Tag Archive for 'collision detection'

PHP OOP Game Coding: Collision Detection

ropeDistance in 2D Space

For a number of years I’ve had David Bourg’s book, Physics for Game Developers (2002, O’Reilly), and I’ve been meaning to translate a set of formulas into OOP classes that could be used as part of a PHP game development library. After spending time on (simple) game development last summer using Python, I decided it was time to get busy with a similar project using more OOP and PHP. I wanted something that was small enough to run on Raspberry Pi computers, but still an animated video game.

On previous posts on this blog I’ve used SVG graphics with PHP, but the examples I used were fairly static. Here I’d like to try them in a more dynamic role to see if PHP could generate code to make them dance. For starters I thought that a simple 2D space game would be appropriate — more on the order of Astroids than Space Aliens.

2D Outer Space on a Grid: Plane Geometry

In order to get anywhere, I decided that the universe (galaxy, solar system, whatever; you choose) would live on a 500 x 400 grid. It can be adjusted for different screens, but the first step is to set up a common grid for clear discussion. Further, I thought that starting with rectangles as ‘space ships’ would make everything else easier. (You can build something more elaborate later in the series.) The two space crafts are Oopz and Titeaz. Oopz is crewed by OOP developers, and Titeaz has a crew of sequential and procedural programmers who keep getting in trouble because of spaghetti knots and tight bindings. The Oopz goes on rescue missions to send them PHP code packages of classes and design patterns. Figure 1 shows the initial positions of the two ships:

Figure 1: Grid with Oopz and Titeaz

Figure 1: Grid with Oopz and Titeaz

Each of the grid squares in Figure 1 is 50 x 50 pixels, and the space ships use conventional a x|y position denotation.

Determining Distance and Collision Detection

The first thing we’ll tackle in Rocket Science 101 is determining the distance between two objects.

Raspberry Pi Users: You will need the Chromium browser for the graphics in this series. You can download it using the following code:
sudo apt-get install chromium

The distance between objects can be used for everything from determining when two objects have collided (distance = 0 + fudge-factor) to when another ship is in rescue range to receive project-saving OOP code. The SVG objects on your screen (without the grid) can be seen in Figure 2:

Figure 2: Determining Distance

Figure 2: Determining Distance

The code for this starting screen is based on the SVG W3 standards and saved as an XML file:

< ?xml version="1.0" standalone="no"?>
< !DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd"> 
<svg width="500" height="400" viewBox="0 0 500 400"
     xmlns="http://www.w3.org/2000/svg" version="1.1">
<desc>Oopz and Titaz</desc>
<!-- Show outline of canvas using 'rect' element -->
<rect x="0" y="0" width="500" height="400"
        fill="#DCDCDC" stroke="blue" stroke-width="1"></rect>
<!-- Space craft Oopz -->
<rect x="100" y="100" width="30" height="20"
        fill="#cf5300" stroke="#369" stroke-width=".4"></rect>
<!-- Space craft Titeaz -->
<rect x="300" y="200" width="30" height="20"
        fill="#369" stroke="#00cc00" stroke-width=".4"></rect>

To see the distance calculation, click the Play button. See if you can figure out what formula is used before you look at the code:


The calculations are based on one of the most fundamental theorems in plane geometry. Before continuing, see if you can figure it out and resolve the solution.
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