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How to resize a contour?

asked 2014-10-16 10:28:16 -0600

thdrksdfthmn gravatar image

updated 2020-11-28 06:42:49 -0600

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?

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

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

Would this suit your needs?

StevenPuttemans gravatar imageStevenPuttemans ( 2014-10-17 04:33:51 -0600 )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 gravatar imagethdrksdfthmn ( 2014-10-17 05:02:20 -0600 )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 gravatar imageStevenPuttemans ( 2014-10-17 05:48:25 -0600 )edit

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

StevenPuttemans gravatar imageStevenPuttemans ( 2014-10-17 08:36:22 -0600 )edit

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answered 2014-10-17 08:21:25 -0600

thdrksdfthmn gravatar image

updated 2015-10-05 14:06:23 -0600

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


    return 0;

result image:

image description

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answered 2018-04-13 10:06:26 -0600

updated 2018-08-30 19:24:08 -0600

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) {
    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(,;
    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;
    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)
    for (int i = 0; i < src.size(); i++)
        scaleContour(src[i], dst[i], scale);
        void main(){
            std::vector<std::vector<cv::Point>> src,dst;

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

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nice to have you back here ;)

berak gravatar imageberak ( 2018-04-13 10:11:17 -0600 )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 gravatar imagefzuffer ( 2018-06-09 03:07:26 -0600 )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 gravatar imageberak ( 2018-06-09 03:14:31 -0600 )edit

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

fzuffer gravatar imagefzuffer ( 2018-06-09 04:03:15 -0600 )edit

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Asked: 2014-10-16 10:28:16 -0600

Seen: 20,988 times

Last updated: Dec 05 '18