CODEWITHSUNDEEP
c++opencvimage-processing
Jonathan
Jonathan

Reputation: 691

Finding the Average Color within a Polygon Bound in OpenCV

Background

I am trying to create a utility that will return the average pixel color within a given polygon using OpenCV. The polygon will be defined via 4 points, but it is not necessarily a rectangle/square. For example, the following structures are to be expected:

     A__________B    A_______B
    /          /     \       \
   /          /       \       \
 D/__________/C       D\_______\C

Given a cv::Mat image in OpenCV and a polygon defined by the points (A, B, C, D). I know points A, B, C, and D, but I want to calc the average pixel color within the polygon. I wanted to get some suggestions from the OpenCV community on how to do this most efficiently.

Research Done

Another post on StackOverflow suggested drawing contours using the drawContours function and then take the mean of the bounding rectangle surrounding the contour. I would obviously have to modify the mean calculation so that it uses the polygons drawn by the fillPoly function instead.

Suggestions/Concerns are much appreciated!

Upvotes: 8

Views: 13052

Answers (1)

Miki
Miki

Reputation: 41765

You can simply use the mean function with a mask, where the mask is your filled polygon.

#include <opencv2\opencv.hpp>
#include <iostream>
using namespace cv;

int main()
{
    // Create a black image with a gray rectangle on top left
    Mat1b img(300, 300, uchar(0));
    rectangle(img, Rect(0, 0, 100, 100), Scalar(100), CV_FILLED);

    // Define a polygon
    Point pts[1][4];
    pts[0][0] = Point(20, 20);
    pts[0][1] = Point(40, 100);
    pts[0][2] = Point(200, 60);
    pts[0][3] = Point(150, 30);

    const Point* points[1] = {pts[0]};
    int npoints = 4;

    // Create the mask with the polygon
    Mat1b mask(img.rows, img.cols, uchar(0));
    fillPoly(mask, points, &npoints, 1, Scalar(255));

    // Compute the mean with the computed mask
    Scalar average = mean(img, mask);

    std::cout << average << std::endl;

    return 0;
}

Upvotes: 14

Related Questions