OpenCV & Python Basics Cheat Sheet

1. Installation

pip install opencv-python opencv-python-headless  

Verify Installation:

import cv2
print(cv2.__version__)  

2. Import OpenCV

import cv2  

3. Reading and Displaying an Image

img = cv2.imread('image.jpg')  
cv2.imshow('Image', img)  
cv2.waitKey(0)  # Wait for a key press  
cv2.destroyAllWindows()  # Close the window  

4. Writing and Saving an Image

cv2.imwrite('output.jpg', img)  

5. Resizing an Image

resized = cv2.resize(img, (300, 200))  
cv2.imshow('Resized', resized)  
cv2.waitKey(0)  

6. Converting to Grayscale

gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)  
cv2.imshow('Grayscale', gray)  
cv2.waitKey(0)  

7. Drawing Shapes

Draw a Line:

cv2.line(img, (50, 50), (200, 50), (0, 255, 0), 3)  

Draw a Rectangle:

cv2.rectangle(img, (60, 60), (200, 150), (255, 0, 0), 2)  

Draw a Circle:

cv2.circle(img, (150, 150), 40, (0, 0, 255), -1)  # -1 fills the circle  

Put Text on Image:

cv2.putText(img, 'Hello OpenCV', (50, 250), cv2.FONT_HERSHEY_SIMPLEX, 1, (255, 255, 255), 2)  

8. Image Filtering

Blurring an Image:

blur = cv2.GaussianBlur(img, (15, 15), 0)  

Edge Detection (Canny):

edges = cv2.Canny(img, 100, 200)  
cv2.imshow('Edges', edges)  
cv2.waitKey(0)  

9. Rotating and Flipping

rotated = cv2.rotate(img, cv2.ROTATE_90_CLOCKWISE)  
flipped = cv2.flip(img, 1)  # 1 = Horizontal, 0 = Vertical  

10. Cropping an Image

cropped = img[50:200, 100:300]  # [y1:y2, x1:x2]  
cv2.imshow('Cropped', cropped)  
cv2.waitKey(0)  

11. Video Capture from Webcam

cap = cv2.VideoCapture(0)  # 0 for webcam  

while True:  
    ret, frame = cap.read()  
    cv2.imshow('Webcam', frame)  
    if cv2.waitKey(1) & 0xFF == ord('q'):  # Press 'q' to exit  
        break  

cap.release()  
cv2.destroyAllWindows()  

12. Video Writing (Save to File)

cap = cv2.VideoCapture(0)  
fourcc = cv2.VideoWriter_fourcc(*'XVID')  
out = cv2.VideoWriter('output.avi', fourcc, 20.0, (640, 480))  

while cap.isOpened():  
    ret, frame = cap.read()  
    if ret:  
        out.write(frame)  
        cv2.imshow('Recording', frame)  
        if cv2.waitKey(1) & 0xFF == ord('q'):  
            break  
    else:  
        break  

cap.release()  
out.release()  
cv2.destroyAllWindows()  

13. Face Detection (Haar Cascade)

face_cascade = cv2.CascadeClassifier(cv2.data.haarcascades + 'haarcascade_frontalface_default.xml')  
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)  
faces = face_cascade.detectMultiScale(gray, 1.1, 4)  

for (x, y, w, h) in faces:  
    cv2.rectangle(img, (x, y), (x + w, y + h), (255, 0, 0), 2)  

cv2.imshow('Face Detection', img)  
cv2.waitKey(0)  

14. Key OpenCV Functions

15. Morphological Operations

kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (5, 5))  
erosion = cv2.erode(img, kernel, iterations=1)  
dilation = cv2.dilate(img, kernel, iterations=1)  

16. Drawing Contours

gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)  
ret, thresh = cv2.threshold(gray, 127, 255, 0)  
contours, _ = cv2.findContours(thresh, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)  
cv2.drawContours(img, contours, -1, (0, 255, 0), 3)  
cv2.imshow('Contours', img)  
cv2.waitKey(0)  

17. Thresholding

_, threshold = cv2.threshold(gray, 127, 255, cv2.THRESH_BINARY)  
cv2.imshow('Threshold', threshold)  
cv2.waitKey(0)  

Example: Image Pipeline (Blurring & Edges)

img = cv2.imread('image.jpg')  
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)  
blur = cv2.GaussianBlur(gray, (5, 5), 0)  
edges = cv2.Canny(blur, 50, 150)  
cv2.imshow('Pipeline Result', edges)  
cv2.waitKey(0)  

OpenCV provides powerful tools for image processing, computer vision, and video analysis.