Why images dimensions goes up when cropping it

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Stelios Fanourakis 2019-1-21

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https://ww2.mathworks.cn/matlabcentral/answers/440748-why-images-dimensions-goes-up-when-cropping-it

评论： Jan 2019-1-28

I enclose two images. Both are the same image.

The original ('A1b.jpg') has the beggining point of the imaging window at [136,120] and end point at [548,120].

The ('A1b(al).jpg') is the same image cropped but the imaging window length is outrageous enlarged with starting point at [180,96] and end point at [1237,96].

Why this difference? I only cropped the image under certain dimensions. Why it took that scaling and stretching?

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Answer 1

Image Analyst 2019-1-21

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https://ww2.mathworks.cn/matlabcentral/answers/440748-why-images-dimensions-goes-up-when-cropping-it#answer_357363

Looks like you're saving the figure window itself instead of the image in the axes. When you save the figure window, you get all that white padding, and the image is screen pixels, which may not be the same as the underlying image pixels because the image may have been zoomed or subsampled to fit your figure window.

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Stelios Fanourakis 2019-1-21

在 MATLAB Online 中打开

Image Analyst.

I am not doing anything more than just cropping the image to the size of the imaging window that we care because of the valuable information. I also make it a bit smaller so the automated algorithm segmentation will be faster.

I use this code

for iter = 1:NumIter
    phi =evolution_LGD(Img,phi,epsilon,Ksigma,KONE,KI,KI2,mu,nu,lambda1,lambda2,timestep,alf);
    if(mod(iter,50) == 0)
        figure(2),
        imagesc(uint8(Img),[0 255]),colormap(gray),axis off;axis equal,title(num2str(iter))
        hold on,[c,h] = contour(phi,[0 0],'r','linewidth',1); hold off
        pause(0.02);
    end
end

The problem is that I cannot use imwrite since if I use imwrite(Img,[]) nothing happens and even if something eventually happens, the outcome image will not be what I want. Since, the active contour I am using for segmentation is not part of the image, how else can I save the image, if not the usual "Save As ...".

I need to give a specific value for imwrite to work. This value needs to be the image AND the contour overlaid. Otherwise, I get error.

When I save it e.g. as jpg (I know I'll try another format, but at this time with jpg format I am most familiar with), the rows and columns are messed.

Original image start from 136 to 548 Columns and after cropping I start from 73 to 508. Why this loss is happening? What is the reason causing this loss? The width of imaging window is the same cropped. Only the height is changing

Stelios Fanourakis 2019-1-22

在 MATLAB Online 中打开

Image Analyst, I use this code for automated segmentation

And the images I attach

clc;clear all;close all;
Img=imread('A6.jpg');
Img = imcrop(Img,[254.25 161.75 445 154.5])
Img = double(Img(:,:,1));
imadjust(Img);
imsharpen(Img);
histeq(Img);
adapthisteq(Img);
NumIter = 1400; %iterations
timestep=0.1; %time step
mu=0.1/timestep;% level set regularization term, please refer to "Chunming Li and et al. Level Set Evolution Without Re-initialization: A New Variational Formulation, CVPR 2005"
sigma = 3;%size of kernel
epsilon = 1;
c0 = 2; % the constant value 
lambda1=1.03;%outer weight, please refer to "Chunming Li and et al,  Minimization of Region-Scalable Fitting Energy for Image Segmentation, IEEE Trans. Image Processing, vol. 17 (10), pp. 1940-1949, 2008"
lambda2=1.0;%inner weight
%if lambda1>lambda2; tend to inflate
%if lambda1<lambda2; tend to deflate
nu = 0.0005*255*255;%length term
alf = 20;%data term weight
figure,imagesc(uint8(Img),[0 255]),colormap(gray),axis off;axis equal
[Height Wide] = size(Img);
[xx yy] = meshgrid(1:Wide,1:Height);
phi = (sqrt(((xx - 64).^2 + (yy - 76).^2 )) - 15);
phi = sign(phi).*c0;
Ksigma=fspecial('gaussian',round(2*sigma)*2 + 1,sigma); %  kernel
ONE=ones(size(Img));
KONE = imfilter(ONE,Ksigma,'replicate');  
KI = imfilter(Img,Ksigma,'replicate');  
KI2 = imfilter(Img.^2,Ksigma,'replicate'); 
figure,imagesc(uint8(Img),[0 255]),colormap(gray),axis off;axis equal,
hold on,[c,h] = contour(phi,[0 0],'r','linewidth',1); hold off
pause(0.5)
tic
for iter = 1:NumIter
    phi =evolution_LGD(Img,phi,epsilon,Ksigma,KONE,KI,KI2,mu,nu,lambda1,lambda2,timestep,alf);
    if(mod(iter,50) == 0)
        figure(2),
        imagesc(uint8(Img),[0 255]),colormap(gray),axis off;axis equal,title(num2str(iter))
        hold on,[c,h] = contour(phi,[0 0],'r','linewidth',1); hold off
        pause(0.02);
    end
end
toc

Check the first row and column of the imaging window at the original image and the same at the cropped one. They are different. Also the length of the image (number of columns) are different among the two images. Under this condition, the total values of pixels in every column will be also different as saved in the xls file. I don't want this. I wish I could keep the cropped image so the active contour should run faster and accurate (if you apply the whole-original image for automated segmentation the contour just get messed with all this information. It doesn't segment right). But having exactly the same columns as the original image where the manual segmentation took place.

Image Analyst 2019-1-28

If you did something to the image, like drew things into the overlay above it, then you'll need to use getframe() to get the image "as you see it".

Jan 2019-1-28

What a pity that evolution_LGD does not contain a useful help text, which explains, what the output is. Using getframe has the drawback, that you work with screen resolution. If the processed image is displayed with a scaling, the data are resampled. Therefore it might be more useful to obtain the output of the contour as a matrix with the original dimensions as the image and use it as overlay to join it with the image.

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