calculate 2D image from 3D data

Now I have the xyz data, and want to calculate the corresponding 2D image. The equations I am using is from shape from shading, I(i,j)=1/sqrt(p*p+q*q+1), p and q are the slope of the surface along the x and y directions, respectively. Here calculate them using discrete approximations, p=Z(i,j)-Z(i,j-1), q=Z(i,j)-Z(i-1,j). The result should be the above image, however what I get is the below one. Could someone give me some advice about this?

for i=1:numr
  for j=1:numc
      if (j==1)
          p(i,j)=z(i,j+1)-z(i,j);
      else
          p(i,j)=z(i,j)-z(i,j-1);
      end
      if i==1
          q(i,j)=z(i+1,j)-z(i,j);
      else
          q(i,j)=z(i,j)-z(i-1,j);
      end  
      I(i,j)=1/sqrt(p(i,j)*p(i,j)+q(i,j)*q(i,j)+1);
  end
end

[jonas]

Could you upload the xyz data?

[MENGZHI DI]

Thank you for replying. I put them in a 3x16384 matrix. The first row is x, second is y, third is z data.