How to find cells corresponding to zero lag in an autocorrelation function

Hi,

I am looking to identify the cells corresponding to zero lag for an autocorrelation function. To do that I wrote the following as input:

I = imread('001.tif');
acf = autocorr2d(I);        
[M,N]=size(acf);                
lag_x = [1:N]'-ceil((N+1)/2);   
lag_y = [1:M]'-ceil((M+1)/2);   
[r_0,c_0] = find_r0c0(acf, lag_x,lag_y);

The response I keep getting is: undefined function or variable find_r0c0. How can this be done?

[KALYAN ACHARJYA]

May be ?

I = imread('2.png');
acf = autocorr2d(I);        
[M,N]=size(acf);                
lag_x = [1:N]'-ceil((N+1)/2);   
lag_y = [1:M]'-ceil((M+1)/2);   
[r_0,c_0] = find(acf,lag_x,lag_y);
%..................still ^^ seems issue here

Answer 1

[Ajay Pattassery]

The zero lag produces the highest value in the autocorrelation output. Hence simply finding the maximum value of the autocorrelation matrix and finding its coordinates gives the pixel location.

In the below example xcorr2 functions compute the 2-D autocorrelation.

I = imread('cameraman.tif');       
acf = xcorr2(I);
maximum = max(max(acf));
[r_0,c_0]=find(acf==maximum);

Also, the following code will work since the maximum value of the autocorrelation output will be at the middle position.

I = imread('cameraman.tif');       
[m,n] = size(I);
r_0 = ceil((m+n-1)/2);
c_0 = ceil((m+n-1)/2);
 

Refer the following link for further information on xcorr2.

https://www.mathworks.com/help/signal/ref/xcorr2.html

[Olalekan Ajayi]

Thanks @ Ajay

[Ajay Pattassery]

My Pleasure!