How to read image from a buffer of YUV422

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I have some image data, its format is yuyv(YUV422), and only have its buffer.
I use Mat::Mat(Size size, int type, void* data, size_t step=AUTO_STEP)
and write cv::Mat img = cv::Mat(cv::Size(640,480), CV_8UC2, imgBuffer);
the step is AUTO_STEP, is it correct?

Then I try to change a zone to black ( 0,44,64 en YUV )

for (int i=0; i<50; i++)
{
    for (int j=0; j<50; j++)
    {
        img.at<cv::Vec2b>(j,i)[0]=0;
        if(j%2==0)
            img.at<cv::Vec2b>(j,i)[1]=44;
        else
            img.at<cv::Vec2b>(j,i)[1]=62;
    }
}

But the output color is green, I don't know why? Please, help me