MATLAB Examples

Tracking a Green Ball using Web Camera

This example shows you how to use MATLAB to process images captured from a web camera on BeagleBone Black® board to track a green ball.

Contents

Introduction

The MATLAB Support Package for BeagleBone Black Hardware allows you to capture images from the web camera and bring those right into MATLAB for processing. Using this capability we will develop an ball tracking algorithm.

Prerequisites

Required Hardware

To run this example you will need the following hardware:

  • BeagleBone Black hardware
  • A 5V power supply
  • A web camera, e.g. Logitech Webcam C600

Create a Web Camera object

Connect a compatible USB camera to the USB host port of your BeagleBone Black hardware. Note that some USB cameras draw too much current from the USB port of BeagleBone Black hardware and may not work properly. Use a powered USB hub in such cases. Make sure that the AvailableWebcam property of the beaglebone object shows the USB camera. If you do not see the USB camera, your USB camera is not recognized properly. Try rebooting your BeagleBone Black with the camera attached.

Create a web camera object by executing the following commands on the MATLAB prompt.

bbb = beaglebone;

Check the AvailableWebcams property of the bbb object to find the web camera name. Create webcam object using this name. Without specifying the camera name, it uses the one found automatically.

cam = webcam(bbb);

The cam is a handle to a webcam object. Let's display the images captured from web camera in MATLAB.

for i = 1:100
    img = snapshot(cam);
    imagesc(img);
    drawnow;
end

Extract color components

We will extract three 2D matrices from the 3D image data corresponding to the red, green, and blue components of the image. Before proceeding with the rest of the example, we will load a saved image. We will make sure our algorithm works on the test image before moving on to live data.

img = imread('tennis_ball.jpg');
imagesc(img);
r = img(:, :, 1);
g = img(:, :, 2);
b = img(:, :, 3);

Calculate green

Next we approximate the intensity of the green component

justGreen = g - r/2 - b/2;

Threshold the green image

We threshold the image to find the regions of image that we consider to be green enough.

bw = justGreen > 40;
imagesc(bw);

Find center

Find the center of the image and mark it with a dot.

[x, y] = find(bw);
if ~isempty(x) && ~isempty(y)
    xm = round(mean(x));
    ym = round(mean(y));
    xx = max(1, xm-5):min(xm+5, size(bw, 1));
    yy = max(1, ym-5):min(ym+5, size(bw, 2));
    bwbw = zeros(size(bw), 'uint8');
    bwbw(xx, yy) = 255;
end
imagesc(justGreen + bwbw);

Run detection algorithm on live data

We can create a MATLAB function, trackball.m, out of the MATLAB code we developed in the previous sections of this example. View the MATLAB function in the editor.

edit('trackball.m');

The function trackball() takes an image and a threshold for green detection and returns the results of green detection algorithm. We will call this function on the images captured in a loop. Before running the MATLAB code snippet below, get hold of a tennis ball and place it in the view of the BeagleBone Black Web Camera. While the MATLAB code is running, move the ball around.

figure;
for i = 1:200
    [img, bw] = trackball(snapshot(cam), 40);
    subplot(2, 1, 1);
    imagesc(img);
    subplot(2, 1, 2);
    imagesc(bw);
    drawnow;
end

Summary

This example introduced the an application example where images coming from BeagleBone Black with a Web Camera are processed with a simple green detection algorithm. This algorithm has subsequently been used to track a tennis ball.