Edge Detection using Sobel, Perwitt and Roberts

[S,P,R]=deal(double(rgb2gray(imread('peppers.png'))));
[r c] = size(S);
for i=1:r-2
    for j=1:c-2
        %Sobel mask
        Sx=((2*S(i+2,j+1)+S(i+2,j)+S(i+2,j+2))-(2*S(i,j+1)+S(i,j)+S(i,j+2)));
        Sy=((2*S(i+1,j+2)+S(i,j+2)+S(i+2,j+2))-(2*S(i+1,j)+S(i,j)+S(i+2,j)));
        S(i,j) = sqrt(Sx.^2+Sy.^2);

        %Perwitt mask
        Px=((P(i+2,j+1)+P(i+2,j)+P(i+2,j+2))-(P(i,j+1)+C(i,j)+P(i,j+2)));
        Py=((P(i+1,j+2)+P(i,j+2)+P(i+2,j+2))-(P(i+1,j)+P(i,j)+P(i+2,j)));
        P(i,j)=sqrt(Px.^2+Py.^2);

        %Roberts mask
        Rx = R(i+2, j+2)-R(i+1,j+1);
        Ry = R(i+1, j+2)-R(i+2, j+1);
        R(i,j)=sqrt(Rx.^2+Ry.^2);
    end
end
figure(1),imshow(uint8(S)); title('Sobel gradient');
figure(2),imshow(uint8(P)); title('Prewitt gradient');
figure(3),imshow(uint8(R)); title('Roberts gradient');