Setting transparency in OpenCV not working - c++

Question

I am setting transparency for a four channeled Mat like this (Based on some calculations). But when I am showing the image the on window there is no change happening for the Image. Any help would be a great support.
void feather_touch(Rect enclosingRect, Mat frame){
Point center(frame.size().width * 0.5, frame.size().height * 0.5);
int inclussive_circle_radius = (sqrt((frame.cols * frame.cols + frame.rows * frame.rows))) / 2;
for(int i = 0; i < frame.rows; i++){
for(int j = 0; j < frame.cols; j++){
Point point(i, j);
if(!inRect(point, enclosingRect)){
Vec4b channels = frame.at<Vec4b>(i, j);
int dx = center.x - point.x;
int dy = center.y - point.y;
int dist = sqrt((dx * dx) + (dy * dy));
float alpha = (float)dist/(float)inclussive_circle_radius;
int a = (int)((1 - alpha) * 255);
frame.at<Vec4b>(i, j)[3] = a;
}
}
}
}
bool inRect(cv::Point p,Rect rect) {
return p.x >= rect.x && p.x <= (rect.x + rect.width) && p.y >= rect.y && p.y <= (rect.y + rect.height);
}

Answer 1

I got the answer though: imshow in OpenCV doesn't support transparency.I replaced it by using addWeighted functionality. Now my function look like this:
float alpha = ((float)dist/(float)inclussive_circle_radius);
//int a = (int)((1 - alpha) * 255);
//frame.at<Vec4b>(i, j)[3] = a;
Rect rect(j, i, 1, 1);
Mat mat = frame(rect);
Mat sub = layer(rect);
if(dist > (enclosingRect.width*0.5)){
addWeighted(mat, alpha, sub, 1 - alpha, 0, mat);
mat.copyTo(frame(rect));
}else{
sub.copyTo(frame(rect));
}