MATLAB: How eccentricity work

eccentricityImage Processing Toolboxregionprops

hi everyone

for regionprps command we have a property that call eccentricity. I searched and studied matlab help about this. but I don't understand how this code work? for example if we have these ellipse :

Is ImgLabel(1).Eccentricity and ImgLabel(2).Eccentricity equal for these objects? why? these objects are like each other but you can imagine that one of them rotated. I mean that is it important that our object be Horizontal or vertical ? thanks

Best Answer

First of all, MATLAB calls white objects as foreground, not black objects. So you'd have to invert your images or call imclearborder() to remove the white surround.

After that it fits each blob to an ellipse, and whatever the eccentricity of the fitted ellipse is, that's what it gives you. It does not depend on the "orientation" (angle) of the ellipse.

Related Solutions

MATLAB: Find the orientation of each arbitrary object in an image

I used this question to practice some of my image processing skills (still in early stages of exploration).

Here's what I did step-by-step to compute the orientation of each object and to compute the linear equation of the major axis of each object.

I downloaded the image and named it 'bwimage.png' (also attached, same image). The result is shown in a figure below where the major axis of each object is drawn through its center point (and extended a bit).

% Is the image binary?
info = imfinfo('bwimage.png'); %including the pull path is better

info.BitDepth  %ans = 24; so it's not a binary image. 
% Read image as RGB
I = imread('bwimage.png'); %including the pull path is better
% Convert RGB --> grayscale --> binary
BW = imbinarize(rgb2gray(I));
% Get rid of white border
BWConv = bwconvhull(~BW);
BW = BW & BWConv;
% Checkout the image to make sure the conversion made sense.
% Keep the figure because we'll add to it later
% Note the orientation of the y axis (reversed)
figure()
h = imshow(BW); 
axis on
% Get the center coordinate for each object, it's orientation [-90:90 deg], 
% and the lenght of its major axis (in pixels).  
stats = regionprops('table',BW,'Centroid','MajorAxisLength','Orientation'
% How many objects were detected?
nObjects = size(stats,1);  % = 74
% take a look at the results
head(stats) %first few objects
% Continuing with the figure created above, add the center points
hold on
ph1 = plot(stats.Centroid(:,1), stats.Centroid(:,2), 'rs'); 
% using orientation, compute slope and y-intercept of each major axis
% Note the reversal of the sign of the orientation!  This is to account 
% for the reversed y axis!            
%           here---v
stats.Slope = atan(-stats.Orientation*pi/180); 
% Compute y interceps
stats.Intercep = stats.Centroid(:,2) - stats.Slope.*stats.Centroid(:,1); 
% Now that we've got the linear eq for each line, compute the bounds of each
% object along its major axis line. Add some extra length so we can see more of the lines. 
% To use the exact major axis lenght, divide by 2 instead of 1.6 (both lines below)
stats.EndpointX = stats.Centroid(:,1) + [-1,1].* (stats.MajorAxisLength/1.6 .* sqrt(1./(1+stats.Slope.^2))); 
stats.EndpointY = stats.Centroid(:,2) + [-1,1].* (stats.Slope .* stats.MajorAxisLength/1.6 .* sqrt(1./(1+stats.Slope.^2))); 
% Plot major axis of each object that spans the length of the obj
mah = plot(stats.EndpointX.', stats.EndpointY.', 'r-')

The orientation of each object is stored in stats.Orientation. The slope and intercept of each major axis is stored in stats.Slope and stats.Intercept. The end points of each line segment draw is stored in stats.EndpointX/Y. And the center points of each object are stored in stats.Centroid.

MATLAB: How to find boundaries of scattered pixels in an image

One way to get the "boundary" is to call bwconvhull():

chImage = bwconvhull(binaryImage);

To find the centroid of the blob(s), you can use regionprops():

props = regionprops(chImage, 'Centroid');
% Plot centroids.
hold on;
for k = 1 : length(props)
    plot(props(k).Centroid(1), props(k).Centroid(2), 'r+', 'MarkerSize', 25);
end

To find the centroid of the points (rather than the solid convex hull blob regions), you can use find

[y, x] = find(binaryImage); % Not [x,y] as in the other answer.
xCentroid = mean(x);  % Column.
yCentroid = mean(y);  % Row.

Best Answer

Related Solutions

MATLAB: Find the orientation of each arbitrary object in an image

MATLAB: How to find boundaries of scattered pixels in an image

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