Collecting pixel values in a line drawn using mouse - c++

Question

I have an image (24 bit bmp) such as the following:
The user draws a line (shown here in red) using mouse. This line can be any where with any angle. Then he clicks right or left mouse button, and the image pixel values across the lines are stored in a file in addition to displaying on the console.
I have used setMouseCallback() for showing the position of the mouse (shown below ). But I need a little more help in understanding an elegant way for finding and storing the pixel values across the line. Kindly help!
void CallBackFunc(int event, int x, int y, int flags, void* userdata)
{
if ( event == EVENT_LBUTTONDOWN )
{
cout << "Left button of the mouse is clicked - position (" << x << ", " << y << ")" << endl;
}
else if ( event == EVENT_RBUTTONDOWN )
{
cout << "Right button of the mouse is clicked - position (" << x << ", " << y << ")" << endl;
}
else if ( event == EVENT_MOUSEMOVE )
{
cout << "Mouse move over the window - position (" << x << ", " << y << ")" << endl;
}
}
int main(int argc, char** argv)
{
Mat img = imread("C:\\Users\\Acme\\Desktop\\image-processing\\2.bmp");
namedWindow(" Window", 1);
setMouseCallback(" Window", CallBackFunc, NULL);
imshow(" Window", img);
waitKey(0);
return 0;
}

Answer 1

Extract the line by warping it to a 1 x (linelength) or (linelength) x 1, vertical or horizontal Mat. Then you can easily read down or across pixel values.

Answer 2

The specific details depend upon your program, but values is populated once two points are clicked. What you do after that is up to you.
cv::Point g_points[2];
int g_pointIndex;
std::vector<cv::Vec3b> values;
bool g_allGood = false;
void onMouse(int e, int x, int y, int d, void* ud)
{
if (e != CV_EVENT_LBUTTONDOWN || g_pointIndex >= 2)
return;
g_points[g_pointIndex++] = cv::Point(x, y);
}
void main()
{
// load image as greyscale
Mat img = imread("C:\\temp\\2.png", CV_8UC1);
namedWindow("img", 1);
setMouseCallback("img", onMouse);
while (1)
{
// all points collected
if (g_pointIndex == 2 && !g_allGood)
{
/*
to save processing, i suggest you remove the mouse callback once all points
are collected. do this with: setMouseCallback("img", NULL,NULL);
*/
// create line iterator, and add pixel values to values vector
cv::LineIterator it(img, g_points[0], g_points[1]);
for (int i = 0; i < it.count; i++, ++it)
values.push_back((Vec3b)*it);
// you now have all pixel values in values;
g_allGood = true;
}
imshow("img", img);
waitKey(100);
}
}