MATLAB: Major axis and minor axis

Image Processing Toolboxregionprops

How can I plot the major axis and minor axis for an object in an image?

Best Answer

regionprops() to get the centroid and orientation (angle). After that it becomes straight trig. The axes length is R, compute R*cos(theta), R*sin(theta), and center those displacements on the centroid.

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MATLAB: Is MinorAxisLength always perpendicular to the MajorAxisLength

The lengths itself are not perpendicular to anything, because they are scalars. The replied lengths are:

 'MajorAxisLength' Length (in pixels) of the major axis of the ellipse that has the same
 normalized second central moments as the region
 (equivalent for "Minor")

The major and minor axes of an ellipse (not their lengths) are perpendicular by definition.

MATLAB: Regionprops returning “wrong” axis lengths

That's because all the ellipse properties are not designed for hollow shapes. Matlab is trying to fit an ellipse just to the 'on' pixels of your shape (so it's trying to fit an ellipse to the walls of the pipe only) and failing. You can tell because the 'Circularity' is only about 0.2. It should be near one for something close to a circle.

The fix is to fill your shape before calling regionprops:

props = regionprops(imfill(mask, 'holes'), 'all');

With that the rest of your code works fine. 'Circularity' is now 0.96.

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Related Solutions

MATLAB: Is MinorAxisLength always perpendicular to the MajorAxisLength

MATLAB: Regionprops returning “wrong” axis lengths

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