Question

I want to stretch an elliptical object in an image until it forms a circle. My program currently inputs an image with an elliptical object (eg. coin at an angle), thresholds and binarizes it, isolates the region of interest using edge-detect/bwboundaries(), and performs regionprops() to calculate major/minor axis lengths.

Essentially, I want to use the ‘MajorAxisLength’ as the diameter and stretch the object on the minor axis to form a circle. Any suggestions on how I should approach this would be greatly appreciated. I have appended some code for your perusal (unfortunately I don’t have enough reputation to upload an image, the binarized image looks like a white ellipse on a black background).

EDIT: I’d also like to apply this technique to the gray-scale version of the image, to examine what the stretch looks like.

code snippet:

rgbImage = imread(fullFileName);
redChannel = rgbImage(:, :, 1);
binaryImage = redChannel < 90;
labeledImage = bwlabel(binaryImage);
area_measurements = regionprops(labeledImage,'Area');
allAreas = [area_measurements.Area];
biggestBlobIndex = find(allAreas == max(allAreas));
keeperBlobsImage = ismember(labeledImage, biggestBlobIndex);
measurements = regionprops(keeperBlobsImage,'Area','MajorAxisLength','MinorAxisLength')

Answer 1

I finally managed to figure out the transform required thanks to a lot of help on the matlab forums. I thought I’d post it here, in case anyone else needed it.

    stats = regionprops(keeperBlobsImage, 'MajorAxisLength','MinorAxisLength','Centroid','Orientation');
    alpha = pi/180 * stats(1).Orientation;
    Q = [cos(alpha), -sin(alpha); sin(alpha), cos(alpha)];
    x0 = stats(1).Centroid.';
    a = stats(1).MajorAxisLength;
    b = stats(1).MinorAxisLength;
    S = diag([1, a/b]);
    C = Q*S*Q';
    d = (eye(2) - C)*x0;
    tform = maketform('affine', [C d; 0 0 1]');
    Im2 = imtransform(redChannel, tform);
    subplot(2, 3, 5); 
    imshow(Im2);