Binary image is a digital image that has only two possible value for each pixel – either 1 or 0, where 0 represents white and 1 represents black. In the complement of a binary image, the image pixel having value zeros become ones and the image pixel having value ones become zeros; i.e white and black color of the image is reversed.

Complementing Binary image using MATLAB library function :

Complementing Binary image without using library function :

We can complement a binary image by subtracting each pixel value from maximum possible pixel value a binary image pixel can have (i.e 1 ), and the difference is used as the pixel value in the complemented image. It means if an image pixel have value 1 then, in complemented binary image same pixel will have value ( 1 – 1 ) = 0 and if Binary image pixel have value 0 then, in complemented binary image same pixel will have value ( 1 – 0 ) = 1.

Below is the Implementation of above idea-

 
Alternate way :

In MATLAB, Arrays are basic data structure.They can be manipulated very easily. For example Array = 1 - Array ;
Above code will subtract each element of the array from 1. So, Instead of using two loops to subtract 1 to each pixel of binary image. We can directly write it as –
comp=1-img
Here ‘img’ is our binary image array.

Below code will also complement a binary Image

Input:

Output:

References : https://in.mathworks.com/help/images/ref/imcomplement.html