OpenCV polyLines() throws error: (-215:Assertion failed) p.checkVector(2, CV_32S) >= 0 in function 'polylines'

I am trying to draw a segmentation mask from a YOLO segmentation mask dataset. The annotation line I am reading looks like this:

36 0.6158357764423077 0.814453125 0.6158357764423077 0.8095703125 0.6070381225961539 0.8095703125 0.6041055721153846 0.8115234375 0.5894428149038462 0.8154296875 0.5747800576923077 0.8125 0.5513196490384615 0.8134765625 0.5483870961538462 0.81640625 0.5923753653846154 0.818359375 0.6158357764423077 0.814453125

I am using cv2.polylines to draw the shape but am getting an error:

image_height, image_width, c = img.shape
isClosed = True
color = (255, 0, 0)
thickness = 2

with open(annotation_file) as f:
    for line in f:
        split_line = line.split()
        class_id = split_line[0]
        mask_shape = [float(numeric_string) for numeric_string in split_line[1:len(split_line)]]
        mask_points = []
        for i in range(0,len(mask_shape),2):
            x,y = mask_shape[i:i+2]
            mask_points.append((x * image_width, y * image_height))
        points = np.array([mask_points])
        image = cv2.polylines(img, points,
                              isClosed, color, thickness)
        break

Error:

OpenCV(4.7.0) /Users/xperience/GHA-OCV-Python/_work/opencv-python/opencv-python/opencv/modules/imgproc/src/drawing.cpp:2434: error: (-215:Assertion failed) p.checkVector(2, CV_32S) >= 0 in function 'polylines'

It's an issue of the .dtype. points.dtype must be np.int32. polyLines wants integers, not floats.

The values you insert into your list are all floats. The array would have had float64 dtype.

Just call this:

# points array for one polyline
points = np.array(mask_points, dtype=np.int32)

# polyLines takes a list of polylines
# you have one polyline, so it'll be a list of one element
image = cv2.polylines(
    img,
    [points], # one polyline
    isClosed, color, thickness)

Putting the outer dimension into the numpy array itself (you did that with [mask_points]), instead of using an outer list, is only acceptable if ALL your polylines have the same number of points.