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added fencer collision project

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Sara Gerretsen 2026-02-11 14:05:24 +01:00
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<p class="project-card-role">C Programmer</p>
</a>
</div>
<!--<h3>Chief Experimentor @ Home</h3>--!>
<h3>Chief Experimentor @ Home</h3>
<div indented>
<div class="project-grid">
<a class="internal" href="projects/fencer-collision.html">
<div class="project-card" fencer-collision></div>
<div class="project-labels">
<p>sdl2</p>
<p>from scratch</p>
<p>solo</p>
</div>
<h2 class="project-title">Fencer Collision</h2>
<p class="project-card-role">C Engine Programmer</p>
</a>
</div>
</div>
</section>
<section>
<h2 id="experience">Experience</h2>

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<!DOCTYPE html>
<html>
<head>
<title>Fencer Collision - Sara Gerretsen</title>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<script type="text/javascript" src="../shared/jquery.min.js"></script>
<link rel="stylesheet" href="../shared/style.css">
<div id="site-header">
<script>$(function(){$("#site-header").load("../shared/header.html");});</script>
</div>
</head>
<body>
<h1>Project Name</h1>
<section class="project">
<h2>Info</h2>
<div indented>
<p>Project Type: SDL2 Custom Toy Engine</p>
<p>Project Timeframe: 2023</p>
</div>
<h2>Project Summary</h2>
<p indented>
The goal of the project was to build, from theory and whitepapers, a simple 2d physics engine that can be used to make games. Mainly as a practice project to teach myself the basics of how physics engines work.
</p>
<h2>Snippets</h2>
<p>
(Linked text directs to the relevant spot in
<a href="https://git.objectionable.solutions/Sara/fencer/src/branch/main/src">the code repository</a>)
</p>
<p>
The start of the project was building a 2-body arbitrary shape collision solver. For this I used the
<a href="https://git.objectionable.solutions/Sara/fencer/src/commit/c3f059eeb78db6a4b60ccdbf5928ad32d3ce55b9/src/collision.c">Separating Axis Theorem</a>,
as it is relatively simple to implement.
</p>
<p>
With a working 2-body solver in hand, the next step was to implement an iterative multi-body solver. But this requires a lot more than a toy SAT implementation. So first I went and figured out world and object representations.
</p>
<h3>World Representation</h3>
<p>
Although I really like using C for quick toy examples, I had now reached the point at which I usually switch to C++, and in hindsight that would've probably been the better option. However, my recent explorations of Rust meant that I had learned of the joy of "Trait"-style polymorphism (most comparable to C++ interfaces). So I wanted to try using those in C. For this I found an approach to implementing typeclasses in C (the article for which isn't online anymore, but I explained my version over on
<a href="https://blog.winter-software.com/2024/02/22/typeclasses-in-c"> my blog</a>).
</p>
<p>
The requirements for any game object to be part of the physics world is that it has to at least implement
<a href="https://git.objectionable.solutions/Sara/fencer/src/commit/c3f059eeb78db6a4b60ccdbf5928ad32d3ce55b9/src/physics_entity.h"><code>PhysicsEntity</code></a>,
which in turn requires
<a href="https://git.objectionable.solutions/Sara/fencer/src/commit/c3f059eeb78db6a4b60ccdbf5928ad32d3ce55b9/src/transformable.h"><code>Transformable</code></a>
to be implemented. As solving collisions requires modifying the transform of the object.
In order to be rendered, and for the update function to be called, the object also has to implement
<a href="https://git.objectionable.solutions/Sara/fencer/src/branch/main/src/behaviour_entity.h"><code>BehaviourEntity</code></a>,
which exposes the object lifetime functions spawn, update, start, and draw. And itself requires
<a href="https://git.objectionable.solutions/Sara/fencer/src/branch/main/src/drop.h"><code>Drop</code></a>
to be implemented. <code>Drop</code> being pretty much the best destructor implementation you can get in C without implementing a garbage collector.
</p>
<p>
With the world represented, the work on the collision solver could start. Objects that implement <code>PhysicsEntity</code> can then register themselves with the
<a href="https://git.objectionable.solutions/Sara/fencer/src/branch/main/src/physics_world.h">physics world</a>.
Once an object is registered to the physics world it will use the SAT to check for collisions, and finally, that comes to the multi-body iterative collision solver.
</p>
<p>
<a href="https://git.objectionable.solutions/Sara/fencer/src/commit/c3f059eeb78db6a4b60ccdbf5928ad32d3ce55b9/src/physics_entity.c#L50">The iterative solver</a>
is implemented in the static functions of the <code>PhysicsEntity</code> typeclass.
</p>
<p>
This all then comes together in something like the
<a href="https://git.objectionable.solutions/Sara/fencer/src/branch/archive/platformer-prototype/src/player.h"><code>Player</code></a>
struct, which implements all these typeclasses and uses them to create a platformer character. Or the
<a href="https://git.objectionable.solutions/Sara/fencer/src/branch/archive/platformer-prototype/src/tilemap.c"><code>Tilemap</code></a>
object that can be loaded from <a href="https://ldtk.io/" target="_blank">LDTK</a> levels to create a renderable physics entity that can be hit and interacted with by the player.
</p>
</section>
</body>
</html>

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<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<script type="text/javascript" src="../shared/jquery.min.js"></script>
<link rel="stylesheet" href="../shared/style.css">
<div id="site-header">
<script>$(function(){$("#site-header").load("../shared/header.html");});</script>
</div>
</head>
<body>
<h1>Project Name</h1>
<section class="project">
<h2>Info</h2>
<div indented>
<p>Project Type: Engine, Genre, Application</p>
<p>Project Timeframe: Length, Year</p>
</div>
<h2>Product Overview</h2>
<p indented>
Description of the game.
</p>
<h2>Project Overview</h2>
<p indented>
Description of role in development.
</p>
<h2>Postmortem</h2>
<p>Thoughts on the development process and what I learned from it.</p>
</section>
</body>
</html>