simple illumination correction in images openCV c++

Question

I have some color photos and the illumination is not regular in the photos: one side of the image is brighter than the other side.

I would like to solve this problem by correcting the illumination. I think local contrast will help me but I don't know how :(

Would you please help me with a piece of code or a pipeline ?

Answer 1

Convert the RGB image to Lab color-space (e.g., any color-space with a luminance channel will work fine), then apply adaptive histogram equalization to the L channel. Finally convert the resulting Lab back to RGB.

What you want is OpenCV's CLAHE (Contrast Limited Adaptive Histogram Equalization) algorithm. However, as far as I know it is not documented. There is an example in python. You can read about CLAHE in Graphics Gems IV, pp474-485

Here is an example of CLAHE in action:

And here is the C++ that produced the above image, based on http://answers.opencv.org/question/12024/use-of-clahe/, but extended for color.

#include <opencv2/core.hpp>
#include <vector>       // std::vector
int main(int argc, char** argv)
{
    // READ RGB color image and convert it to Lab
    cv::Mat bgr_image = cv::imread("image.png");
    cv::Mat lab_image;
    cv::cvtColor(bgr_image, lab_image, CV_BGR2Lab);

    // Extract the L channel
    std::vector<cv::Mat> lab_planes(3);
    cv::split(lab_image, lab_planes);  // now we have the L image in lab_planes[0]

    // apply the CLAHE algorithm to the L channel
    cv::Ptr<cv::CLAHE> clahe = cv::createCLAHE();
    clahe->setClipLimit(4);
    cv::Mat dst;
    clahe->apply(lab_planes[0], dst);

    // Merge the the color planes back into an Lab image
    dst.copyTo(lab_planes[0]);
    cv::merge(lab_planes, lab_image);

   // convert back to RGB
   cv::Mat image_clahe;
   cv::cvtColor(lab_image, image_clahe, CV_Lab2BGR);

   // display the results  (you might also want to see lab_planes[0] before and after).
   cv::imshow("image original", bgr_image);
   cv::imshow("image CLAHE", image_clahe);
   cv::waitKey();
}