Why is the returned value of cv2.HoughLines of OpenCV for Python need to be accessed with index?

Question

I hope I wrote the question title right because I don't know how to exactly explain it. Consider below's code:

lines = cv2.HoughLines(edges,1,np.pi/180,200)
for rho,theta in lines[0]:
    a = np.cos(theta)
    b = np.sin(theta)
    x0 = a*rho
    y0 = b*rho
    x1 = int(x0 + 1000*(-b))
    y1 = int(y0 + 1000*(a))
    x2 = int(x0 - 1000*(-b))
    y2 = int(y0 - 1000*(a))

    cv2.line(img,(x1,y1),(x2,y2),(0,0,255),2)

Why it has to be wrote for rho,theta in lines[0]:? By this kind of code, I can only obtain one line. I have tried to remove the indexing in lines but I got ValueError: need more than 1 value to unpack. I have tried to print the returned value and it look something like this:

[[[ 287.            1.97222209]]

[[ 885.            1.20427716]]

[[ 881.            1.22173047]]]

I have kinda solved this problem my making the code look like this:

lines = cv2.HoughLines(edges,1,np.pi/180,200)
for i in range(10):
    for rho,theta in lines[i]:

I wonder, what is really happening? Or did I do something wrong here?

Answer 1

lines=cv2.HoughLines(canny,1,numpy.pi/180,120)
for i in lines:
    # print(i)
    rho=i[0][0]
    theta=i[0][1]
    a=numpy.cos(theta)
    b=numpy.sin(theta)
    x0=a*rho
    y0=b*rho
    x1=int(x0+1000*(-b))
    y1=int(y0+1000*(a))
    x2=int(x0-1000*(-b))
    y2=int(y0-1000*(a))
    cv2.line(img,(x1,y1),(x2,y2),(0,255,0),1)
cv2.imshow('ss',img)

Answer 2

I believe it should be this:

for line in lines:
    rho, theta = line[0]
    ...

this way you loop through all of the values in the lines array, each of which is a line consisting of rho and theta.

It would of course be much bettwe if they structure this as

[ [r0,t0], [r1,t1], ... ,[rn,tn] ]

but instead they made it confusing by using the extra nested

[ [[r0,t0]], [[r1,t1]], ... ,[[rn,tn]] ]

form.

The line in lines: loops through giving [[ri,ti]] terms, which you can then make into [ri,ti] via line[0], which you then pass into rho and theta.

Answer 3

for line in lines:
    rho, theta = line[0]
    a = np.cos(theta)
    b = np.sin(theta)

This works for me on Python2.7 (Anaconda) and OpenCV3.1.0. There seems to be a mismatch between the example in the online documentation provided by OpenCV (1XnX2) and what it actually returns in the HoughLines function (nX1X2).