Most useful OpenCV functions to know for image analytics

opencv tutorial python

If you want to work or learn computer vision with Python, you must know OpenCV. It is an open Source Computer Vision Library that has 2500 plus algorithms. In this OpenCV tutorial, I will show you some basic OpenCV functions which you must know if you are starting python OpenCV.

Install OpenCV: Before you start as you know you need to install OpenCV by typing pip install opencv-python

How to read image python OpenCV

Let’s say you have an image in a local folder and you want to read that image and you also want to show that image using OpenCV.

# OpenCV imread Python
import cv2
# Load image using 'IMREAD'
img = cv2.imread("data/test_image.jpg")
# DISPLAY
cv2.imshow("Opencv show image python",img)
cv2.waitKey(0) # 0 for wait image window forever, 1 for wait 1 miliseconds, 2 for 2 miliseconds...

Read video OpenCV python

The below code is to read video frames in OpenCV and play the video. Now the idea of reading video is to read video frame by frame as image and show those images frame by frame. That is how an entire video will play.

# Play video in python
import cv2

# Below function will read video frames
cap = cv2.VideoCapture('data/test_video.mp4')

while True:
    read_ok, img = cap.read()
    cv2.imshow("Play video in python", img)

    # Close video window by pressing 'x'
    if cv2.waitKey(1) & 0xFF == ord('x'):
        break

line: 8 => Read each video frame from vcap variable and store into img. If reading is successful, it will declare read_ok variable as true else it will declare read_ok as false.

line: 9 => Show each each video frame as image

line: 12 => Close video window by pressing ‘x

Read webcam OpenCV python

If you want to capture your webcam video, you can use same above code. You just need replace video file path with 0.

0 for your primary or default webcam. If you have two cameras, the value will be 1, 2 etc. I am using my default webcam of my laptop.
So I am keeping this value as 0.

# Play video in python
import cv2

# Below function will read video frames
cap = cv2.VideoCapture(0)

while True:
    read_ok, img = cap.read()
    cv2.imshow("Play video in python", img)

    # Close video window by pressing 'x'
    if cv2.waitKey(1) & 0xFF == ord('x'):
        break

Morphological Transformation of image in OpenCV python

Morphological transformation is a commonly used feature extraction technique in image processing. Two basic morphological operators are Dilation and Erosion.

  • Dilation: Expand edges with white. convert a pixel element to ‘1’ if at least one pixel under the kernel is ‘1’. It is done by kernel slide (2d convolution)
  • Erosion: It is opposite to dilation. It is used for sharping the edges. It erodes away the boundaries of the foreground object. It converts a pixel to 1 only if all the pixels under the kernel are 1, otherwise convert that pixel to 0.

To do those morphological operations you can first convert normal image to grayscale image. In this OpenCV tutorial, I will also show you how you can detect edges of an image using canny edge detection.

Note: There are so many algorithms to detect edge, in this tutorial I will only show you canny edge detection.

import cv2
import numpy as np
# LOAD AN IMAGE USING 'IMREAD'
img = cv2.imread("data/Cristiano_Ronaldo_256.jpg")
# DISPLAY
cv2.imshow("Normal image",img)

# Convert image to grayscale python opencv
img_Gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
cv2.imshow("Grayscale image",img_Gray)

# gaussianblur opencv python
img_Blur = cv2.GaussianBlur(img_Gray,(7,7),0)
cv2.imshow("Blur image",img_Blur)

# canny edge
img_Canny = cv2.Canny(img_Blur,100,100)
cv2.imshow("Canny edge image",img_Canny)

# Dilation of edges
kernal_mat = np.ones((5,5),np.uint8)
img_dilate = cv2.dilate(img_Canny,kernal_mat , iterations=1)
cv2.imshow("Dilation image",img_dilate)

# Erosion of edges
img_erode = cv2.erode(img_dilate, kernal_mat, iterations=1)
cv2.imshow("Erosion image", img_erode)
print(img.shape)

cv2.waitKey(0)

Resize image in OpenCV

Before starting resizing of image in opencv, let me explain how opencv see an image.

Normally in mathematics +ve X in the east direction and +ve Y in the north direction (which is upward). But in Opencv +ve X in the same direction (east) and +ve Y in the opposite direction (south), which is downwards.

Now if shape of our image is (256, 256).

python opencv tutorial

Now to resize image, let’s say you have an image of any shape and you want to reshape that image to (256, 256) pixel. Same thing you can do to increase size of your image. But it will not increase quality of the image, it will only increase size of the image.

import cv2

# # Load Normal image using 'IMREAD'
img = cv2.imread("data/Cristiano_Ronaldo.jpg")

# Defining desired shape
fWidth = 256
fHeight = 256

# Resize image in opencv
img_resize = cv2.resize(img, (fWidth, fHeight))

print("Normal image shape")
print(img.shape)

print("Resized image shape")
print(img_resize.shape)

cv2.imshow("Normal image", img)
cv2.imshow("Resized image", img_resize)

cv2.waitKey(0)

Crop image in OpenCV

Now to crop image in Python OpenCV you don’t need any function as image itself is a matrix. To crop image in OpenCV you just need to define starting and ending value of X and starting and ending value of Y from that image matrix.

Let’s say you want to crop your image vertically half. That means you have to keep all values of X (width) but half of the value of Y (height) from the image matrix.

Crop image opencv python
# OpenCV imread Python
import cv2
# LOAD AN IMAGE USING 'IMREAD'
img = cv2.imread("data/Cristiano_Ronaldo_256.jpg")

# Shape of the image
print(img.shape)

# First value of shape is height of the image
img_height = img.shape[0]
# Half height
half_height = int(img_height/2)

# Second value of shape is height of the image
img_width = img.shape[1]

# Cropping image vertically
img_cropped = img[0:half_height, 0:img_width]

cv2.imshow("Normal image", img)
cv2.imshow("Cropped image", img_cropped)

cv2.waitKey(0)

Conclusion

In this OpenCV tutorial you learned most useful basic functions which you need to use everyday if you want to learn computer vision with Python.

  • How to read image python OpenCV
  • Read video OpenCV python
  • Read webcam python OpenCV
  • Morphological Transformation of image in python OpenCV
  • Resize image in Opencv python
  • Crop image in OpenCV

If you have any question or suggestion regarding this topic see you in comment section. I will try my best to answer.

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