find gradient using sobel mask operator in matlab

Question

I am using following code to mask image using sobel mask .

for i=1:size(C,1)-2
    for j=1:size(C,2)-2
        %Sobel mask for x-direction:
        Gx=((2*C(i+2,j+1)+C(i+2,j)+C(i+2,j+2))-(2*C(i,j+1)+C(i,j)+C(i,j+2)));
        %Sobel mask for y-direction:
        Gy=((2*C(i+1,j+2)+C(i,j+2)+C(i+2,j+2))-(2*C(i+1,j)+C(i,j)+C(i+2,j)));

        %The gradient of the image
        %B(i,j)=abs(Gx)+abs(Gy);
        B(i,j)=sqrt(Gx.^2+Gy.^2);
        direction = atan(Gy./Gx)

    end
end

My question is that sometimes gradient direction value is giving as "NaN", how to avoid it?

Second, how to quantize gradient direction into eight zone and find feature vector for the image? Please someone help me.

Answer 1

Here is the implementation :

clc;clear;close;

% Read the Image
image=imread('girl.png');
f=rgb2gray(image);

%   Initializations
Gx=zeros(size(f,1),size(f,2));
Gy=zeros(size(f,1),size(f,2));
Theta=zeros(size(f,1),size(f,2));
Edges=zeros(size(f,1),size(f,2));

% Sobel Filtering
for x=2:size(f,1)-2
    for y=2:size(f,2)-2

    Gy(x,y)= ( f(x-1,y+1) + 2*f(x,y+1) + f(x+1,y+1) )...
        -( f(x-1,y-1) + 2*f(x,y-1) + f(x+1,y-1) );

    Gx(x,y)= ( f(x+1,y-1) + 2*f(x+1,y) + f(x+1,y+1) )...
        -( f(x-1,y-1) + 2*f(x-1,y) + f(x-1,y+1) );   

    Theta(x,y)= atan(Gx(x,y)/Gy(x,y));  % Direction

    Edges(x,y)=sqrt( Gx(x,y)^2 + Gy(x,y)^2);

    end
end

Results :