How to resize a contour?

thdrksdfthmn
2170 ●5 ●18 ●46

6772 ●3 ●48 ●79 https://github.com/stu...

I have an application that detects an object by finding its contour. Because the image where I search for the object may be at a very big scale is affecting my detection because the contour can be too long and I skip the contours longer than a threshold. For fixing this, I have thought of resizing the image larger than a maximum size to that maw size. Doing so, I am detecting the object in the smaller image, and when drawing the contour on the initial image, I am getting a "wrong detection". Is there a possibility to resize the contour?

Comments

Hmm afaik contours are actually for visualisation purposes and for creating masks. Resizing them will not be that easy. How about the following approach

Use the contour to make a binary mask.
Apply a resize operation on that binary mask, which is possible since it is a data matrix.
Now use findContours again on that binary image to change it back to an active contour.

Would this suit your needs?

StevenPuttemans (Oct 17 '14)edit

If I just do the (x, y) -> (resizer * x, resizer * y) is not correct? Actually I am trying to test this. (x and y are both positive)

thdrksdfthmn (Oct 17 '14)edit

That was my first tought. But I am not sure if this will actually work for each point. You should test and report back :)

StevenPuttemans (Oct 17 '14)edit

Great! Yeah my approach suffers from artefacts ... hadn't thought on that...

StevenPuttemans (Oct 17 '14)edit

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answered Oct 17 '14

thdrksdfthmn
2170 ●5 ●18 ●46

updated Oct 5 '15

sturkmen

6772 ●3 ●48 ●79 https://github.com/stu...

Nice, it is working, and it is working very nice. More, your idea of resizing the mask is introducing errors, because the cv::fillPoly is introducing errors (small "stairs") and resizing it is just making the errors to appear in the new contour, and they are even bigger.

Sample code :

#include "opencv2/imgproc.hpp"
#include "opencv2/highgui.hpp"

using namespace cv;

int main( int argc, char** argv )
{
    vector<vector<Point> > contours;
    Mat img = Mat::zeros( 500, 500, CV_8UC1 );

    circle( img, Point(250,250), 100, Scalar(255) );

    findContours( img, contours, RETR_LIST, CHAIN_APPROX_SIMPLE);

    fillConvexPoly( img, Mat( contours[0] ) / 2, Scalar(255)); // draws contour resized 1x/2

    polylines( img, Mat( contours[0] ) * 2, true, Scalar(255)); // draws contour resized 2x

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

    return 0;
}

result image:

image description

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answered Apr 13 '18

Mostafa Sataki

2529 ●12 ●18 ●48 http://www.7khatcode.com/

updated Aug 31 '18

sturkmen

6772 ●3 ●48 ●79 https://github.com/stu...

You can use the following method to resize the contour by keeping the contour center like the morphology operation.

    void contourOffset(const std::vector<cv::Point>& src, std::vector<cv::Point>& dst, const cv::Point& offset) {
    dst.clear();
    dst.resize(src.size());
    for (int j = 0; j < src.size(); j++)
        dst[j] = src[j] + offset;

}
void scaleContour(const std::vector<cv::Point>& src, std::vector<cv::Point>& dst, float scale)
{
    cv::Rect rct = cv::boundingRect(src);

    std::vector<cv::Point> dc_contour;
    cv::Point rct_offset(-rct.tl().x, -rct.tl().y);
    contourOffset(src, dc_contour, rct_offset);

    std::vector<cv::Point> dc_contour_scale(dc_contour.size());

    for (int i = 0; i < dc_contour.size(); i++)
        dc_contour_scale[i] = dc_contour[i] * scale;

    cv::Rect rct_scale = cv::boundingRect(dc_contour_scale);

    cv::Point offset((rct.width - rct_scale.width) / 2, (rct.height - rct_scale.height) / 2);
    offset -= rct_offset;
    dst.clear();
    dst.resize(dc_contour_scale.size());
    for (int i = 0; i < dc_contour_scale.size(); i++)
        dst[i] = dc_contour_scale[i] + offset;
    }

void scaleContours(const std::vector<std::vector<cv::Point>>& src, std::vector<std::vector<cv::Point>>& dst, float scale)
{
    dst.clear();
    dst.resize(src.size());
    for (int i = 0; i < src.size(); i++)
        scaleContour(src[i], dst[i], scale);
}
        void main(){
            std::vector<std::vector<cv::Point>> src,dst;
            scaleContours(src,dst,0.95);
         }

In the sample below, the green contour is main contour and the red contour is scaled contour with a coefficient of 0.95. image description

Comments

nice to have you back here ;)

berak (Apr 13 '18)edit

Is there an equivalent python code for this? If not would be very grateful if you could please explain the process on the code above.

fzuffer (Jun 9 '18)edit

i don't think, we have a python example,but the algorithm is:

find the center of the contour (moments() or getBoundingRect())
subtract it from each point in the contour
multiply contour points x,y by a scale factor
add the center again to each point

berak (Jun 9 '18)edit

Thank You very much for the quick response.. Appreciate it a lot.

fzuffer (Jun 9 '18)edit

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How to resize a contour?

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