rectangle not showing up in opencv

Why is rectangle not showing up in my code?

import cv2 im = cv2.imread('players.bmp') #im.shape >>returns (765,1365,3) cv2.rectangle(im, (64,1248), (191,1311), (0,255,0), 2) cv2.namedWindow("image", cv2.WINDOW_NORMAL) cv2.imshow('image', im) cv2.waitKey(0) cv2.destroyAllWindows() 

3 Answers

It does not show the rectangle, because you are drawing it outside the image.

Why? you may be asking. It is simple. You have this:

#im.shape >>returns (765,1365,3) 

This means

rows/height = 765 cols/width = 1365 channels = 3 

Then you do

cv2.rectangle(im, (64,1248), (191,1311), (0,255,0), 2) 

Here you use 2 points which are tuples (x,y), but you are writing them as if they where tuples (y,x). I know that OpenCV uses in a lot of functions the order (y,x), but this is due that they see the image as a matrix, which commonly is accessed with (row, column) which translates to (y,x). In the case of this rectangle they require Points which are expressed in the typical Cartesian way (x,y).

In conclusion, just change it to:

cv2.rectangle(im, (1248, 64), (1311, 191), (0,255,0), 2) 

And it should work.

I had a completely different cause for the issue

for me it wasn't showing up because I was using a numpy view from the image, so instead of using cv2.cvtColor(frame, cv2.COLOR_RGB2BGR), I instead used frame[..., ::-1] to convert RGB and BGR.

Somehow this makes the result immutable and when cv2.rectangle tries to write to it, it just doesn't get changed.

I stumbled upon this question while facing the same problem. In my case, I was correctly passing x and y along with x+w and y+h. I had made 2 mistakes though:

• I had this part in a try and except block (so I couldn't see the exception being raised)
• My code was changing h to an array prior to me drawing the rectangle. (I had extracted the values of x,y,w,h from x,y,w,h = cv2.boundingRect(cnt) before mistakenly changing h to [1,-1,-1,-1])