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Editorial Team
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Asked: 2026-05-31T08:07:25+00:00

i am trying to write a zoom function which looks something like this: centre

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i am trying to write a zoom function which looks something like this:

centre = ((im.width-1)/2, (im.height-1)/2)
width = int(im.width/(2.0*level))
height = int(im.height/(2.0*level)) 
rect = (centre[0]-width, centre[1]-height, width*2, height*2)
dst = cv.GetSubRect(im, rect)
cv.Resize(dst, im)

when I use exactly what is written above, I get an odd result where the bottom half of the resultant image is distorted and blurry. However when I replace the line cv.Resize(dst, im) with

size = cv.CloneImage(im)
cv.Resize(dst, size)
im = size

it works fine. Why is this? is there something fundamentally wrong with the way i am performing the zoom?

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1 Answer

  1. Editorial Team

    cv.Resize requires source and destination to be separate memory locations.

    Now in the first snippet of your code, you are using cv.GetSubRect to generate an object pointing to area of image which you wish to zoom in. Here the new object is NOT pointing to a new memory location. It is pointing to a memory location which is a subset of original object.

    Since cv.Resize requires both the memory locations to be different, what you are getting is a result of undefined behavior.

    In the second part of your code you are fulfilling this criteria by using cv.CloneImage.
    you are first creating a copy of im (i.e. size. however you could have used a blank image aswell) and then you are using cv.Resize to resize dst and write the resulting image in size.

    My advice is to go through the function documentation before using them.

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