ginput() in opencv-python

The Image Processing Toolbox in MATLAB has this very useful utility called as ginput, which allows manually picking co-ordinate points from an image. I tried to recreate that functionality in python based OpenCV, and the result is the following code:


import cv2
import numpy as np
from matplotlib import pyplot as plt
import os

a = np.array([0,0], dtype='float32')
def getXY(imgPath):

 try:
   os.environ["DEBUG"]
   DEBUG = 1

 print "-------------------------------"
 print "Using OpenCV version : ",cv2.__version__
 print "-------------------------------"

 print "TASK : Mimic getxy() feature of MATLAB"
 print "***************************************"

 except KeyError:
    #print "For verbose mode, set DEBUG flag"
    DEBUG = 0

 #define the event
 def getxy(event, x, y, flags, param):
   global a
   if event == cv2.EVENT_LBUTTONDOWN :
      a = np.vstack([a, np.hstack([x,y])])
      if (DEBUG):
      print "(row, col) = ", (x,y)

   #Read the image
   img = cv2.imread(imgPath)
   if (DEBUG):
      print "Reading the image..."

   #Set mouse CallBack event
   cv2.namedWindow('image')
   cv2.setMouseCallback('image', getxy)
   if (DEBUG):
      print "Set MouseCallback functionality..."

   #show the image
   print "Click to select a point OR press ANY KEY to continue..."
   cv2.imshow('image', img)
   cv2.waitKey(0)
   cv2.destroyAllWindows()

   #obtain the matrix of the selected points
   b = a[1:,:]
   if (DEBUG):
      print ""
      print "The clicked points..."
      print b, b.dtype, b.shape

   print "The selected points are returned as a float64 MATRIX..."
   return b

Steps to use it :

  • save the above source code as getXY.py in your working directory.
  • In your python code, import this file, i.e.,

from getXY import *

  • Pass the image path to getXY, and start clicking on the displayed image. The no. of times you click, those many points will get saved.
  • If you want a verbose version, set the environment variable DEBUG = 1 from the terminal before running the code, i.e.,

DEBUG=1; export $DEBUG

That’s it! I’ll upload the code onto a git repo as well soon and provide a link here as well.

  • To remove the verbose code, unset the DEBUG variable, i.e, :

unset $DEBUG

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