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

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]

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]

And here is a second hint:

 >> I = imread('board.tif');
 >> 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 ..

Answer 1

[Image Analyst]

Try this

inverseGrayImage = uint8(255) - grayImage;

[Mark Quesada]

You animal, that was spot on; Worked right out of the gate!

Answer 2

[Roche de Guzman]

I = imcomplement(I)

[Victor Mtsimbe Norrild]

You animal, that was spot on; Worked right out of the gate!