Computes the major axis of a region in an image.

image—A 2D array containing a region.

pixels—A long array containing the element numbers of the pixels in image that compose the region.

result—A double scalar value that is the angle defined by the horizontal axis and the major axis through the region.

Eigval—Specifies a variable to receive the eigenvalues of the region.

Eigvect—Specifies a variable to receive the eigenvectors of the region.

The major axis of a region describes the direction of maximal dispersion. Specifically, the major axis of a region is determined from the eigenvectors of the covariance matrix of the pixel location array, where the pixel location array is composed of the row and column values of the region pixel locations. The eigenvector corresponding to the maximum eigenvalue of the covariance matrix is the major axis angle.

; Segment the image using thresholding.

image = IMAGE_READ(!IP_Data + 'blobs.tif')

one_blob = THRESHOLD(image('pixels'), 20, 25, /Binary)

; Find the major axis of the remaining blob.

blob_pixels = WHERE(one_blob NE 0)

major = MAJOR_AXIS(one_blob, blob_pixels)

PRINT, 'Major axis angle = ', major

CENTROID,   PERIMETER