Chad Chad - 1 year ago 310
Python Question

Measure edge strength in OpenCV, magnitude of gradient

I have an application where I need to check the focus of a camera. For this, I want to measure edge strength (magnitude of gradient) in several predefined locations on a single axis (1D). The image target will be a simple printout of black objects on a while background.

I am using OpenCV with Python. I know there are several edge detection algorithms within OpenCV like Canny, Sobel, laplace but all of these are to filter the image. I want to actually measure the strength of an edge. Are there any algorithms within OpenCV that can provide this? Or do I just write my own algorithm to measure edge strength?

Answer Source

You can compute the magnitude like:

  1. Compute dx and dy derivatives (using cv::Sobel)
  2. Compute the magnitude sqrt(dx^2 + dy^2) (using cv::magnitude)

This is a simple C++ code that compute the magnitude of the gradient. You can easily port to Python, since it's just a few calls to OpenCV functions:

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

int main()
    //Load image
    Mat3b img = imread("path_to_image");

    //Convert to grayscale
    Mat1b gray;
    cvtColor(img, gray, COLOR_BGR2GRAY);

    //Compute dx and dy derivatives
    Mat1f dx, dy;
    Sobel(gray, dx, CV_32F, 1, 0);
    Sobel(gray, dy, CV_32F, 0, 1);

    //Compute gradient
    Mat1f magn;
    magnitude(dx, dy, magn);

    //Show gradient
    imshow("Magnitude", magn);

    return 0;
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