Errore Array dimensions must match for binary array op.

% --- Executes on button press in grain.
function grain_Callback(hObject, eventdata, handles)
% hObject    handle to grain (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

X = handles.image;

B1 = handles.B1;
B2 = handles.B2;
B3 = handles.B3;

sigma = handles.sigma;
seed = handles.seed;

U(:,:,1)  = grain(X, B1, sigma, seed);
U(:,:,2)  = grain(X, B2, sigma, seed);
U(:,:,3)  = grain(X, B3, sigma, seed);

Y = X+U;

axes(handles.box_immagine);

imshow(Y)

guidata(hObject, handles);

function [ U ] = grain( X, B , sigma, seed)
%GRAIN Summary of this function goes here
%   Detailed explanation goes here

sigma = sigma/255;


randn('state', seed);

X(:,:,1) = randn(size(X(:,:,1))) * sigma;
X(:,:,2) = randn(size(X(:,:,2))) * sigma;
X(:,:,3) = randn(size(X(:,:,3))) * sigma;

U = conv2(X(:,:,1),B);
U = conv2(X(:,:,2),B);
U = conv2(X(:,:,3),B);


end