Gerchberg–Saxton algorithm/ Fast Fourier transform/ Phase Retreiveal algorithm

Hi, this is my Gerchberg–Saxton algorithm codes that perform fft and ifft with the purpose of retrieve the phase of the images. I need an error function in my codes to exit the for loop but I am not sure how to do it. Can anyone help me on this? I really appreciate your help, thank you.

These are the codes:

clear all; close all; clc

%Read and display source image

imgA = imread('void.jpg');

imgA = rgb2gray(imgA);

figure(1); imshow(imgA); title('Image from Source');

%Convert source image to Grayscale

imgB = imread('void2.jpg');

imgB = rgb2gray(imgB);

figure(2); imshow(imgB); title('Grayscale Image');

%Make both images the same size

%Get the size of image A

[rowsimgA colsimgA numberOfColorChannelsimgA] = size(imgA);

%Get the size of image B

[rowsimgB colsimgB numberOfColorChannelsimgB] = size(imgB);

%Check if lateral sizes match

if rowsimgB ~= rowsimgA || colsimgA ~= colsimgB

imgB = imresize(imgB, [rowsimgA colsimgA]);

end

imgA = im2double(imgA);

imgB = im2double(imgB);

error = [];

iteration = 200;

image_void = fftshift(imgB);

image_void = ifft2(image_void);

image_void = fftshift(image_void);

%Decompose into amplitude and phase (image is converted into double)

for i=1:iteration

image_fft_recon = abs(imgA) .* exp(1i*angle(image_void));

image_recon = fftshift(image_fft_recon);

image_recon = fft2(image_recon);

image_recon = fftshift(image_recon);

image_fft_recon2 = abs(imgB) .* exp(1i*angle(image_recon));

image_void = fftshift(image_fft_recon2);

image_void = ifft2(image_void);

image_void = fftshift(image_void);

end

figure(3);

imagesc(abs(image_void)),colorbar;

title('exponential img');

figure(4);

imagesc(abs(image_fft_recon)), colorbar;

title('constructed img');