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Python crop not working

asked 2013-06-05 07:33:21 -0500

Rohan12111 gravatar image

Hello, I am relatively new to OpenCV. I have gleaned from tutorials and such that you can crop by using this script:

import cv2
import numpy as np
import video

cam = cv2.VideoCapture(0)
ret,vis =    
crop = vis[100:400, 100:300]    

And this works fine. I get no errors.

However, when i put it into my main script, it doesn't work, i've narrowed it down to this section of code:

def PicTake(self):
    ret,vis =
    x1,y1 = self.selection[0]
    x2,y2 = self.selection[1]
    a = 0
    taken = 0
    while taken == 0:
        if cv2.imread("C:\Python27\opencv\samples\python2\Major\Test"+str(a)+".png") == None:
            crop = vis[x1:y1, x2:y2]
            print crop
            taken = 1
    return ("Picture Taken")

where self.selection is just a list of two tuples [(x1,y1),(x2,y2)]. After the first if statement, print crop returns "[]" and empty list.

So yeah, why does it work with numbers and other situations fine, but not here?!

Any help is greatly appreciated, thanks!

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answered 2013-06-07 04:15:25 -0500

DanMaynard gravatar image

I assume that you would like to crop out a rectangle where the tuples (x1,y1) and (x2,y2) define the corners? Also to be clear, I assume origin at top left, x across and y down.

The [] notation is really array slicing in numpy and not an OpenCV function, so the first pair is rows and the second pair columns. This means you need

crop = vis[y1:y2, x1:x2]

Numpy slicing is very flexible, in general with 'basic slicing' like this it returns a view to the same memory buffer so if you modify crop the values in vis will change. This is probably not a problem here, but if it is you can force a copy easily with

crop = vis[y1:y2, x1:x2].copy()

... although obviously this is slightly slower to run.

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How would you crop multiple ROIs from a np.array? With the coords arranged this way. Rect=([x1,y1,x2,y2],[x1,y1,x2,y2]...[n]).

Firelife gravatar imageFirelife ( 2019-01-30 07:29:31 -0500 )edit
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Asked: 2013-06-05 07:33:21 -0500

Seen: 11,709 times

Last updated: Jun 07 '13