Index exceeds the number of array elements in image processing.

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Puru Raj 2022-6-24

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回答： DGM 2022-6-24

The given code below shows an error that index exceeds the number of array elements. And for different video sample it runs for different no. of frames. How to fix this?

		% Calculate the diameter.
		grayImage = rgb2gray(thisFrame);%to convert image from rgb to gray
Unrecognized function or variable 'thisFrame'.
        EdgeFrame = edge(grayImage,"sobel");%to detect edges
        Graycrop = imcrop(EdgeFrame,[60 100 150 10]); %To crop the image
		t = zeros(2,10);
        for i=1:10
            r = find(EdgeFrame(:,i));
            L = length(r);
            t(1,i)=r(1);
            t(2,i)=r(L);
        end
        upper_edge = max(t(1,:));
        lower_edge = min(t(2,:));
        Diameter(frame) = abs(upper_edge - lower_edge);

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Answer 1

Glenn 2022-6-24

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To me it seems like the image 'thisFrame', is not always the same size, can that be correct?

It seems like you could either (1) use the cropped version inside your for-loop, or (2) deal with the variable image size in the initiation of your for-loop.

Possible solutions:

(1)

r = find(Graycrop(:,i));

(2)

dimSize = size(EdgeFrame, 2);
Graycrop = imcrop(EdgeFrame,[60 100 150 dimSize]); %To crop the image
t = zeros(2, dimSize);
for i=1:dimSize
    r = find(EdgeFrame(:,i));
    % moreover, I think you can add some efficiency here:
    t(:,i) = r([1, end]);
end

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Answer 2

DGM 2022-6-24

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acircle.png

Obviously, not all columns of the edgemap will contain nonzero elements, so r will often be zero-length. Also, Graycrop isn't used for anything, so I don't know if that's intended.

Either way, this isn't exactly an efficient or robust way to find the diameter of an object. Consider the example:

A = imread('acircle.png');
A = rgb2gray(A);
mk = A>128; % binarize
% using regionprops
S = regionprops(mk,'equivdiameter');
d = S.EquivDiameter
d = 190.8529
% this is the simplified version of your method
h = nnz(any(mk,1)) % object height
h = 190
% or likewise
w = nnz(any(mk,2)) % object width
w = 190