Vito Vito - 1 year ago 119
C++ Question

Gamma Correction with pow

I would use gamma correction to a image.
So, I have to pow every pixel intensity of my source image with a G = 0.6.
I have problem cause the destination image is completely wrong.
Maybe I have a casting problem when I take pixel from source image.
Here my code:

#include <opencv2/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <iostream>
#include <opencv2/imgproc/imgproc.hpp>

using namespace cv;
using namespace std;

int main() {

Mat src = imread("spine1.jpeg");

Mat dst = Mat(src.rows, src.cols, CV_8UC1);
cvtColor(src, src, CV_8UC1);
dst = Scalar(0);

for (int x = 0; x < src.rows; x++) {
for (int y = 0; y < src.cols; y++) {
int pixelValue = (int)<uchar>(x, y);<uchar>(x, y) = pow(pixelValue, 0.6);


namedWindow("Input", CV_WINDOW_AUTOSIZE);
namedWindow("Output", CV_WINDOW_AUTOSIZE);

imshow("Input", src);
imshow("Output", dst);

return 0;

Edit: change
cvtColor(src, src, CV_8UC1);
cvtColor(src, src, COLOR_BGR2GRAY);

Answer Source

The call to cvtColor is wrong. You should use:

cvtColor(src, src, COLOR_BGR2GRAY);

Also, you can make your code much simpler, and less error prone:

#include <opencv2/opencv.hpp>

int main()
    // Load the image as grayscale
    cv::Mat1b src = cv::imread("path_to_img", cv::IMREAD_GRAYSCALE);

    // Convert to double for "pow"
    cv::Mat1d dsrc;
    src.convertTo(dsrc, CV_64F);

    // Compute the "pow"
    cv::Mat1d ddst;
    cv::pow(dsrc, 0.6, ddst);

    // Convert back to uchar
    cv::Mat1b dst;
    ddst.convertTo(dst, CV_8U);

    // Show results
    imshow("SRC", src);
    imshow("DST", dst);

    return 0;
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