Question

This is my source image (ignore the points, they were added manually later):

My goal is to get a rough polygon approximation of the two hands. Something like this:

I have a general idea on how to do this; I want to use cvCanny to find edges, cvFindContours to find contours, and then cvApproxPoly.

The problem I’m facing is that I have no idea on how to properly use cvCanny, particularly, what should I use for the last 3 parameters (threshold1&2, apertureSize)? I tried doing:

cvCanny(source, cannyProcessedImage, 20, 40, 3);

but the result is not ideal. The left hand looks relatively fine but for the right hand it detected very little:

In general it’s not as reliable as I’d like. Is there a way to guess the “best” parameters for Canny, or at least a detailed explanation (understandable by a beginner) of what they do so I can make educated guesses? Or perhaps there’s a better way to do this altogether?

Answer 1

Maybe one of the easiest solution is make Otsu thresholding on grayscale image, find contours on the binary image and than approximate them. Here’s code:

Mat img = imread("test.png"), gray;
vector<Vec4i> hierarchy;
vector<vector<Point2i> > contours;

cvtColor(img, gray, CV_BGR2GRAY);
threshold(gray, gray, 0, 255, THRESH_OTSU);
findContours(gray, contours, hierarchy, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);

for(size_t i=0; i<contours.size(); i++)
{
    approxPolyDP(contours[i], contours[i], 5, false);
    drawContours(img, contours, i, Scalar(0,0,255));
}

imshow("result", img);
waitKey();

And this is result: