OpenCV Won't draw circle - Python

Question

What's wrong with this - why won't it draw the circle in the window I create?

    threeDWinName = "3D View"
    cv2.namedWindow(threeDWinName, cv2.CV_WINDOW_AUTOSIZE)
    img2 = cv2.imread('white.png', 0)
    cv2.imshow(threeDWinName,img2)
    cv2.circle(img2, (100,100),100,255,-1)
    cv2.imshow(threeDWinName,img2)

fraxel · Accepted Answer · 2013-01-27 13:32:21Z

7

Your code works for me. I see the file you are reading is called white.png, and by loading it with the 0 you are loading it in grayscale, so the circle you are drawing is also white. That could be the problem ;)

img2 = cv2.imread('black.png', 0)
cv2.circle(img2, (100,100),100,255,-1)
cv2.imwrite('circle.png', img2)

enter image description here

or if you use img2 = cv2.imread('black.png'), then you get this:

enter image description here

answered Jan 27, 2013 at 13:32

fraxel

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2 Comments

Sam Heather Over a year ago

thanks for your help again. Why does the grayscale representation of a blue circle go white? I gave up on trying to make a background because I couldn't get the circle draw function work, so figured I'd use my own background image. Not efficient I know,...

fraxel Over a year ago

@Sam Heather - Hey, happy to help. For grayscale images the value between 0 and 255 is a shade of grey, from black to white. For colour images, usually you would pass a tuple of three values, like this (255,0,0) This represents the luminousity of each of the channels Blue, Green, and Red (or BGR). But in your case you've only passed a single value, and the function has interpreted it as the value for the first (Blue) channel. If you pass (0,0,255), you'll get a red circle. Try changing the values to generate different colours.

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