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?

Answer 1

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.

Answer 2

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.

Answer 3

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.