Loading image as gray and conversion color image into gray produces different results (Bug #2332)

Loading color image from file as gray (by imread function) and conversion color image into gray (by cvtColor function) produces different results.

In example bellow the number of pixels with different values is 362 for 512x512 standard "lena.jpg" image.

• OS: Mac OS X 10.7 Lion
• OpenCV: 2.4.2 from MacPorts repository
• G++: i686-apple-darwin11-llvm-g++-4.2 (GCC) 4.2.1

Note that same behavior were obtained for older versions of OpenCV too.

Example code:

#include <opencv2/opencv.hpp>

using namespace cv;

int main() {
    Mat img_color = imread("lena.jpg");
    Mat img_gray1;
    cvtColor(img_color, img_gray1, CV_BGR2GRAY);
    imwrite("gray1.png", img_gray1);

    Mat img_gray2 = imread("lena.jpg", 0);
    imwrite("gray2.png", img_gray2);

    return 0;
}

MD5 sums of result:

MD5 (gray1.png) = 0aa370384ee44502d381f4af07b3bd7d
MD5 (gray2.png) = b28471468d1c6b7550c7e96ee9c22670