OpenCV-Cheatsheet

Sample code and examples for OpenCV, Python

• Python
• Pre processing
• Post processing
• Examples

Python

Basic Operations

Checking the version

import cv2
print cv2.__version__
  

Loading Image in Grayscale

 import cv2
 # Load an color image in grayscale
 img = cv2.imread('messi5.jpg',0)
  

Resize Image

  small = cv2.resize(image, (0,0), fx=0.5, fy=0.5) 
  #and this will resize the image to have 100 cols (width) and 50 rows (height):

  resized_image = cv2.resize(image, (100, 50)) 

Display Image

 cv2.imshow('image',img)
 cv2.waitKey(0)
 cv2.destroyAllWindows()

Check keypress and write Image

import numpy as np
import cv2

img = cv2.imread('messi5.jpg',0)
cv2.imshow('image',img)
k = cv2.waitKey(0) & 0xFF
if k == 27: # wait for ESC key to exit
  cv2.destroyAllWindows()
elif k == ord('s'): # wait for 's' key to save and exit
  cv2.imwrite('messigray.png',img)
  cv2.destroyAllWindows()

Using Matplotlib

Its always handy to display images using matplotlib library. So here is an example of doing this

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

img = cv2.imread('messi5.jpg',0)
plt.imshow(img, cmap = 'gray', interpolation = 'bicubic')
plt.xticks([]), plt.yticks([]) # to hide tick values on X and Y axis
plt.show()

Pre-processing

Grayscale to binary Threshold method

  
  ret,thresh = cv2.threshold(frame_threshed,127,255,0)
  

Median Blur Filter

  img = cv2.medianBlur(img,5)

Gaussian Filter

blur = cv2.GaussianBlur(img,(5,5),0)

Otsu's Thresholding

  # Otsu's thresholding
  ret2,th2 = cv2.threshold(img,0,255,cv2.THRESH_BINARY+cv2.THRESH_OTSU)

Erosion

import cv2
import numpy as np

img = cv2.imread('j.png',0)
kernel = np.ones((5,5),np.uint8)
erosion = cv2.erode(img,kernel,iterations = 1)

Cuntour Detection

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

Post-processing

Display Text on Image

  cv2.putText(image,"Hello World!!!", (x,y), cv2.FONT_HERSHEY_SIMPLEX, 2, 255)

Crop Image with x,y,w,h

import cv2
img = cv2.imread("lenna.png")
crop_img = img[y:y+h, x:x+w]
cv2.imshow("cropped", crop_img)
cv2.waitKey(0)

Draw a Line on Image

lineThickness = 2
cv2.line(image, (x1, y1), (x2, y2), (0,255,0), lineThickness)

Examples

Find Biggest Color Object OpenCV-2.4

import numpy as np
import cv2

im = cv2.imread('shot.bmp')
hsv_img = cv2.cvtColor(im, cv2.COLOR_BGR2HSV)
COLOR_MIN = np.array([20, 80, 80],np.uint8)
COLOR_MAX = np.array([40, 255, 255],np.uint8)
frame_threshed = cv2.inRange(hsv_img, COLOR_MIN, COLOR_MAX)
imgray = frame_threshed
ret,thresh = cv2.threshold(frame_threshed,127,255,0)
contours, hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)

# Find the index of the largest contour
areas = [cv2.contourArea(c) for c in contours]
max_index = np.argmax(areas)
cnt=contours[max_index]

x,y,w,h = cv2.boundingRect(cnt)
cv2.rectangle(im,(x,y),(x+w,y+h),(0,255,0),2)
cv2.imshow("Show",im)
cv2.waitKey()
cv2.destroyAllWindows()

Capture Live Webcam Feed

import numpy as np
import cv2

cap = cv2.VideoCapture(0)

while(True):
  # Capture frame-by-frame
  ret, frame = cap.read()
  
  # Our operations on the frame come here
  gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
  # Display the resulting frame
  cv2.imshow('frame',gray)

  if cv2.waitKey(1) & 0xFF == ord('q'):
      break
    
# When everything done, release the capture
cap.release()
cv2.destroyAllWindows()

Playing video from file

import numpy as np
import cv2

cap = cv2.VideoCapture('vtest.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()