Python opencv crop not working

Question

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 = cam.read()    
crop = vis[100:400, 100:300]    
cv2.imshow("Img",vis)
cv2.imshow("Crop",crop)    
cv2.waitKey(0)

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 = self.cam.read()
    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
            cv2.imshow("crop",crop)
            cv2.imwrite("C:\Python27\opencv\samples\python2\Major\Test"+str(a)+".png",crop)
            taken = 1
        else:
            a+=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!

My code for getting the values is: `def onmouse(self, event, x, y, flags, param): self.onwindow = 1 # Mouse is on screen x,y = np.int16([x, y]) # BUG self.currentxy = (x,y) if event == cv2.EVENT_LBUTTONDOWN: self.RectOn = 0 #Del old rectangle and make new one self.click = 1 self.x1y1 = (x,y) self.selection[0] = self.x1y1 self.selection[1] = (0,0) if event == cv2.EVENT_LBUTTONUP: self.click = 0 self.x2y2 = (x,y) self.selection[1] = self.x2y2 self.RectOn = 1 print self.selection` — Rohan Williams, Jun 06 '13 at 07:24

score 4 · Accepted Answer · answered Jun 05 '13 at 16:40

4

vis is just a numpy array.
Check the result of print vis.shape to check the dimensions of your input image.
In case of a color image this would be something like this:

(367, 550, 3)

Which represents the dimensions in the order of height, width and then color depth.
So, if you want to select a portion from (x1,y1) to (x2,y2) when x2>x1 and y2>y1:

vis[y1:y2, x1:x2]

and this would also cover the depth dimension too.
Note that if y2<y1 or x2<x1, the result would be an empty array.

answered Jun 05 '13 at 16:40

Kamyar Infinity

2,711
1
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32

1

Thank you! It's working, I had the `[y1:y2, x1:x2]` the opposite way. And your right, I have to do something about the two points being selected in different positions. Thanks! – Rohan Williams Jun 06 '13 at 07:40

score 1 · Answer 2 · edited May 23 '17 at 12:23

1

you need to use

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

see this answer to get a detailed explanation.

also check to see if the camera actually outputted anything. after the line

ret, vis = self.cam.read()

add these lines

if not ret:
 print 'No captured images'

edited May 23 '17 at 12:23

Community

1
1

answered Jun 05 '13 at 23:38

samkhan13

3,315
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score 0 · Answer 3 · answered Jul 20 '14 at 01:14

0

Croping can be very confusing. The CV world runs on corner pairs (X1,Y1), while python is working by slicing arrays. A correct cropping might look like:

crop = original[y1:y2, x1:x2] # pay attention to your Y's and X's

answered Jul 20 '14 at 01:14

MountainLogic

308
2
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Python opencv crop not working

3 Answers3