How can I reverse black and white in a grayscale image?

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Clara on 2 Jul 2014
I have a grayscale image and I'm trying to reverse the black and white in it as an exercise. I think I'm supposed to use for loops in some way to access the colormap so the entire image matrix is composed of 1's and 0's (at which point I could switch the two by subtracting 1 from all values) but I don't know how to get this matrix in the first place. Thank you!
Cedric Wannaz on 2 Jul 2014
Here is a hint
>> A = randi(5 , 3, 4)
A =
5 5 2 5
5 4 3 1
1 1 5 5
>> 5-A
ans =
0 0 3 0
0 1 2 4
4 4 0 0
Here you see that 5-A operates on the whole array A, without the necessity to implement a loop.
Cedric Wannaz on 2 Jul 2014
Edited: Cedric Wannaz on 2 Jul 2014
And here is a second hint:
>> J = rgb2gray(I);
>> size(J)
ans =
648 306
>> min(J(:))
ans =
0
>> max(J(:))
ans =
255
so pixels' "grayscale" level seem to be coded with (unsigned) integers in the range 0 to 255.
Note that you can visualize J with
>> imshow( J ) ;
Now maybe there is an operation that you could perform on J which would reverse the scale ..

Image Analyst on 2 Jul 2014
Try this
inverseGrayImage = uint8(255) - grayImage;
Mark Quesada on 26 Mar 2019
You animal, that was spot on; Worked right out of the gate!

Roche de Guzman on 14 Jan 2021
I = imcomplement(I)
Victor Mtsimbe Norrild on 17 Mar 2021
You animal, that was spot on; Worked right out of the gate!