OpenCV and MATLAB gray scale values differ for the same image

Question

In MATLAB, i read a color video , extract a certain frame and convert it to a gray scale image using the rgb2gray function.But when I load the same video with OpenC

Answer 1

I get this problem too , in MATLAB documents i can found rgb2gray implementation and it was so easy as follow

gray_value = 0.2989 * R + 0.5870 * G + 0.1140 * B

So i implement this algorithm in OpenCV as follow

cv::Mat rgb_image = imread("/what/ever/directory/that/was/optional.jpg" );

int nrows = rgb_image.rows;  // number of columns
int ncols = rgb_image.cols;  // number of rows

cv::Mat gray_image( nrows , ncols , CV_8UC1 ); // define one channel Mat with same size as rgb_image

for(int row = 0; row < rgb_image.rows ; row++)
{
    for(int col = 0 ; col < pic.cols ; col++)
    {

        //matlab algorithm for rgb2gray
        gray.at( row , col ) =
                0.2989 * rgb_image.at( row , col )[0]+
                0.5870 * rgb_image.at( row , col )[1]+
                0.1140 * rgb_image.at( row , col )[2];        
   }

}

and this code will give same result as matlab, and in OpenCV you can use below code to regenerate it:

cv::cvtColor( rgb_image , gray_image , CV_BGR2GRAY );  //BLUE+GREEN+RED

but if you use below code

cv::cvtColor( rgb_image , gray_image , CV_RGB2GRAY ); //RED+GREEN+BLUE

then this algorithm will be in reverse order as follows:

gray_value = 0.2989 * R->B + 0.5870 * G->G + 0.1140 * B->R

and the output not same as MATLAB output

---

using cv::transform function

I found a handy and useful function in opencv named cv::transform that implement above things in easiest way . if we have three Mat matrix named src for source image and gray for destination Matrix and m is a matrix that affect a transportation to every channel. by this matrixs we can implement CV_BGR2GRAY and CV_RGB2GRAY as follows

1-CV_BGR2GRAY

Mat src, gray, m ;
src=imread(" ");
m=(Mat_(1,3)<<0.1140,0.5870,0.2989);
cv::transform(src,       //src
              gray,      //dst
              m );       //mtx

output will like as follow image

2-CV_RGB2GRAY

Mat src, gray, m ;
src=imread(" ");
m=(Mat_(1,3)<<0.2989,0.5870,0.1140);
cv::transform(src,       //src
              gray,      //dst
              m );       //mtx

and output is like as follow