Tips and typical application examples for using OpenCV in Python

This article isn't specifically about Python, but rather about using OpenCV within the Python environment. It will cover the most relevant basic operations in the data processing phase of deep learning and walk you through four interesting and practical examples:

  • A time-lapse photography app
  • A small program for capturing screenshots from a video
  • An image data augmentation tool
  • An object detection annotation utility

6.1 Introduction to OpenCV

OpenCV is the most widely used open-source computer vision library today. Originally developed in C/C++, it supports multiple platforms including Linux, Windows, macOS, Android, and iOS. Additionally, it offers interfaces for Python, MATLAB, and Java, which has contributed to its popularity in both academic research and industrial applications. Its rich functionality, high performance, and permissive licensing have made it a go-to tool in the field of computer vision.

The project was initiated by Intel in 1998. Gary Bradski, a computer vision engineer at Intel, noticed that many students were developing their own internal code or libraries for vision tasks. This inspired him to create a general-purpose library that could be easily accessed and used by researchers and developers alike. The first alpha version of OpenCV was released at CVPR in 2000, followed by several beta versions before the official release in 2006.

In 2009, when Gary joined Willow Garage, OpenCV received more active development and support. Version 1.1 was released, and by 2010, OpenCV 2.0 introduced a comprehensive C++ interface. Since then, OpenCV has continued to evolve, with major updates like OpenCV 3.0 in 2015, which brought architectural improvements, more algorithms, better performance, and enhanced GPU support. Today, OpenCV is widely used in both research and commercial settings.

6.1.1 Structure of OpenCV

Currently, OpenCV has two main versions: OpenCV2 and OpenCV3. While OpenCV3 offers more features and better usability, this section focuses on OpenCV2 due to its compatibility with deep learning frameworks and ease of use for beginners.

The OpenCV library is divided into several modules based on functionality. Some of the most commonly used ones include:

  • Core: Contains fundamental data structures (like matrices, lines, and shapes) and basic operations.
  • Imgproc: Provides image processing functions such as filtering, resizing, and edge detection.
  • Highgui: Offers basic GUI and file I/O functions, like displaying images and videos.

For video and more specialized visual tasks, OpenCV includes additional modules such as:

  • Video: Includes optical flow and object tracking functions.
  • Calib3d: Used for 3D reconstruction and camera calibration.
  • Features2d: Contains feature detection and matching algorithms like ORB.
  • Objdetect: Provides object detection tools, including cascade classifiers and Latent SVM.
  • ML: Implements traditional machine learning algorithms for vision tasks.
  • Flann: A library for approximate nearest neighbor search, often used with key points.
  • Gpu: Offers GPU-accelerated functions using CUDA.
  • Photo: Includes image restoration and noise reduction techniques.
  • Stitching: Enables panoramic image creation.
  • Nonfree: Contains patented algorithms like SIFT and SURF.
  • Contrib: Experimental modules that may be included in future releases.
  • Lagacy: Old interfaces retained for backward compatibility.
  • Ocl: Uses OpenCL for parallel computing.
  • Superres: Implements super-resolution algorithms.
  • Viz: A 3D visualization module built on VTK.

From a user perspective, OpenCV3 brings more features and finer module organization compared to OpenCV2.

6.1.2 Installing and Using OpenCV

Installing OpenCV on Linux is straightforward. Most distributions offer package managers that can install it directly. For example, on Ubuntu 16.04 LTS, you can run the following commands in the terminal:

sudo apt install libopencv-dev python-opencv

Alternatively, you can download the source code from the official website and compile it manually. First, install the required dependencies:

sudo apt install build-essential cmake git libgtk2.0-dev pkg-config libavcodec-dev libavformat-dev libswscale-dev
sudo apt-get install python-dev python-numpy libtbb2 libtbb-dev libjpeg-dev libpng-dev libtiff-dev libjasper-dev libdc1394-22-dev

Next, clone the OpenCV repository and prepare the build:

git clone https://github.com/opencv/opencv.git
cd opencv
mkdir release
cd release
cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local
make
sudo make install

On Windows, you can download the installer from the OpenCV official website. After installation, locate the `cv2.pyd` file under `/build/python/2.7` and copy it to `\Lib\site-packages`. If you're just experimenting with OpenCV in Python, you can also download the appropriate wheel file from Christoph Gohlke’s site and install it via pip:

https://www.lfd.uci.edu/~gohlke/pythonlibs/#opencv

To use OpenCV in Python, simply import it with:

import cv2

Once imported, you’re ready to start working with images and videos using OpenCV's powerful tools.

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