How to get the indexes of contours' array to choose the N largest contours using opencv and Python?

Question

I am trying to find the 2 largest contours using python and opencv.

I have tried to get the indexes and then call a drawContour function, but something goes wrong.

This is my code

im2, contours, hierarchy = cv.findContours(roi, cv.RETR_TREE, cv.CHAIN_APPROX_NONE)

largest_area = 0
second_area = 0
l_index = 0
s_index = 0
for i, c in enumerate(contours):
    area = cv.contourArea(c)
    if (area > largest_area):
        if (area > second_area):
            second_area = largest_area
            largest_area = area
            l_index = i
    elif (area > second_area):
        second_area = area
        s_index = i

cv.drawContours(frame, contours[l_index], -1, (0, 255, 0), 2)
cv.imshow('frame',frame)

This is the error:

cv.drawContours(frame, contours[l_index], -1, (0, 255, 0), 2) IndexError: list index out of range

A second question, if I could make it, I don't know how to draw both of them, how could I do that?

Answer 1

First answer.

You're using drawContours function in a wrong way.
2nd parameter of drawContours is a list of contour (=a list of Point )s and 3rd parameter is the index of the contour you want to draw.
So your code should be:

cv.drawContours(frame, contours, l_index, (0, 255, 0), 2)

Second answer.

If you want to draw both contours, just call drawContours twice.

cv.drawContours(frame, contours, l_index, (0, 255, 0), 2)
cv.drawContours(frame, contours, s_index, (0, 0, 255), 2)