're wanting to write is:
combinedFrame(Rect(0, 0, camMiddleSize.width, camMiddleSize.height)).copyTo(leftSideOfScreen);
That is:
create a cv::Mat from another one (call to cv::Mat::copyTo(cv::Mat&))
the "source matrix" is obtained by extracting a rectangle from combinedFrame : combinedFrame(Rect(0, 0, camMiddleSize.width, camMiddleSize.height))