Why .^2 does not work as expected?

Hi,

I'm doing some math calculations in a code. I noticed something strange in the code below:

a = imread('C:\tqPixels.png'); I = rgb2gray(a);

exp = I.^2

As tqPixels.png is an image with only 4 white pixels (all have 255 value), I thought that exp variable value would be each value powered to itself (255 * 255):

exp = [65025, 65025; 65025, 65025]

However, when I saw exp value in the results, it had the same values variable "I" had:

exp = [255, 255; 255, 255]

Sorry if this is very simple, but why .^2 does not work as it should?

I'm attaching the image and the code here for you to test. I really appreciate any suggestions on this.

a = imread('C:\tqPixels.png');
I = rgb2gray(a);

exp1 = I.^2;

Answer 1

[Roger Stafford]

I think you need to convert I to ‘double’ before doing the exponentiation on it. As it is, I believe it is ‘uint8’ type and anything above 255 is set down to just 255. Do this:

   exp1 = double(I).^2

[Image Analyst]

Note how Roger doesn't use exp for the variable name like you did. exp is a built-in function and you should not use it as a variable name.

[Steven Lord]

This saturation behavior is described in the "Largest and Smallest Values for Integer Classes" section on this documentation page.

[Queila Martins]

That's true, Image Analyst (about variable's name). Thank you for pointing that :D

Also thank you Steven for the link. Very useful!!

[Queila Martins]

Roger, it worked! Now it makes sense. Thank you so much!