OpenCV - Gamma Correction

Gamma correction (also known as power transformation) is used in image processing in several situations. The classic example where gamma correction is needed is in CRT monitors. In CRTs the amount or intensity of light from the phosphor in the screen is related to voltage as

Output = Voltage ^crt_gamma

Hence if we give a linear or normal image to the CRT, the image would be look much too dark. In order to mitigate this effect, we “gamma correct” the image such a way that the image output looks well. All we do is something opposite to what the CRT does.

gamma_correc_image = image^{(1/crt_gamma)}

For most CRTs, the crt_gamma is somewhere between 1.0 and 3.0.

Now lets delve into the code. The code given below is just to explicit to understand (I believe).

#include<stdio.h>
#include<highgui.h>
#include<cv.h>
#include<math.h>
#include<stdlib.h>

int main(int argc, char **argv)

{	const char* imagename = argc > 1 ? argv[1] : "lena.jpg";
	IplImage* img = 0;
	int h, w, step, channels, i, j, k;
	uchar *data;
        //read the input image
	img = cvLoadImage(imagename, CV_LOAD_IMAGE_UNCHANGED);
	if(!img)
		printf("Could not load image file: %s\n", imagename);
	h = img->height;
	w = img->width;
	step = img->widthStep;
	channels = img->nChannels;
	data = (uchar *)img->imageData;
        //assign the gamma inverse value, here 2
	int gamma = 2;

        //new image with gamma correction
	IplImage* trans = cvCreateImage(cvGetSize(img), 8, channels);
	int trans_step;
	trans_step = trans->widthStep;
        //gamma correction
	cvPow(img, trans, gamma);
	cvConvertScaleAbs(trans, trans, 1, 0);
	cvNamedWindow("OpenCV", CV_WINDOW_AUTOSIZE);
	cvShowImage("OpenCV", trans);
	cvWaitKey(0);
	cvReleaseImage(&img);
	cvReleaseImage(&trans);

	return 0;
}

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