# How can i use threshold to convert a gray-scaled image into binary image ?

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How can I use threshold to convert a gray-scaled image into binary image , I mean to get the image just black and white ?

Does Binarization help ??

Thx

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Simon
on 19 Nov 2013

Hi!

If you load an image into matlab, you get a matrix A (for example) of size (XxYx3) with X and Y being the number of pixels in x- and y-direction. Usually this matrix is for RGB images. Look at imread

If it is a grayscale image the values for all three colors are the same, they range between 0 and 255. You may now apply a threshold

% threshold

t = 128;

% find values below

ind_below = (A < t);

% find values above

ind_above = (A >= t);

% set values below to black

A(ind_below) = 0;

% set values above to white

A(ind_above) = 255;

##### 4 Comments

Image Analyst
on 23 Mar 2023

@Nayana again (see my answer) there is no need to do all that and make A a double matrix of 0 and 255. You can simply do

mask = A >= t;

And of course he messes up the size of A. It's not "of size (XxYx3)". It's of size YxXx3 which is rows x columns x 3.

DGM
on 25 Aug 2023

(i don't remember why i parked my browser on this page, but I'll bite anyway)

One thing to start with:

% find values below

ind_below = (A < t);

% find values above

ind_above = (A >= t);

This whole baloney is unnecessary. Do the comparison once. If you need, negate the logical result, that way you know the union of masks spans the entire input. Comparing it twice just wastes time and invites room for error. That said, there's no need to do so at all.

A logical image as would result from a comparison operator (e.g. >=) is a properly-scaled image itself. It can be rescaled for viewing or saving without problem. Creating a floating-point image in uint8-scale, or otherwise blindly presuming that the input is uint8 is to create an improperly-scaled image which won't display or save correctly without intervention. Learn how images are scaled according to their numeric class.

There's no reason to create two masks. There's no reason to overwrite the input. There's no reason to need two operations to do so. Stop creating problems for yourself. As IA and Jan have posted, this whole thing reduces to a single line of code, and that single line of code is more robust than this answer.

Image Analyst
on 19 Nov 2013

Yes you binarize the image by thresholding:

binaryImage = grayImage > thresholdValue;

There is no need to ever multiply by 255 that I've ever encountered. Displaying the binary (logical) image will show it as black and white even without multiplying by, or directly setting to, a value of 255.

##### 2 Comments

Jan
on 19 Nov 2013

Right, in general (and especially for binarized images) the scaling is obsolete...

Ali nafaa
on 29 Nov 2022

Edited: Image Analyst
on 29 Nov 2022

x = imread('cameraman.tif');

figure,imshow(x);

[r,c] = size (x);

output=zeros(r,c);

for i = 1 : r

for j = 1 : c

if x(i,j) > 128

output(i,j)=1;

else

output(i,j)=0;

end

end

end

figure,imshow(output);

##### 4 Comments

Image Analyst
on 29 Nov 2022

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