find gradient using sobel mask operator in matlab

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

%------------------- C is an image
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

[Image Analyst]

Why? Why not simply use imgradient() to get the Sobel filtered image?

[DIWAKAR KUMAR]

please tell me how to quantize gradient direction into eight zone and find feature vector for the image?

[Image Analyst]

What features do you want to measure?