opencv 图像透明

cv::Mat transparentImage;
cv::cvtColor(image, transparentImage, CV_BGR2BGRA);

// find all white pixel and set alpha value to zero:
for (int y = 0; y < transparentImage.rows; ++y)
{
    for (int x = 0; x < transparentImage.cols; ++x)
    {
        cv::Vec4b & pixel = transparentImage.at<cv::Vec4b>(y, x);
        // if pixel is white
        if (pixel[0] == 255 && pixel[1] == 255 && pixel[2] == 255)
        {
            // set alpha to zero:
            pixel[3] = 0;
        }
    }
}