Implementing Sobel Gradient without image package

I am supposed to apply a Sobel filter on an image in Octave, but I am not allowed to use any functions from the image package. I wrote the code but my output is just a black image. Here is what I have so far: (Thank you in advance!)

    kx= [1 ,0 ,-1; 2,0,-2; 1, 0 ,-1];
    ky= [1,2,1; 0,0, 0; -1, -2 ,-1];

    H = conv2(kx,im2double(my_img),'same');
    V = conv2(ky,im2double(my_img),'same');
    E = sqrtm(H.*H + V.*V);
    figure 4
    imshow(E, [])

Answer 1

[Andrew Davies]

    kx= [1 ,0 ,-1; 2,0,-2; 1, 0 ,-1];
    ky= [1,2,1; 0,0, 0; -1, -2 ,-1];

    H = conv2(im2double(my_img),kx,'same');
    V = conv2(im2double(my_img),ky,'same');
    E = sqrt(H.*H + V.*V);
    imshow(E, [])

conv2(... 'same') uses the size of the first input. Also don't you want sqrt and not sqrtm?