# Plotting Specific Points in Matlab - A Detailed Guide and Its Applications

In this comprehensive article, we will delve into the world of plotting specific points using the powerful software tool, Matlab. We will uncover its key features for point visualization, discuss diff …

Updated November 26, 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 comprehensive article, we will delve into the world of plotting specific points using the powerful software tool, Matlab. We will uncover its key features for point visualization, discuss different types of plots and techniques involved in plotting, and provide practical code samples to demonstrate how you can utilize these methods effectively. Furthermore, we will examine how this knowledge can be leveraged to enhance our understanding in the field of Computer Vision.

## Introduction

Matlab is an extensive programming environment for technical computing, which offers various tools for data analysis, visualization, and algorithm development. It has become increasingly popular due to its advanced features that cater to a wide range of applications in fields such as engineering, finance, and computer vision. One such feature that we will explore in depth here is plotting specific points using the software’s graphing capabilities.

## Why Plot Specific Points?

Plotting specific points can serve multiple purposes, depending on your area of work. In the context of computer vision, plotting key points, or landmarks, such as corners in an image, can be useful for tracking motion and understanding the structure of a scene. These points also play a critical role in various geometric transformations like rotations, scaling, or translations that are required when dealing with digital images.

In this article, we will cover several aspects related to plotting specific points using Matlab:

- Types of plots for point visualization
- Understanding coordinate systems and their relevance in plotting points
- Techniques for generating different types of graphs
- Practical examples to demonstrate the implementation process
- How the knowledge can be applied to Computer Vision applications

## Types of Plots for Point Visualization

Matlab offers several plotting options that can aid in effectively visualizing points and other data:

Scatter Plot: A scatter plot is a graphical representation where each data point is connected by lines or curves, typically with no specific order between the pairs of points. The scatter plot is useful for illustrating relationships between variables, highlighting clusters within datasets, and identifying outliers.

Line Plot: A line plot displays a series of data points connected sequentially by straight lines. It is helpful in depicting trends and changes over time, as well as providing insights into the underlying dynamics of the given dataset.

Polar Plot: Polar plots display data along the radial axis, with values increasing or decreasing uniformly from the origin (center) to the outer edge of a polar coordinate system. These plots are useful for visualizing periodic phenomena and identifying patterns that may not be evident in other types of graphs.

Stem Plot: In a stem plot, data points are connected by lines, and each point is represented as a vertical line with an associated value above it. This type of plot is particularly beneficial for understanding the distribution of values within discrete or continuous datasets.

Pie Chart: A pie chart illustrates how different parts contribute to a whole, displaying data in the form of arcs or slices of a circle. It can effectively visualize proportions between various elements and their relative size compared to other components within a dataset.

## Coordinate Systems for Point Plotting

To plot specific points using Matlab, you need to understand the basic coordinate system: the Cartesian Plane. The Cartesian plane is a two-dimensional space where each point can be represented as an ordered pair of real numbers (x, y) or complex numbers (z, w). In this article, we will primarily work with two-dimensional points, which means the coordinates consist of x and y values.

To plot a point in Matlab, you need to convert these coordinates into a format that can be understood by the software. This can be done by specifying the x and y coordinates as separate vectors or combining them in a single vector. In both cases, the first element corresponds to the x-coordinate, while the second element represents the y-coordinate.

## Techniques for Generating Different Types of Graphs

Now that you are familiar with plot types and coordinate systems, let’s delve into how you can use Matlab functions to generate different types of graphs:

- Scatter Plot: To create a scatter plot in Matlab, you can utilize the ‘plot’ function along with the ‘.’, which represents points by default. For instance, consider the following code to create a scatter plot of a given set of data using x and y coordinates:

```
x = [1 2 3 4];
y = [5 8 7 6];
plot(x,y,'.', 'MarkerSize', 10)
```

The above code creates a scatter plot connecting the given set of data points by lines and makes each point have a size of 10.

- Line Plot: To create a line plot using Matlab, you can use the ‘plot’ function without any additional parameter, as it uses the default option which is to connect points by straight lines. For instance, consider the following code for creating a line plot using x and y coordinates:

```
x = [1 2 3 4];
y = [5 8 7 6];
plot(x,y)
```

- Polar Plot: Creating a polar plot in Matlab is similar to the other types of plots but with an additional parameter for specifying the ‘LineWidth’ and a different function called ‘polar’. Here is an example of generating a simple polar plot using theta and r values:

```
theta = linspace(0,2*pi,10);
r = sin(theta);
polar(theta,r,'LineWidth',3)
```

In this code, we use the ‘linspace’ function to generate an equidistant vector of values in the interval [0, 2*pi]. The sine function generates a corresponding set of y-values (r), and then we plot these on a polar graph with a line width of 3.

- Stem Plot: To create a stem plot in Matlab, you can utilize the ‘stem’ function that has various options to specify the data input format, color, line style, etc. Here is an example code for generating a basic stem plot using x and y coordinates:

```
x = [1 2 3 4];
y = [-5 -3 7 1];
stem(x,y)
```

In this case, the stem plot creates a line for each point in the given set of data (x and y), connected by lines and with different marker symbols to distinguish between individual points.

- Pie Chart: Matlab has a specific function called ‘pie’ that can be used to create pie charts. To generate a basic pie chart, you need to have a vector containing the relative frequencies of each element or a matrix with corresponding values for each category in the dataset. In this example, we create a simple pie chart using the following code:

```
values = [5 7 3 6];
labels = ['A' 'B' 'C' 'D'];
pie(values, labels)
```

In this case, the given vector ‘values’ represents the relative frequencies of elements ‘A’, ‘B’, ‘C’, and ‘D’. The second argument, ‘labels’, contains their corresponding category names. Matlab will create a pie chart with each value as a proportionate slice in the circle, and label each slice using the associated category name from the given vector.

## Practical Examples and Applying Knowledge to Computer Vision Applications

In addition to the examples presented above, let’s look at a couple of more complex scenarios where plotting specific points comes into play in computer vision applications:

Visualizing Edge Points: Edges are discontinuities in images that can help identify objects or boundaries within an image. Matlab provides various functions and algorithms for edge detection, such as the Canny algorithm, which can be used to identify these edge points in images. Once the edges have been found, you can plot them using the techniques outlined earlier (i.e., using a scatter plot, line plot, or other relevant options).

Detecting Interest Points: Feature detection is a crucial step in understanding the structure of an image and tracking objects over time. Matlab provides several functions for detecting feature points within images, such as SIFT (Scale-Invariant Feature Transform) and SURF (Speeded-Up Robust Features), which can be used to identify significant points that may serve as landmarks or reference points in the scene. By plotting these detected points on an image, we can better visualize their locations for understanding geometric transformations or tracking motion more effectively.

In summary, this detailed article has provided a comprehensive guide on plotting specific points using Matlab’s graphing capabilities. This knowledge is instrumental in numerous technical and scientific fields, especially when it comes to data analysis, computer vision applications, and visualizing crucial information for improved decision-making and problem-solving. By exploring the different types of plots and techniques available within Matlab, you are now equipped with the necessary tools to make use of these points for enhancing your work in various domains.