Detecting objects with a certain surface area

asked 2015-08-26 04:55:27 -0600

Vizier87
31 ●1 ●4

updated 2015-08-26 04:57:08 -0600

Hi guys,

I'd like to detect small but undefined shapes with a certain value of surface area, say 25 pixels worth. Now, I tried coding by calculating an array of pixels with adjacency but it seems pretty tedious and demanding for the debugging.

Let's say I want to detect an image with at least 3 x 1 green pixels (where I have performed threshold the image to a binary black & green form):

  for (int i = 0; i < Image.rows; i++){

        if (Image.at<cv::Vec3b>(i , 200)[1] == 255){                

           if (Image.at<cv::Vec3b>(i+1 , 200)[1] == 255){

                if (Image.at<cv::Vec3b>(i+2 , 200)[1] == 255){              
                   putText(Image, "Detected", Point(20, 20), FONT_HERSHEY_PLAIN, 1, Scalar::all(255), 2, 8);
                }

           }

        }
    }

Now, this seemed to work but occasionally it'll crash due to the memory.

I suppose there are workarounds to this? How do I do this?

Thanks bros.

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answered 2015-08-26 12:01:01 -0600

Guyygarty
687 ●6 ●17

You can use "findContours" to find all blobs (binary large objects). BinaryImage needs to be an 8-bit single-channel image. It is helpful if you blur the image a bit before binarizing so that you don't get many 1 pixel objects.

vector<vector<Point>> contours;
vector<Vec4i> hierarchy;

findContours(BinaryImage, contours, hierarchy, CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE); //find all blobs
for (int i = 0; i < contours.size(); i++)
{
    vector<Point> contour_poly;
    approxPolyDP( Mat(contours[i]), contour_poly, 3, true );
    double Area=contourArea(contour_poly); //find area of each blob
    if (Area>25)
       //do whatever
}

guy

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Thanks Guy. I'll try this out and if will get back here to update.

Vizier87 ( 2015-08-26 22:51:54 -0600 )edit

Hi bro, it seems that there's a problem. I only added these lines:

vector<vector<Point>> contours;
vector<Vec4i> hierarchy;
findContours(Black_Green, contours, hierarchy, CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE);

......and it returns:

cv::Exception at memory location 0x00000000002DEAC0

Where did I go wrong?

Vizier87 ( 2015-08-27 03:02:35 -0600 )edit

Make sure that Black_Green is a Mat of type CV_8C1? if it is CV_8C3, the program will crash.

also, just to be sure that there are no ambiguities, you can change vector to std::vector, Point to cv::Point and Vec4i to cv::Vec4i guy

Guyygarty ( 2015-08-27 10:57:36 -0600 )edit

Yeah Guy, you're right it was CV_8C3. You really know your stuff man! :)

However, when I changed it to CV_8C1 the Black_Green didn't appear. I tried reading up some docs on these types but I don't really understand. Where can I get good references on this?

Vizier87 ( 2015-09-01 23:30:15 -0600 )edit

You really know your stuff man I use findContours a lot and have had more than my fair share of exceptions...

what do you mean by "didn't appear"? can you show how you are generating that Mat? guy

Guyygarty ( 2015-09-02 13:58:29 -0600 )edit

Here's how I did it:

Mat Black_Green(current_frame.size(), CV_8UC1, Scalar(0, 255, 0));

I took the current_frame from running webcam.

It worked for CV_8UC3 though.

Vizier87 ( 2015-09-02 23:39:25 -0600 )edit

That would generate a black image - since the Mat is one channel, only the first element of the Scalar is used.

Mat Black_Green(current_frame.size(), CV_8UC1, Scalar(255)); would initialize a white image.

In any case I don't see how you get a binary image with objects this way.

guy

Guyygarty ( 2015-09-04 14:47:56 -0600 )edit

Perhaps I needed to post the whole thing:

capture >> current_frame;
        if (current_frame.empty()) break;
        cvtColor(current_frame, Gray_frame, CV_RGB2GRAY);
        threshold(Gray_frame, Black_White, threshold_value, max_value, threshold_type);
        Mat Black_Green(current_frame.size(), CV_8UC3, Scalar(0, 255, 0));
        Black_Green.setTo(Scalar::all(0), Black_White);

With this in a loop I managed a binary green and black. Well it's a noob code so I'd really appreciate it if you can point out any bad coding on my part.

Whaddaya think? Thanks bro.

Vizier87 ( 2015-09-06 21:27:51 -0600 )edit

In this code, you would need to apply findContours to Black_White, not to Black_Green, which is a three channel image.
To generate the green image, I would use:

Mat Black_Green(current_frame.size(), CV_8UC3, Scalar(0, 0, 0));
Black_Green.setTo(Scalar(0,255,0), Black_White);

Guyygarty ( 2015-09-09 13:41:13 -0600 )edit

Aaahh I see now you've hit the nail. I missed the part where it needed to be a Black and White only. It works now. Thanks.

Vizier87 ( 2015-09-09 22:49:07 -0600 )edit

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