HoughLines() - Drawing the lines and getting their position - c++

Question

Im new to openCv and image processing in general. I need to draw the lines and their position in real time from an camera input like this:
i already have the image from the canny edge detection, but when applying hough line and trying to draw it to that image using the following code i found:
int main(int argc, char* argv[]){
Mat input;
Mat HSV;
Mat threshold;
Mat CannyThresh;
Mat HL;
//video capture object to acquire webcam feed
cv::VideoCapture capture;
capture.open(0);
capture.set(CV_CAP_PROP_FRAME_WIDTH, 640);
capture.set(CV_CAP_PROP_FRAME_HEIGHT, 480);
//start an infinite loop where webcam feed is copied to cameraFeed matrix
//all operations will be performed within this loop
while (true){
capture.read(input);
cvtColor(input, HSV, COLOR_BGR2HSV); //hsv
inRange(HSV, Scalar(H_MIN, S_MIN, V_MIN), Scalar(H_MAX, S_MAX, V_MAX), threshold);//thershold
MorphOps(threshold);//morph operations on threshold image
Canny(threshold, CannyThresh, 100, 50); //canny edge detection
std::vector<Vec4i> lines;
HoughLines(CannyThresh, lines, 1, CV_PI/180, 150, 0, 0 );
for( size_t i = 0; i < lines.size(); i++ )
{
float rho = lines[i][0], theta = lines[i][1];
Point pt1, pt2;
double a = cos(theta), b = sin(theta);
double x0 = a*rho, y0 = b*rho;
pt1.x = cvRound(x0 + 1000*(-b));
pt1.y = cvRound(y0 + 1000*(a));
pt2.x = cvRound(x0 - 1000*(-b));
pt2.y = cvRound(y0 - 1000*(a));
line( input, pt1, pt2, Scalar(0,0,255), 3, CV_AA);
}
imshow("camera", input);
waitKey(30);
}
return 0;
}
i get the following exception:
1- I cant say i really understand that code yet, but can you tell me why it isnt working?.
2- if i manage to make it work, how can i get the Y coordinate of the horizontal lines? i need to know if another object is inside, below or above this one. so i need the position on the Y axis of the 2 horizontal lines on this image (the ones roughlines detected), so i can determine where the other object is regarding this "rectangle".
EDIT #1
I copied the complete code. as you can see in the second image, the debugger doesn't throw any errors. but in the console of the program it says OpenCV Error:Assertion failed (channels() == CV_MAT_CN(dtype)) in cv::Mat::copyTo, file C:\builds\master_packSlave-Win32-vc12-shared\opencv\modules\core\src\copy.cpp, line 281. Also the last call in the call stack is this: > KernelBase.dll!_RaiseException#16() Unknown, im starting to thing is an opencv problem and not a code problem, maybe something with that dll.
EDIT #2
i changed the line
std::vector<Vec4i> lines; // this line causes exception
for
std::vector<Vec2f> lines;
and now it enters the for loop. but it now gives another run time error (another segmentation fault. i think it has to do with these values:
i think they may be going off range, any ideas?

Answer 1

I'm not sure, but it could be the fact that you're trying to draw a line as you had a 3-channel image (using Scalar(b,g,r)), but what you really have is a single-channel image (I suppose that CannyThresh is the output of Canny()).
You can try to change the image to a colored version, using something like this:
Mat colorCannyThresh = CannyThresh.clone();
cvtColor(colorCannyThresh, colorCannyThresh, CV_GRAY2BGR);
or you can draw a line using Scalar([0~255]), changing your line() call to:
line(CannyThresh, pt1, pt2, Scalar(255), 3, CV_AA);
Again, since it's not a complete code, I'm not sure this is the case, but it could be.
About Question2, what you mean by "Y coordinate of the horizontal lines?"
Edit
After inRange(), threshold is a Mat of the same size as HSV and CV_8U type (inRange()), which means it is CV_8U*3 (3-channel -> H.S.V). Are you sure threshold, after MorphOps(), is CV_8U*1 (single-channel), as Canny() expects?
If your goal is to find what is inside a rectangle, you might want to read about minAreaRect().