How to crop an image in OpenCV using Python

Question

How can I crop images  like I ve done before in PIL  using OpenCV    Working example on PIL  im   Image open  0 png   convert  L   im   im crop  1  1  98  33   im save   0 png     But how I can do it on OpenCV    This is what I tried   im   cv imread  0 png   cv CV LOAD IMAGE GRAYSCALE   thresh  im bw    cv threshold im  128  255  cv THRESH OTSU  im   cv getRectSubPix im bw   98  33    1  1   cv imshow  Img   im  cv waitKey 0    But it doesn t work   I think I incorrectly used getRectSubPix  If this is the case  please explain how I can correctly use this function

User · Answer

i had this question and found another answer here: copy region of interest

If we consider (0,0) as top left corner of image called im with left-to-right as x direction and top-to-bottom as y direction. and we have (x1,y1) as the top-left vertex and (x2,y2) as the bottom-right vertex of a rectangle region within that image, then:

roi = im[y1:y2, x1:x2]

here is a comprehensive resource on numpy array indexing and slicing which can tell you more about things like cropping a part of an image. images would be stored as a numpy array in opencv2.

:)

User · Answer

to make it easier for you here is the code that i use   w  h   image shape top 10 right 50 down 15 left 80 croped image   image top   w-down  top   right   h-left  right   plt imshow croped image  cmap  quot gray quot   plt show

User · Answer

Alternatively  you could use tensorflow for the cropping and openCV for making an array from the image   import cv2 img   cv2 imread  YOURIMAGE png     Now img is a  imageheight  imagewidth  3  shape array  Crop the array with tensorflow   import tensorflow as tf offset height 0 offset width 0 target height 500 target width 500 x   tf image crop to bounding box      img  offset height  offset width  target height  target width     Reassemble the image with tf keras  so we can look at it if it worked   tf keras preprocessing image array to img      x  data format None  scale True  dtype None     This prints out the pic in a notebook  tested in Google Colab      The whole code together   import cv2 img   cv2 imread  YOURIMAGE png    import tensorflow as tf offset height 0 offset width 0 target height 500 target width 500 x   tf image crop to bounding box      img  offset height  offset width  target height  target width    tf keras preprocessing image array to img      x  data format None  scale True  dtype None

User · Answer

here is some code for more robust imcrop   a bit like in matlab    def imcrop img  bbox        x1 y1 x2 y2   bbox     if x1  lt  0 or y1  lt  0 or x2  gt  img shape 1  or y2  gt  img shape 0           img  x1  x2  y1  y2   pad img to fit bbox img  x1  x2  y1  y2      return img y1 y2  x1 x2      def pad img to fit bbox img  x1  x2  y1  y2       img   np pad img    np abs np minimum 0  y1    np maximum y2 - img shape 0   0                    np abs np minimum 0  x1    np maximum x2 - img shape 1   0     0 0    mode  constant       y1    np abs np minimum 0  y1       y2    np abs np minimum 0  y1       x1    np abs np minimum 0  x1       x2    np abs np minimum 0  x1       return img  x1  x2  y1  y2

User · Answer

This code crops an image from x 0 y 0 to h 100 w 200  import numpy as np import cv2  image   cv2 imread  download jpg   y 0 x 0 h 100 w 200 crop   image y y h  x x w  cv2 imshow  Image   crop  cv2 waitKey 0

User · Answer

It s very simple  Use numpy slicing   import cv2 img   cv2 imread  lenna png   crop img   img y y h  x x w  cv2 imshow  cropped   crop img  cv2 waitKey 0

User · Answer

Robust crop with opencv copy border function   def imcrop img  bbox      x1  y1  x2  y2   bbox    if x1  lt  0 or y1  lt  0 or x2  gt  img shape 1  or y2  gt  img shape 0           img  x1  x2  y1  y2   pad img to fit bbox img  x1  x2  y1  y2     return img y1 y2  x1 x2      def pad img to fit bbox img  x1  x2  y1  y2       img   cv2 copyMakeBorder img  - min 0  y1   max y2 - img shape 0   0                               -min 0  x1   max x2 - img shape 1   0  cv2 BORDER REPLICATE     y2    -min 0  y1     y1    -min 0  y1     x2    -min 0  x1     x1    -min 0  x1     return img  x1  x2  y1  y2

User · Answer

Note that  image slicing is not creating a copy of the cropped image but creating a pointer to the roi  If you are loading so many images  cropping the relevant parts of the images with slicing  then appending into a list  this might be a huge memory waste    Suppose you load N images each is  gt 1MP and you need only 100x100 region from the upper left corner    Slicing   X      for i in range N       im   imread  image i       X append im 0 100 0 100     This will keep all N images in the memory                                   Because they are still used    Alternatively  you can copy the relevant part by  copy    so garbage collector will remove im   X      for i in range N       im   imread  image i       X append im 0 100 0 100  copy      This will keep only the crops in the memory                                          im s will be deleted by gc    After finding out this  I realized one of the comments by user1270710 mentioned that but it took me quite some time to find out  i e   debugging etc   So  I think it worths mentioning

User · Answer

Below is the way to crop an image   image path  The path to the image to edit  coords  A tuple of x y coordinates  x1  y1  x2  y2  open the image in  mspaint and check the  ruler  in view tab to see the coordinates   saved location  Path to save the cropped image  from PIL import Image     def crop image path  coords  saved location          image obj   Image open  Path of the image to be cropped               cropped image   image obj crop coords              cropped image save saved location              cropped image show     if   name         main         image    image jpg      crop image   100  210  710 380     cropped jpg

[python] How to crop an image in OpenCV using Python

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