# Corner detection algorithm gives very high value for slanted edges?

I have tried implementing a basic version of shi-tomasi corner detection algorithm. The algorithm works fine for corners but I came across a strange issue that the algorithm also gives high values for slanted(titled) edges.

Here's what i did

• Took gray scale image
• computer dx, and dy of the image by convolving it with sobel_x and sobel_y
• Took a 3 size window and moved it across the image to compute the sum of the elements in the window.
• computed sum of the window elements from the dy image and sum of window elements from the dx image and saved it in sum_xx and sum_yy.
• created a new image (call it result) where that pixel for which the window sum was computed was replaced with min(sum_xx, sum_yy) as shi-tomasi algorithm requires.

I expected it to give maximum value for corners where dx and dy both are high, but i found it giving high values even for titled edges.

Here are the some outputs of the image i received: so far so good, corners have high values.

Here's where the problem lies. edges have high values which is not expected by the algorithm. I can't fathom how can edges have high values for both x and y gradients (sobel being close approximation of gradient).

I would like to ask your help, if you can help me fix this issue for edges. I am open to any suggestions and ideas .

Here's my code (if it helps):

def shi_tomasi(image, w_size):
ans = image.copy()
dy, dx = sy(image), sx(image)

ofset = int(w_size/2)
for y in range(ofset, image.shape-ofset):
for x in range(ofset, image.shape-ofset):

s_y = y - ofset
e_y = y + ofset + 1

s_x = x - ofset
e_x = x + ofset + 1

w_Ixx = dx[s_y: e_y, s_x: e_x]
w_Iyy = dy[s_y: e_y, s_x: e_x]

sum_xx = w_Ixx.sum()
sum_yy = w_Iyy.sum()

ans[y][x] = min(sum_xx, sum_yy)
return ans

def sy(img):
t = cv2.Sobel(img,cv2.CV_8U,0,1,ksize=3)
return t
def sx(img):
t = cv2.Sobel(img,cv2.CV_8U,1,0,ksize=3)
return t

• if i had to use a custom algorithm i would have used the opencv version of it. I want to implement it on my own to understand each and everything – Hissaan Ali Jun 10 '20 at 11:01