OpenCV Python Tutorial For Beginners

OpenCV is a computer vision library free and open-source that can quickly create image recognition applications in real-time. this OpenCV python tutorial covers various operations for dealing with images and Videos.

Install OpenCV in python

# pip install
pip install opencv-python
# conda install
conda install -c conda-forge opencv

Read and Write image in OpenCV

First thing first for performing any operation on OpenCV the images should be loaded and after loading images it could be rewritten.

Read Image

For loading an image OpenCV provides imread() function.it takes the first argument as the path of the image and the second one is the color flag.

• Flag 1 cv2.IMREAD_COLOR: Load the image in color. Any image transparency is ignored. It’s the default flag.
• Flag 0 cv2.IMREAD_GRAYSCALE: Loads in grayscale image mode.
• Flag -1 cv2.IMREAD_UNCHANGED: Load the image, including the alpha channel.

import cv2
img = cv2.imread('imagePath.jpg',0)

Display image

Function cv2.imshow() is used to display image.first argument is image name and second is image variable.

cv2.imshow('image',img)
    # wait till window close key pressed.
cv2.waitKey(0)
    # destroy all window
cv2.destroyAllWindows()

Write image

Function cv2.imwrite()  is used to write or save image.it take file name and flag type to save image.

import cv2

img = cv2.imread('imagePath.jpg',0)
cv2.imshow('image',img)
k = cv2.waitKey(0)
if k == 27:         # wait for ESC key to exit
    cv2.destroyAllWindows()
elif k == ord('q'): # wait for 'q' key to save and exit
    cv2.imwrite('imageSave.jpg',0)
    cv2.destroyAllWindows()

Read and Write Videos in OpenCV

For executing any operation on videos in OpenCV we have to load video either from web came or by giving the path of the video and then save it.

Read Video from Camera

Loading video from camera cv2.VideoCapture() is used and camera index is passed like 0 and 1 etc.

import cv2

cap = cv2.VideoCapture(0)
while(True):
    # Capture frames
    ret, frame = cap.read()

    # Operations on the frame
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    # Display frame
    cv2.imshow('frame',gray)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

# Release the capture
cap.release()
cv2.destroyAllWindows()

Video from file

Loading video from camera cv2.VideoCapture() is used but this time pass the path.

import cv2

cap = cv2.VideoCapture('mtest.avi')
while(cap.isOpened()):
    ret, frame = cap.read()
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    cv2.imshow('frame',gray)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

cap.release()
cv2.destroyAllWindows()

Save Video

Save Video from cv2.VideoWriter() function.pass the out put file name, FourCC  code, frames per second (fps) and frame size.

import cv2

cap = cv2.VideoCapture(0)

# Define the codec and create VideoWriter object
fourcc = cv2.VideoWriter_fourcc(*'XVID')
out = cv2.VideoWriter('output.avi',fourcc, 20.0, (640,480))

while(cap.isOpened()):
    ret, frame = cap.read()
    if ret==True:
        frame = cv2.flip(frame,0)

        # write the flipped frame
        out.write(frame)

        cv2.imshow('frame',frame)
        if cv2.waitKey(1) & 0xFF == ord('q'):
            break
    else:
        break

# Release everything if job is finished
cap.release()
out.release()
cv2.destroyAllWindows()

Drawing in OpenCV

In OpenCV we can draw line, circle, rectangle, ellipse and putText by using these functions : cv2.line(), cv2.circle() , cv2.rectangle(), cv2.ellipse(), cv2.putText() for more info on Drawing visite (Drawing Functions).

import cv2

# Create a black image
img = np.zeros((512,512,3), np.uint8)
   
  # Drawing Line   
# Draw a diagonal blue line with thickness of 5 px
img = cv2.line(img,(0,0),(511,511),(255,0,0),5)

  # Drawing Rectangle
img = cv2.rectangle(img,(384,0),(510,128),(0,255,0),3)
  
  # Drawing Circle
img = cv2.circle(img,(447,63), 63, (0,0,255), -1)

  # Drawing Ellipse
img = cv2.ellipse(img,(256,256),(100,50),0,0,180,255,-1)
  
  # Drawing Polygon
pts = np.array([[10,5],[20,30],[70,20],[50,10]], np.int32)
pts = pts.reshape((-1,1,2))
img = cv2.polylines(img,[pts],True,(0,255,255))

  # Text
font = cv2.FONT_HERSHEY_SIMPLEX
cv2.putText(img,'OpenCV',(10,500), font, 4,(255,255,255),2,cv2.LINE_AA)

If you have questions, feel free to ask, if you liked the post comment and share.

Thank you!