How to draw lines between points in OpenCV?

Question

I have an array of tuples:

a = [(375, 193)
(364, 113)
(277, 20)
(271, 16)
(52, 106)
(133, 266)
(289, 296)
(372, 282)]

How to draw lines between points in OpenCV?

Here is my code that isn't working:

for index, item in enumerate(a): 
    print (item[index]) 
    #cv2.line(image, item[index], item[index + 1], [0, 255, 0], 2) 

Solution 1

Using draw contours, you can draw the shape all at once.

img = np.zeros([512, 512, 3],np.uint8)
a = np.array([(375, 193), (364, 113), (277, 20), (271, 16), (52, 106), (133, 266), (289, 296), (372, 282)])
cv2.drawContours(img, [a], 0, (255,255,255), 2)

If you don't want the image closed and want to continue how you started:

image = np.zeros([512, 512, 3],np.uint8)
pointsInside = [(375, 193), (364, 113), (277, 20), (271, 16), (52, 106), (133, 266), (289, 296), (372, 282)]

for index, item in enumerate(pointsInside): 
    if index == len(pointsInside) -1:
        break
    cv2.line(image, item, pointsInside[index + 1], [0, 255, 0], 2) 

Regarding your current code, it looks like you are trying to access the next point by indexing the current point. You need to check for the next point in the original array.

A more Pythonic way of doing the second version would be:

for point1, point2 in zip(a, a[1:]): 
    cv2.line(image, point1, point2, [0, 255, 0], 2) 

Solution 2

If you just want to draw lines, how about cv2.polylines? cv2.drawContours would be preferred when you already have a contours object.

cv2.polylines(image, 
              a, 
              isClosed = False,
              color = (0,255,0),
              thickness = 3, 
              linetype = cv2.LINE_AA)

Comments

[w-m]

There's a tutorial in the official documentation for drawing. docs.opencv.org/3.1.0/dc/da5/tutorial_py_drawing_functions.h‌​tml - what have you tried so far?

[OPV]

I know, but tutorial tells to use two points: cv2.line(img,(0,0),(511,511),(255,0,0),5), but I have some points

[alkasm]

You can also draw polygons or contours using your point list. What exactly do you want to achieve?

[OPV]

I can use polygon, but I dont need to close polygon

[Zev]

Please show some of your code. If you have nothing coded yet, it is hard to meet Stack Overflow's requirements that the question be specific.

[OPV]

pointsInside =[] for index, item in enumerate(pointsInside): print (item[index]) #cv2.line(image, item[index], item[index + 1], [0, 255, 0], 2)

[Zev]

cv2.drawContours(img, [a], 0, (255,255,255), 2) which I posted is one line as well but I do think your solution is a good fit.

[bfris]

@Zev: my bad. I didn't read your code that carefully. cv2.drawContours is probably just as good, especially if you already have a contours object to work with.

[Martin]

Great. Doesnt close lines and its loopless

[spaceman]

This worked for me cv2.polylines(image, [np.array(a)], isClosed = False, color = (0,255,0), thickness = 3, linetype = cv2.LINE_AA) a = [(375, 193) (364, 113) (277, 20) (271, 16) (52, 106) (133, 266) (289, 296) (372, 282)]

[mattsmith5]

after you draw the contours, how do you get the all the contours and store them, of what you just drew, while ignoring everything else in the image?

[mattsmith5]

hi, I left a question here, stackoverflow.com/questions/66923488/…

[bfris]

@mattsmith5, you can make a blank white image with ` image = 255*np.ones((512,512)) ` for a 512x512 pixel image. Then draw on that.