How to Detect Color of a detected blob?

C++
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1

Hi im new with opencv, currently i need to detect a beans from a live feed camera, im able to detect it with SimpleBlobDetector function. Now i want to know the average color inside of the blob to do some calculation. What is the best way to do it?

            Mat im_with_keypoints;
                   drawKeypoints( capture, keypoints, im_with_keypoints, Scalar(0,0,255), DrawMatchesFlags::DRAW_RICH_KEYPOINTS );
                   size_t i, k;
                   Point Coordinate;
                    for( i = k = 0; i < keypoints.size(); i++ )
                    {
                        Coordinate =    keypoints[i].pt ;
                    qDebug ()<< "x " << Coordinate.x << "y " <<Coordinate.y;
                    qDebug ()<< "s " << keypoints[i].size ;

                    }

This is my Code to detect the coordinate of each blob and the diameter of it

3

since you already have the location and the size, you can sample the mean from a rectangular crop, centered around the keypoint position:

 Point p = keypoints[i].pt;
 int w = keypoints[i].size;
 Rect r(p.x-w/2, p.y-w/2, w, w);
 Scalar m,d; // mean and stddev
 meanStdDev( img(r), m, d);
1 Like
4

Thanks for the reply this help me alot! I tried to change a little bit of the code

              Mat im_with_keypoints;

                   drawKeypoints( capture,  keypoints, im_with_keypoints, Scalar(0,0,255), DrawMatchesFlags::DRAW_RICH_KEYPOINTS );
                   size_t i, k;
                   Point Coordinate;

                    for( i = k = 0; i < keypoints.size(); i++ )
                    {
                        Coordinate =    keypoints[i].pt ;
                    Rect r(Coordinate.x-(keypoints[i].size /2), Coordinate.y-(keypoints[i].size/2), keypoints[i].size, keypoints[i].size) ;
                    Mat roi(im_with_keypoints, r);
                    cv::Scalar mean;
                    mean = cv::mean(roi);
                    qDebug() << "Blue " << mean[0] ;
                    qDebug() << "Green " << mean[1] ;
                    qDebug() << "Red " << mean[2] ;
                    }

This code above also the same right? also i have another question, if i use cvtColor to change the color from BGR to HSV is it better to detect the color or it will be just the same with BGR color?

5

yep, looks like :wink:

please explain the reason to do so.

no, results will not be the same, it’s another colorspace

6

please explain the reason to do so.
I read some journal that said its better to do image processing in HSV color space, is this also work in my case?

7

sure, but you still have to explain “my case”, please. which is ? what are you trying to do ?

is it still: " detect a beans" ? if so, you might profit from doing it in HSV space, since most likely you only have to look at hue for classification (not 3 r,g,b vaues)

8

Yess its still about detecting the beans. Since the result of the color is not exactly the same with the real color but i know there is a factor from my camera and lighting as well, but maybe i can optimize it if i change the color space, maybe?

9

using hsv will make it less sensitive against variation in lighting, so try if you can exploit it ;9

10

Okay one last question if you dont mind :grinning:
cvtColor(capture, im_with_keypoints, COLOR_BGR2HSV);

i just to need to do this right to change it?
i dont need to change other line?

11

you probably do not want to use im_with_keypoints for this

12

so i make a new output then use it for my input in drawKeypoints?

13

yep, exactly … :wink:

(actually, drawing the keypoints is irrelevant for the detection, it’s just visualization for humans)

1 Like
14

ohh okayyy got it, thank you so much for your help !