MATLAB: Fill a border of a matrix with 0’s

Image Processing Toolboxmatrix border

Lets say I have a matrix A. I want to replace all the values at the border meaning, the first row, the last row, the first column, and the last column with 0s, How do I do this

Best Answer

Try this:

grayImage(1,:) = 0;
grayImage(end,:) = 0;
grayImage(:,1) = 0;
grayImage(:,end) = 0;

Related, if you want to add a layer of 0's all the way around, you can use padarray() if you have the Image Processing Toolbox:

paddedImage = padarray(grayImage,[1 1]);

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MATLAB: How to change pixel value of boundary pixel

It won't be binary if you set a value of 2. Binary images are only true/false, or 1/0 if viewing as a number. However if you use an int32, uint8, or double image you can set the edges to 2. Assuming your definition of boundary is the top and bottom lines and the left and right columns, you can do this:

grayImage(:,1) = 2;
grayImage(:,end) = 2;
grayImage(1,:) = 2;
grayImage(end,:) = 2;

If, instead, you want to set the white pixels of the image to 2, assuming you can do this

binaryImage = grayImage > 0; % All white pixels.
grayImage(binaryImage) = 2; % Change from 1 or 255 or whatever, into 2.

MATLAB: Creating border of an image

Mayank - is you have a colour image, then you could do something like

myImage = uint8(randi(255,256,256,3));
border = 2;
myImage(1:border,:,:) = 0;
myImage(end-border+1:end,:,:) = 0;
myImage(:,1:border,:) = 0;
myImage(:,end-border+1:end,:) = 0;
image(myImage)

The above can probably be simplified though...

With your code, the problem seems to be with

I(end:end+border,end:end+border)=0;

In the above, you are trying to add 2 to the last row and last column. Are you trying to augment your 256x256 matrix to be 258x258? Or do you just want to set the last two rows and last two columns to all zeros? If the latter, you need to do something like

I(end-border+1:end,:) = 0;

which will set all columns in the last two rows to be all zeros. If we just do

I(end-border+1:end,end-border+1:end) = 0;

then we are only setting the last two columns of the last two rows to be all zeros. We need to use the : to indicate all. Your code could then become

I(end-border+1:end,:) = 0; 
I(:, end-border+1:end) = 0; 
I(1:border,:) = 0; 
I(:, 1:border) = 0;

Best Answer

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MATLAB: How to change pixel value of boundary pixel

MATLAB: Creating border of an image

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