display rectangle around components in original image

Question

I am using OpenCV for a C++ application.I'm using connected component for object detection.I want to draw a rectangle around object in original frame.I can draw rectangle in comonent window.can I draw a color rectangle in gray scale image ?in below I write part of my code.thanks for your help.

Mat frame;
Mat stat, centroid;
int threshval = 100;
static void on_trackbar(int, void*){
 Mat bw = threshval < 128 ? (frame < threshval) : (frame > threshval);
 Mat labelImage(frame.size(), CV_32S);
 int nLabels = connectedComponentsWithStats(bw, labelImage, stat, centroid, 8);
 std::vector<Vec3b> colors(nLabels);
      colors[0] = Vec3b(0, 0, 0);//background
   for (int label = 1; label < nLabels; ++label) {
 colors[label] = Vec3b((rand() & 255), (rand() & 255), (rand() & 255));}
 at dst(frame.size(), CV_8UC3);
 for (int r = 0; r < dst.rows; ++r) {
     for (int c = 0; c < dst.cols; ++c) {
         int label = labelImage.at<int>(r, c);
         Vec3b &pixel = dst.at<Vec3b>(r, c);

         pixel = colors[label];} 
for (int i = 0;i < nLabels;i++)
    {

        vector<Rect> rComp;
        rComp.push_back(Rect(Point((stat.at<int>(i, CC_STAT_LEFT) ), (stat.at<int>(i, CC_STAT_TOP) )), Size((stat.at<int>(i, CC_STAT_WIDTH) ), (stat.at<int>(i, CC_STAT_HEIGHT)))));
    //  

            rectangle(dst, Rect(Point(stat.at<int>(i, CC_STAT_LEFT  ) , stat.at<int>(i, CC_STAT_TOP  ) ), Size(stat.at<int>(i, CC_STAT_WIDTH   ) , stat.at<int>(i, CC_STAT_HEIGHT  ))), Scalar(0, 255, 255));}
}

    for (int i = 0;i < nLabels;i++) {
        int x = stat.at<int>(i, CC_STAT_LEFT);
        int y = stat.at<int>(i, CC_STAT_TOP);
        int w = stat.at<int>(i, CC_STAT_WIDTH) ;
        int h = stat.at<int>(i, CC_STAT_HEIGHT);
        rectangle(frame, Rect(x,y,w,h), Scalar(0, 255, 255));
    }
}
imshow("Connected Components", dst);

You can't draw the rectangle because you are essentially not drawing a rectangle on frame2 in the above code. — Rick M., Jul 25 '17 at 12:03
May be because frame2 is gray scale? Have you validated the values in stat before drawing on frame2? — Rick M., Jul 25 '17 at 12:39
`for (int i = 0;i < nLabels;i++) { int x = stat.at(i, CC_STAT_LEFT) ; int y = stat.at(i, CC_STAT_TOP) ; int w = stat.at(i, CC_STAT_WIDTH); int h = stat.at(i, CC_STAT_HEIGHT) ; rectangle(frame2, Rect(x,y,w,h), Scalar(0, 255, 255));}` ؟ — louis89, Jul 25 '17 at 12:42
Can you update your code in the question how and where you try to draw the rectangle on frame2/frame? — Rick M., Jul 25 '17 at 13:12
8UC1? 32FC1? or what? It should actually be color right because `dst` is color and you can see the rectangles in that? Try saving the frame and then checking if the rectangle is being drawn or not. — Rick M., Jul 25 '17 at 14:22
have a look at [this answer](https://stackoverflow.com/questions/35780054/drawing-a-colored-rectangle-in-a-grayscale-image-using-opencv) — Rick M., Jul 25 '17 at 14:59
thanks a lot.I solve it by change Scale(0,255,255) to Scale(255,255,255) — louis89, Jul 25 '17 at 15:09
Glad it helped you. I see you were struggling with this for a while. That is exactly what I said, you can't draw color rectangles on gray scale images. Should I add this as an answer and you'll accept it? — Rick M., Jul 25 '17 at 15:10

score 1 · Accepted Answer · answered Jul 25 '17 at 15:29

1

As mentioned here and here, you can't draw colored rectangles on gray scale images. You could either use Scalar(255, 255, 255) - white / Scalar(0, 0, 0) or follow the hack in the first link.

answered Jul 25 '17 at 15:29

Rick M.

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display rectangle around components in original image

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