A simple implementation of sobel filtering in Python


One can directly use ‘ndimage’ of scipy to compute the sobel filtering of the input image as follows:

dx = ndimage.sobel(im, 0)  # horizontal derivative
dy = ndimage.sobel(im, 1)  # vertical derivative
mag = np.hypot(dx, dy)  # magnitude
mag *= 255.0 / np.max(mag)  # normalize

Or your can write the function by yourself and add more features to it.

def sobel_filter(im, k_size):
    
    im = im.astype(np.float)
    width, height, c = im.shape
    if c > 1:
        img = 0.2126 * im[:,:,0] + 0.7152 * im[:,:,1] + 0.0722 * im[:,:,2]
    else:
        img = im
    
    assert(k_size == 3 or k_size == 5);
    
    if k_size == 3:
        kh = np.array([[-1, 0, 1], [-2, 0, 2], [-1, 0, 1]], dtype = np.float)
        kv = np.array([[1, 2, 1], [0, 0, 0], [-1, -2, -1]], dtype = np.float)
    else:
        kh = np.array([[-1, -2, 0, 2, 1], 
                   [-4, -8, 0, 8, 4], 
                   [-6, -12, 0, 12, 6],
                   [-4, -8, 0, 8, 4],
                   [-1, -2, 0, 2, 1]], dtype = np.float)
        kv = np.array([[1, 4, 6, 4, 1], 
                   [2, 8, 12, 8, 2],
                   [0, 0, 0, 0, 0], 
                   [-2, -8, -12, -8, -2],
                   [-1, -4, -6, -4, -1]], dtype = np.float)
    
    gx = signal.convolve2d(img, kh, mode='same', boundary = 'symm', fillvalue=0)
    gy = signal.convolve2d(img, kv, mode='same', boundary = 'symm', fillvalue=0)

    g = np.sqrt(gx * gx + gy * gy)
    g *= 255.0 / np.max(g)
  
    #plt.figure()
    #plt.imshow(g, cmap=plt.cm.gray)      
  
    return g

2 thoughts on “A simple implementation of sobel filtering in Python

  1. JT Hiquet

    Hello, How did you read your image in? I don’t want to use opencv. Did you use ndimage.imread to open the image?

    Reply

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