OpenCV's RotatedRect angle does not provide enough information

Question

From my experiments, the angle returned by RotatedRect's angle variable goes from -90 to 0 degrees, which is not sufficient to determine if the object is leaned to the left or right.

For example, if the angle is -45 degrees, we cannot say if we need to rotate +45 or -45 degrees to deskew it.

An excerpt of the code I'm using:

RotatedRect rotated_rect = minAreaRect(contour);
float blob_angle_deg = rotated_rect.angle;
Mat mapMatrix = getRotationMatrix2D(center, blob_angle_deg, 1.0);

Leaning the object in one direction I get angles from 0 to -90 degrees, while leaning the object to the other direction I get angles from -90 to 0 degrees.

How can I find the angle by which I should rotate my image to deskew it?

Possible duplicate of [MinAreaRect angles - Unsure about the angle returned](https://stackoverflow.com/questions/15956124/minarearect-angles-unsure-about-the-angle-returned) — jdhao, Feb 22 '19 at 09:49

score 17 · Accepted Answer · edited May 22 '18 at 21:10

17

After learning from Sebastian Schmitz and Michael Burdinov answers this is how I solved it:

RotatedRect rotated_rect = minAreaRect(contour);
float blob_angle_deg = rotated_rect.angle;
if (rotated_rect.size.width < rotated_rect.size.height) {
  blob_angle_deg = 90 + blob_angle_deg;
}
Mat mapMatrix = getRotationMatrix2D(center, blob_angle_deg, 1.0);

So, in fact, RotatedRect's angle does not provide enough information for knowing an object's angle, you must also use RotatedRect's size.width and size.height.

edited May 22 '18 at 21:10

Daniel

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answered Jun 06 '14 at 15:34

João M. S. Silva

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In his answer he adds 180, you add 90. Why is there this difference? – Schütze Feb 13 '19 at 12:19
@Schütze, could you elaborate? It's been four and a half years. – João M. S. Silva Feb 15 '19 at 16:08
You gave a link in which the solution adds 180 degrees, then you wrote your own solution which adds 90. Something is inconsistent, either remove the link or use the same angle to add. – Schütze Feb 18 '19 at 10:01
I think they are different things. As I said, I learned from the two answers, then solved my problem the way describe it. – João M. S. Silva Feb 19 '19 at 22:08

score 4 · Answer 2 · answered Jun 06 '14 at 06:34

4

I explained how you can convert the angle of the rectangle into [0-180] in this thread.

The Angle is always calculated along the longer side.

answered Jun 06 '14 at 06:34

Sebastian Schmitz

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score 3 · Answer 3 · answered Jun 06 '14 at 05:57

Switching values of width and height of rectangle is the same as rotating it by 90 degrees. So if the range of angles was 180 degrees instead of 90 than same rectangle would have 2 representations (width, height, angle) and (height, width, angle+90). Having range of 90 degrees you can represent every rectangle and you can do that in only one way.

score 0 · Answer 4 · answered Oct 28 '20 at 09:34

This is what I use (c is my contour). Basically, I get the longest line's du and dv, and then use atan2()

rect = cv2.minAreaRect(c)
box = cv2.boxPoints(rect)
origin = box[0]
rect_width, rect_height = rect[1]
if rect_width > rect_height:
     target = box[3]
else:
     target = box[1]
dv = target[1] - origin[1]
du = target[0] - origin[0]
angle_rads = math.atan2(dv, du)

OpenCV's RotatedRect angle does not provide enough information

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