Using drawContours OpenCV function in python

Question

I have installed OpenCV 2.2 and when I try to use drawContours I get the following error:

cv.drawContours(frame, contours, 0, cv.RGB(255, 0, 0))
TypeError: <unknown> is not a numpy array

The code related to this error is the following:

storage = cv.CreateMemStorage(0)
contours = cv.FindContours (color_mask, storage, method = cv.CV_CHAIN_APPROX_SIMPLE)
cv.drawContours(frame, contours, 0, cv.RGB(255, 0, 0))

The python documentation does not correspond with the correct order of parameters (I know the correct order thank to IDLE) and the C++ documentation for this function does not help me very much

Here is the full code (relevant code):

    cv.NamedWindow("MyWindow", 1)
    capture = cv.CaptureFromCAM(0)

    while 1:
        frame = cv.QueryFrame(capture)

        color_mask = cv.CreateImage(cv.GetSize(frame), 8, 1)

        cv.InRangeS(frame, cv.Scalar(*min_color), cv.Scalar(*max_color), color_mask)

        cv.CvtColor(frame, frame, cv.CV_BGR2HSV)

        storage = cv.CreateMemStorage(0)
        contours = cv.FindContours (color_mask, storage, method = cv.CV_CHAIN_APPROX_SIMPLE)
        cv.drawContours(image = frame, contours = contours, contourIdx = 0, color = cv.RGB(255, 0, 0))

        cv.ShowImage("MyWindow", frame)

Thanks in advance

Zebreu · Accepted Answer · 2011-05-09 20:33:11Z

6

You should be aware that drawContours and DrawContours are two different functions. They actually do the same thing, but they accept different parameters. I believe the first one only accepts numpy arrays instead of CvMat or other arrays from openCV.

answered May 9, 2011 at 20:33

Zebreu

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Comments

Christoph Rackwitz · Accepted Answer · 2024-02-16 10:35:48Z

2

You should check the function parameters in the python reference of DrawContours, and try not to rely on the order of the parameters when calling a function that takes multiple arguments, you should use labels.

In other words :

cv.DrawContours(img=frame, contour=contours, ...)

If you check the documentation of DrawContours:

DrawContours(img, contour, external_color, hole_color, max_level, thickness=1, lineType=8, offset=(0, 0))

You will notice that the function accepts 8 arguments:

5 needed (img, contour, external_color, hole_color, max_level)
3 optional (thickness, lineType, offset)

and there are no arguments called contourIdx or color

eg:

cv.DrawContours(img=frame, contour=contours, external_color=cv.RGB(255, 0, 0), hole_color=cv.RGB(0, 255, 0), max_level=1 )

edited Feb 16, 2024 at 10:35

Christoph Rackwitz

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answered Apr 6, 2011 at 11:49

P2bM

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4 Comments

Manuel Over a year ago

The right function parameters does not correspond with the official reference (I know this sounds stupid). If I do as you said the following error is raised: TypeError: Required argument 'contourIdx' (pos 3) not found

Manuel Over a year ago

After a small break, I think the error could be in the contours generation

P2bM Over a year ago

i think you are looking at the wrong documentation follow the link on the answer, its the documentation for the python API

Manuel Over a year ago

I have read this documentation, but after download OpenCV 2.2.0 and compile & install it, if I open Idle, import OpenCV module and write cv.DrawContours, the parameters showed by Idle are not the same documented, I think the doc was generated before the final version. If I try to label parameters like 'img = wherever' the python interpreter raise and error saying there are any parameter with label 'img'

Martijn Pieters · Accepted Answer · 2012-05-31 14:30:54Z

1

Use the function cv2array and array2cv given on OpenCV Cheatsheet to convert the image to the specific format.

Something like this:

imgray = array2cv(cv2.cvtColor(cv2array(image), cv.CV_RGB2GRAY))
storage = cv.CreateMemStorage(0)
contours = cv.FindContours(imgray, storage,cv.CV_RETR_LIST, cv.CV_CHAIN_APPROX_SIMPLE,(0,0))

edited May 31, 2012 at 14:30

Martijn Pieters

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answered May 31, 2012 at 14:09

Anurag

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Comments

Manuel · Accepted Answer · 2011-04-06 15:15:40Z

0

I have found some errors in Python wrapper of OpenCV 2.2. Examples like "camshift.py" run with OpenCV 2.1 but not with OpenCV 2.2. I suppose my problems are derivated from this bug (now I will use 2.1 version)

I have reported this bug as well as the documentation error

@P2bM thanks for your help

answered Apr 6, 2011 at 15:15

Manuel

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