# Effortlessly Plot an Ellipse using MATLAB - A Detailed Guide for Computer Vision Enthusiasts

In this article, we will delve into the fascinating world of computer vision and its applications with the help of a powerful programming language, MATLAB. We will cover the fundamentals of how to cre …

Updated October 29, 2023

*Hey! If you love Computer Vision and AI, let's connect on Twitter or LinkedIn. I talk about this stuff all the time!*In this article, we will delve into the fascinating world of computer vision and its applications with the help of a powerful programming language, MATLAB. We will cover the fundamentals of how to create and understand ellipses as an essential tool for analyzing images, and then progress towards plotting them using MATLAB’s various graphical functions and visualization techniques. The aim is to provide you with valuable insights on implementing computer vision concepts in MATLAB, particularly when working with 2D shapes like ellipses.

Introduction

An ellipse is a conic section, a generalization of the circle that is created by intersecting a plane and a cone. It plays an essential role in computer vision due to its unique characteristics as a curved shape that can be described by various parameters. Ellipses are widely used for modeling 2D shapes in images, such as ovals or elongated circles, which often occur in scenes involving natural objects like planets, the sun, and biological structures.

In this article, we will learn how to utilize MATLAB’s capabilities for plotting and analyzing ellipses effectively. We will explore key concepts of computer vision and the role it plays in image processing. Along the way, we will incorporate various graphical functions provided by MATLAB to create an interactive, informative environment that demonstrates the use of 2D shapes like ellipses in MATLAB programs.

Plotting Ellipses Using MATLAB: Fundamentals and Concepts

Before diving into the process of plotting an ellipse using MATLAB, it is crucial to grasp a basic understanding of its components and mathematical properties. An ellipse can be defined by a set of parameters such as semi-major axis (a), semi-minor axis (b), eccentricity (e), eccentric anomaly (E), true anomaly (v), and focal distances (f1, f2). These parameters help in specifying the size, shape, orientation, and position of an ellipse.

- Ellipse Equation in Polar Form

An ellipse can be represented mathematically as a set of points that satisfy the equation:

(x/a)^2 + (y/b)^2 = 1

where (a, b) are the lengths of semi-major and semi-minor axes, respectively. This formulation shows how an ellipse is essentially an affine transformation of a circle in polar coordinates.

- Elliptic Functions for Ellipse Properties

Elliptic functions are mathematical operations used to describe the relationship between angles within an ellipse. They are essential for calculating various properties of an ellipse, such as its area, eccentric anomaly (E), and true anomaly (v). Some common elliptic functions include:

a) Elliptic integral of the first kind: F(t) or F(u, v) b) Elliptic integral of the second kind: E(u) c) Incomplete elliptic integrals: Pi(x), Psi(x, y) d) Complete elliptic integral of the first and second kinds: K and E

- Ellipse Properties for Computer Vision Applications

In the realm of computer vision, ellipses are primarily used as a modeling tool to analyze various shapes and patterns within images. They play an essential role in object detection, shape recognition, and image segmentation tasks. Some common applications of elliptical analysis include:

a) Identifying and tracking objects with elongated or oval-shaped characteristics (e.g., insects, planets, etc.) b) Measuring the size and shape parameters to understand an object’s characteristics c) Comparing ellipse properties to find similarities between different shapes d) Segmenting images based on the presence of specific elliptical objects or patterns

Plotting Ellipses in MATLAB: Code Samples and Techniques

To plot an ellipse using MATLAB, we will leverage its graphical functions, such as the “plot” command, which draws lines and curves, and the “patch” function, which plots filled shapes. Furthermore, the “axis equal” command can be used to ensure that all axes are drawn with the same scale.

- Create an Ellipse Using MATLAB’s “plot” Function

To begin plotting ellipses in MATLAB, let’s take advantage of its built-in plot function for drawing curves and lines. By defining the major and minor axes (a and b), along with the orientation angle (theta), we can create an arbitrary ellipse shape. Here is a basic code snippet to create an ellipse using the “plot” function:

% Define semi-major axis, semi-minor axis, and orientation angle a = 4; % Length of semi-major axis b = 2; % Length of semi-minor axis theta = pi/3; % Orientation angle in radians (optional to determine shape)

% Create an ellipse using the “plot” function
ellipse_handles = plot(a*cos(0:pi/180:2*pi), b*sin(0:pi/180:2*pi), ‘r’, …
‘LineWidth’, 3, ‘DisplayName’, ‘Ellipse’);
set(get(ellipse_handles,‘Parent’),‘Units’,‘Normalized’,…
‘Position’,[0.16,0.16,0.75,0.75]); % Adjust the position and size of plot

- Create a Filled Ellipse Using MATLAB’s “patch” Function

To create a filled ellipse in MATLAB, we can use its “patch” function with appropriate parameters for shape, face color, and edge color:

% Define semi-major axis, semi-minor axis, orientation angle, and other properties a = 4; % Length of semi-major axis b = 2; % Length of semi-minor axis theta = pi/3; % Orientation angle in radians (optional to determine shape) color_face = [1 0.6 0.5]; % Color for filled area (RGB format) color_edge = ‘k’; % Color for the ellipse’s boundary

% Create a filled ellipse using the “patch” function
filled_ellipse_handles = patch([a*cos(0:pi/180:2*pi), b*sin(0:pi/180:2*pi)],…
[a*sin(0:pi/180:2*pi), -b*cos(0:pi/180:2*pi)], color_face, ‘FaceColor’, color_face,…
‘EdgeColor’, color_edge, ‘LineWidth’, 3);

% Adjust the position and size of filled ellipse plot set(get(filled_ellipse_handles,‘Parent’),‘Units’,‘Normalized’,… ‘Position’,[0.16,0.16,0.75,0.75]);

Conclusion

In this comprehensive article on plotting ellipses using MATLAB, we have explored the fundamentals of computer vision concepts and how ellipses are utilized in various image processing tasks. We went through the mathematical properties and definitions behind ellipses and delved into their applications within computer vision. We also provided detailed code samples illustrating different techniques for creating ellipses using both the “plot” and “patch” functions in MATLAB, along with additional graphical options like adjusting the position and size of the plots.

Ultimately, understanding how to leverage the power of MATLAB’s tools to work with shapes such as ellipses will open a world of possibilities for advanced image processing, analysis, and object recognition in your projects and research endeavors.