Python 2.7 OpenCV3 tutorial

Reading Images

To read an image as it is

            >>>import cv2

            >>> a=cv2.imread('D:/1.jpg')

                                    (or)

            >>> a=cv2.imread('D:/1.jpg',-1) #the default

                                    (or)

            >>> a=cv2.imread('D:/1.jpg',cv2.IMREAD_UNCHANGED)

To  show an image

            >>> cv2.imshow('window_name',a)

            >>>cv2.waitKey(0)

window_name can be any name u like. cv2.waitKey(milliseconds) returns ASCII value of the key pressed on the Keyboard

To destroy the window created through imshow

To destroy all windows,

            >>>cv2.destroyAllWindows()

To destroy a specific window only,

            >>>cv2.destroyWindow('window_name')

To check the dimensions of the image read

>>> import cv2

>>> image=cv2.imread('1.png')

>>> image.shape

(450, 300, 3)

#450 rows, 300 columns and 3 planes

>>> (rows,columns,channels)=image.shape

Converting an image into gray scale  

To find different flags available in opencv

To find the list of color conversion possible

            >>> flags= [i for i in dir(cv2) if i.startswith('COLOR_')]

            >>> print flags

To find the flags for events in opencv

            events=[i for i in dir(cv2) if 'EVENT' in i]

            print events

To find the different line flags available

            >>> flags= [i for i in dir(cv2) if i.startswith('LINE_')]

            >>> print flags

To find the different FONT flags available

            fonts=[i for i in dir(cv2) if 'FONT' in i]

            print fonts

To draw a Line 

To draw a rectangle 

 To Draw an Ellipse

                                                                               

To add text to images 

 To resize an image

  

To copy an image

image=cv2.imread(‘C:/1.jpg’)

copied=image.copy()

Convert image to binary image, based on threshold

>>> gray=cv2.cvtColor(image,cv2.COLOR_BGR2GRAY)

>>> ret1,thresh1=cv2.threshold(gray,0,255,cv2.THRESH_BINARY+cv2.THRESH_OTSU)

>>> ret1

149.0 # threshold value decided by OTSU and all the pixels intensity above this threshold is made 255

#thresh1 is the binary image.

>>> ret,thresh=cv2.threshold(gray,50,255,cv2.THRESH_BINARY)

>>> ret

50.0# threshold value decided by u i.e, second argument

To find contours

_,contours,hierarchy=cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)

To flip an image

rimg=cv2.flip(img,1) #vertical flip

fimg=cv2.flip(img,0) #horizontal flip

Image Arithmetic

Adding two images

added=cv2.add(img1,img2)

added=cv2.addWeighted(img1,0.7,img2,0.3,0)

Absolute difference of two images

difference= cv2.absdiff(img1,img2) 

                                                                               

Creating the negative of an binary image

binaryInvert=cv2.bitwise_not(binary_imag)

 Find the edges of an image using Canny

edges = cv2.Canny(image,100,200)

Gradient Filters

1. sobelx=cv2.Sobel(image,cv2.CU_64F,1,0,5)

   sobely=cv2.Sobel(image,cv2.CU_64F,0,1,5)

2. lap=cv2.Laplacian(image,cv2.CU_64F,0,1,5)