How to handle Data, and Images(4) Introduction To OpenCV

Lesson Notes in .ipynb file

How to handle Data, and Images(4) - Introduction To OpenCV

Topics

• How to upload files in Colab
• OpenCV’s basic functions
• How to output an Image in CoLab
• Summary

How to upload files in Colab

Before we dive into how to use OpenCV library, we should know how to upload images in Google CoLab. There are various methods to upload a file, but this instance we’re going to be using this method

from google.colab import files
uploaded = files.upload()

Once you run the code, you should be able to upload a file

OpenCV’s basic functions

OpenCV is an open source library that is commonly used for computer vision, image handling, video handling. You can use this library in C, C++, and Python.

There are 3 basic functions that we must know to use OpenCV.

• cv2.imread(file_name, flag): Loads an image from a file
  • file_name: a name of a file that we’re tyring to read
  • flag: sets flag on how to read images
    • IMREAD_COLOR: If set, always convert image to the 3 channel BGR color image.
    • IMREAD_GRAYSCALE: If set, always convert image to the single channel grayscale image (codec internal conversion).
    • IMREAD_UNCHANGED: If set, return the loaded image as is (with alpha channel, otherwise it gets cropped). Ignore EXIF orientation.
• cv2.imshow(title, image): Displays an image
  • title: title of the window file that pops up
  • image: image that you want to output
• cv2.cvtColor(image, flag): converts the image’s color
  • image: image file
  • flag: sets flag on converting color

How to output an Image in CoLab

Now that we know how to load, display, and convert the color of image, we must know how to display this image in Google CoLab. If you’re just using Python to do this, then we can just use cv2.imshow, but CoLab does not allow this.

We must use Matplotlib to output an image for CoLab.

However, since OpenCV uses BGR for images, and Matplotlib uses RGB, we must convert the color to output an image using Matplotlib

import cv2
import matplotlib.pyplot as plt

# laods an image file 'chanel.jpg' and name it img_basic
img_basic = cv2.imread('chanel.jpg', cv2.IMREAD_COLOR)
# since CV2 used BGR, we need to convert the color to RGB using cvtColor for Matplotlib
plt.imshow(cv2.cvtColor(img_basic, cv2.COLOR_BGR2RGB))
plt.show()

# convert the color to gray
img_basic = cv2.cvtColor(img_basic, cv2.COLOR_BGR2GRAY)
# since CV2 uses BGR, we need to convert the color to RGB using cvtColro for Matplotlib
plt.imshow(cv2.cvtColor(img_basic, cv2.COLOR_GRAY2RGB))
plt.show()

Output:

Summary

• To upload a file in CoLab use:

  from google.colab import files
  uploaded = files.upload()
  

• cv2.imread: Loads an image from a file

• cv2.imshow: Displays an image

• cv2.cvtColor: converts the image’s color

• Since CoLab must use Matplotlib to output an image, we must convert the color of BGR from OpenCV to RGB for Matplotlib