Image Approximation (imshow) trouble -- I believe I'm doing Image Compression??

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Jessica 2024-2-23

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2085993-image-approximation-imshow-trouble-i-believe-i-m-doing-image-compression

评论： Jessica 2024-2-26

continued scroll.pdf

I have been asked to do the following (which I've done successfully):

Here is the beginning of what I entered, and I'm attaching a PDF for the rest.

I get confused when it gets to the second set of directions where I am to show an "image approximation". After all I've researched, again never have coded before, I cannot seem to enter anything correct because all I get is a white image. I used this help page to try to get me on the correct path: https://www.mathworks.com/help/matlab/math/image-compression-with-low-rank-svd.html but have had no luck. Here are the 2nd set of directions specifically that I'm struggling with. The math part of all the coding, I'm golden! It's the image part, and displaying that image where I'm getting the 1st image to display properly, but not the compressed/approximated images to come up as anything other than white.

Thank you!

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DGM 2024-2-24

As to why the image is white, it's probably because your floating-point images are improperly scaled for their class because of the manner in which you handled them or cast them.

https://www.mathworks.com/matlabcentral/answers/171253-how-can-convert-a-image-from-type-of-double-unit16-to-a-grayscale-image

https://www.mathworks.com/help/images/convert-image-data-between-classes.html

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Answer 1

Karl 2024-2-23

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2085993-image-approximation-imshow-trouble-i-believe-i-m-doing-image-compression#answer_1415428

编辑：Karl 2024-2-24

在 MATLAB Online 中打开

The code below builds on code given by Torsten in relation to a related question: https://mathworks.com/matlabcentral/answers/2084643-how-to-do-a-rank-1-approximation.

% Read test image, which has data type uint8.

im1 = imread('moon.tif');

imshow(im1)

title(['original image']);

% Set rank of approximation.

k = 10;

% Convert to and from type double.

% The svd() function used in rank_k_approximation()

% requires data of type single or double.

im2 = uint8(rank_k_approximation(double(im1), k));

imshow(im2)

title(['rank-',num2str(k),' approximation']);

function Ak = rank_k_approximation(A, k)

%RANK_K_APPROXIMATION Calculate rank-k approximation of matrix A.

% The matrix A must be of type single or double

% (required by the function svd).

[U, S, V] = svd(A);

Ak = zeros(size(A));

for i=[1:k]

Ak = Ak + S(i,i)*U(:,i)*V(:,i).';

end

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Karl 2024-2-25

编辑：Karl 2024-2-25

If you post the syntax errors that you're seeing, and the code that produced them, someone might be able to suggest changes for getting things to work with a MATLAB release from 2006.

In case it could be useful, MathWorks offers a free version of MATLAB Online:

https://mathworks.com/products/matlab-online/matlab-online-versions.html

Jessica 2024-2-26