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What is the correct way to find and implement a bounding box mask?

asked 2018-09-04 08:29:38 -0600

bfialkoff gravatar image

Hi I'm trying to find the arbitrarily oriented bounding box of a binary image. This seems like a pretty trivial problem but I've spent 2 days trying to find the correct way to use minAreaRect() and to use it to create a simple binary mask.

Can anyone please help, in java please?

Thank you to all.

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answered 2018-09-04 08:46:18 -0600

berak gravatar image

updated 2018-09-04 09:46:33 -0600

java is somewhat clumsy here, but it's just a few lines:

Mat mask = Mat.zeros(400,400,CvType.CV_8U); // all black

// your RotatedRect will come from minAreaRect(), this is just demo data !
RotatedRect rc = new RotatedRect(Point(200,200), new Size(200,100), 23);

// retrieve corner points:
Point[] p = Point[4];

// draw white, filled poly:
MatOfPoint mp = new MatOfPoint(p);
Imgproc.fillConvexPoly(mask, mp, new Scalar(255));

image description


  • usually you need a contour as input for minAreaRect(), see tutorial and some conversion code (sorry, untested):

    List<MatOfPoint> contours = new ArrayList<>();
    Imgproc.findContours(cannyOutput, contours, ...);
    Point [] pts = contours.get(i).toArray();
    RotatedRect rc = Imgproc.minAreaRect(new MatOfPoint2f(pts));
  • sorry, i was just trying out latest 4.0, where all drawing stuff was moved to Imgproc for consistency, but yes, it's likely in Core with your version

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Thanks! It works like a charm but what is the correct way to get a RotatedRect from minAreaRect? Currently this is what I'm trying to do.

MatOfPoint2f Points = new MatOfPoint2f();
RotatedRect minRect=Imgproc.minAreaRect(Points);

and its not working. paper_mat is a binary image that looks basically like the image you posted just not quite so neat.

Also for brevity fillConvexPoly is in Core not Imgproc

Thanks again

bfialkoff gravatar imagebfialkoff ( 2018-09-04 09:03:14 -0600 )edit

When you write

Point [] pts = contours.get(i).toArray();

Do you mean contours.size () in place of i In the tutorial i is iterating through contours.size () but here we're turning the whole thing into an array. It would make sense...

bfialkoff gravatar imagebfialkoff ( 2018-09-04 10:08:14 -0600 )edit

yea, sorry. findContours() returns a list of contours, and get(i) retrieves one of those.

you probably want get(0) (the first, and ?only?)

berak gravatar imageberak ( 2018-09-04 10:21:14 -0600 )edit

I'll need to double check but if I do get(0) it only draws a point.

So what set pts=contours.toArray () ?

bfialkoff gravatar imagebfialkoff ( 2018-09-04 10:25:35 -0600 )edit

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Asked: 2018-09-04 08:29:38 -0600

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Last updated: Sep 04 '18