change the range of angle for regionprops

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kittu 2012-11-13

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Any idea, how can we change the range of angle for the orientation which we get from regionprops?

Currently, the regionprops returns the angles in [-90,90], but i need in [-180,180]. How can it be achieved?

Thanks

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Matt J 2012-11-13

How would you distinguish between an orientation of -180 and 180?

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Walter Roberson 2012-11-13

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https://ww2.mathworks.cn/matlabcentral/answers/53624-change-the-range-of-angle-for-regionprops#answer_65261

在 MATLAB Online 中打开

'Orientation' — Scalar; the angle (in degrees ranging from -90 to 90 degrees) between the x-axis and the major axis of the ellipse that has the same second-moments as the region. This property is supported only for 2-D input label matrices.

But take note that the major axis of an ellipse is not oriented, so it is not possible to tell the difference between (say) "up and right-wards" and "down and left-wards" as would be required to distinguish a full 360 degree range. An ellipse rotated by 180 degrees around its center is exactly the same as the unrotated ellipse.

If you have information such as an arrowhead that gives direction, then you will need to work that out some other way.

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Matt J 2012-11-14

编辑：Matt J 2012-11-14

Suppose you have the major axis of the of the bounding ellipse and suppose, just as an example, that it is horizontal. How do you decide whether the axis is pointing to the left or to the right? What is the criterion you would use to say which of 2 possible orientations a given axis has?

Walter Roberson 2012-11-14

With the information you have provided, I do not understand why you do not just atan2() the displacement vector? Whether a blob is going up or down is independent of which way the blob is "facing".

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