Why rgb2gray(d​ouble(imre​ad('pictur​e.jpg')) gives a matrix of value 1?

Hi, I am a beginner in Matlab, I try to read my picture that I cropped with paint and convert it to double, and I obtain:

135x194x3 double with

sans=double(imread('aft2.jpg'));

I want to use it with the function normxcorr2, and I have the error message:

Error using normxcorr2
Expected input number 1, T, to be two-dimensional.

Then, I try to convert it with rgb2gray to eliminate the "x3" :

sans=rgb2gray(double(imread('aft2.jpg')));

but it gives me 135x194 double of 1. (my matrix is only composed of the value 1). I obtain this error message:

The values of TEMPLATE cannot all be the same.

So I tried to change my line :

sans=double(rgb2gray(imread('aft2.jpg')));

and it seems that it is working but I dont understand why and what is the difference if I invert rgb2gray and double.

Thank you for your help Aude

Answer 1

[Image Analyst]

You're trying to do too much at once. Simply read it in - it will most likely be a uint8 - then call rgb2gray:

sans = imread('aft2.jpg');
sans2 = rgb2gray(sans); % Convert uint8 color to uint8 gray scale.
sans3 = double(sans2); % Convert to double in the range 0-255.

If you want it to be in the range 0-1 then define sans3 this way:

% Scale min to 0 and max to 1
sans3 = mat2gray(sans2);

or

% Scale 0 to 0 and max to 1
sans3 = double(sans2) / max(sans2(:));

[Aude Rapet]

Ok I see, thank you for your reply!