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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<link rel="stylesheet" href="/index.css">
<title>Physics for Games</title>
<link rel="shortcut icon" href="/favicon/favicon.ico" type="image/x-icon">
<meta name="description" content="A place to learn all about real-time physics simulations through descriptions, code snippets, and example programs all written in C++ and OpenGL.">
<meta name="og:description" content="A place to learn all about real-time physics simulations through descriptions, code snippets, and example programs all written in C++ and OpenGL.">
</head>
<body>
<header>
<h1><a title="physicsforgames.com" href="/">Physics for Games</a></h1>
</header>
<main>
<nav>
<ul class="outer-tree">
<li><a href="/">Introduction</a></li>
<li>
<span>🏀<span>2D</span></span>
<ul class="inner-tree">
<li><label>Rigidbody</label></li>
<li><a title="/2d/rigidbody/rigidbody_1.html" href="/2d/rigidbody/rigidbody_1.html">Linear Forces</a></li>
<li><a title="/2d/rigidbody/rigidbody_2.html" href="/2d/rigidbody/rigidbody_2.html">Rotational Forces</a></li>
<li><a title="/2d/rigidbody/rigidbody_3.html" href="/2d/rigidbody/rigidbody_3.html">Collisions</a></li>
<li><label>Collisions</label></li>
<li><a title="/2d/_collisions/rectangle_rectangle.html" href="/2d/_collisions/rectangle_rectangle.html">Rectangle-Rectangle</a></li>
<li><a title="/2d/_collisions/polygon_polygon.html" href="/2d/_collisions/polygon_polygon.html">Separating Axis Theorem</a></li>
<li><label>Softbody</label></li>
<li><a title="/2d/softbody/softbody_1.html" href="/2d/softbody/softbody_1.html">Springs</a></li>
<li><a title="/2d/softbody/softbody_2.html" href="/2d/softbody/softbody_2.html">Springy Rectangle</a></li>
</ul>
</li>
<li>
<span>🌠<span>3D</span></span>
<ul class="inner-tree">
<li><label>Rigidbody</label></li>
<li><a title="/3d/rigidbody.html" href="/3d/rigidbody.html">Rigidbody in 3D</a></li>
</ul>
</li>
<li>
<span>🔧<span>WebAssembly</span></span>
<ul class="inner-tree">
<li><a title="/intro/intro.html" href="/intro/intro.html">Introduction</a></li>
</ul>
</li>
<li>
<span>🛈<span>About</span></span>
<ul class="inner-tree">
<li><a title="/roadmap.html" href="/roadmap.html">Roadmap</a></li>
</ul>
</li>
</ul>
</nav>
<script src="./softbody_1/dist/output.js"></script>
<script>
window.onload = function() {
// -- Play/Stop Logic
function addButtonListener(pPlay, pStop, pDisableElementList) {
var lPlayElement = document.getElementById(pPlay),
lStopElement = document.getElementById(pStop);
lPlayElement.addEventListener('click', function() {
lPlayElement.style.display = 'none';
lStopElement.style.display = 'block';
pDisableElementList.forEach(function(element) {
element.disabled = true;
});
});
lStopElement.addEventListener('click', function() {
lStopElement.style.display = 'none';
lPlayElement.style.display = 'block';
pDisableElementList.forEach(function(element) {
element.disabled = false;
});
});
}
addButtonListener('gl_canvas_play_undamped', 'gl_canvas_stop_undamped', [
document.getElementById('undamped_spring_length'),
document.getElementById('undamped_start_position'),
document.getElementById('undamped_spring_constant'),
document.getElementById('undamped_spring_mass')
]);
addButtonListener('gl_canvas_play_damped', 'gl_canvas_stop_damped', [
document.getElementById('damped_spring_length'),
document.getElementById('damped_start_position'),
document.getElementById('damped_spring_constant'),
document.getElementById('damped_viscous_constant'),
document.getElementById('damped_spring_mass')
]);
// -- Slider logic
var sliderList = document.querySelectorAll('input[type=range]'),
attachListener = function(element) {
var output = element.nextElementSibling;
var updateOutput = function() {
var value = Number(element.value);
output.innerHTML = value;
}
element.addEventListener('input', updateOutput);
element.addEventListener('change', updateOutput);
updateOutput();
};
for(var i = 0; i < sliderList.length; i++) {
attachListener(sliderList[i]);
}
// -- Add callbacks for sliders
Module.onRuntimeInitialized = function() {
var Undamped_SetLength = Module.cwrap('Undamped_SetLength', 'void', ['number']),
Undamped_SetDisplacement = Module.cwrap('Undamped_SetDisplacement', 'void', ['number']),
Undamped_SetK = Module.cwrap('Undamped_SetK', 'void', ['number']),
Undamped_SetMass = Module.cwrap('Undamped_SetMass', 'void', ['number']),
Damped_SetLength = Module.cwrap('Damped_SetLength', 'void', ['number']),
Damped_SetDisplacement = Module.cwrap('Damped_SetDisplacement', 'void', ['number']),
Damped_SetK = Module.cwrap('Damped_SetK', 'void', ['number']),
Damped_SetMass = Module.cwrap('Damped_SetMass', 'void', ['number']),
Damped_SetC = Module.cwrap('Damped_SetC', 'void', ['number']),
Undamped_lengthSlider = document.getElementById('undamped_spring_length'),
Undamped_displacementSlider = document.getElementById('undamped_start_position'),
Undamped_kSlider = document.getElementById('undamped_spring_constant'),
Undamped_massSlider = document.getElementById('undamped_spring_mass'),
Damped_lengthSlider = document.getElementById('damped_spring_length'),
Damped_displacementSlider = document.getElementById('damped_start_position'),
Damped_kSlider = document.getElementById('damped_spring_constant'),
Damped_massSlider = document.getElementById('damped_spring_mass'),
Damped_cSlider = document.getElementById('damped_viscous_constant'),
undampedSetLength = function(value) {
value = Number(value);
Undamped_SetLength(value);
var currentDisplacementValue = Undamped_displacementSlider.value;
var bound = value;
Undamped_displacementSlider.setAttribute('max', bound);
Undamped_displacementSlider.setAttribute('min', -bound);
if (currentDisplacementValue < -bound) currentDisplacementValue = -bound;
else if (currentDisplacementValue > bound) currentDisplacementValue = bound;
var event = new Event('change');
Undamped_displacementSlider.value = currentDisplacementValue;
Undamped_displacementSlider.dispatchEvent(event);
},
dampedSetLength = function(value) {
value = Number(value);
Damped_SetLength(value);
var currentDisplacementValue = Damped_displacementSlider.value;
var bound = value;
Damped_displacementSlider.setAttribute('max', bound);
Damped_displacementSlider.setAttribute('min', -bound);
if (currentDisplacementValue < -bound) currentDisplacementValue = -bound;
else if (currentDisplacementValue > bound) currentDisplacementValue = bound;
var event = new Event('change');
Damped_displacementSlider.value = currentDisplacementValue;
Damped_displacementSlider.dispatchEvent(event);
};
Undamped_lengthSlider.addEventListener('change', function(event) { undampedSetLength(Number(event.target.value)); });
Undamped_displacementSlider.addEventListener('change', function(event) { Undamped_SetDisplacement(Number(event.target.value)); });
Undamped_kSlider.addEventListener('change', function(event) { Undamped_SetK(Number(event.target.value)); });
Undamped_massSlider.addEventListener('change', function(event) { Undamped_SetMass(Number(event.target.value)); });
Damped_lengthSlider.addEventListener('change', function(event) { dampedSetLength(Number(event.target.value)); });
Damped_displacementSlider.addEventListener('change', function(event) { Damped_SetDisplacement(Number(event.target.value)); });
Damped_kSlider.addEventListener('change', function(event) { Damped_SetK(Number(event.target.value)); });
Damped_massSlider.addEventListener('change', function(event) { Damped_SetMass(Number(event.target.value)); });
Damped_cSlider.addEventListener('change', function(event) { Damped_SetC(Number(event.target.value)); });
};
}
</script>
<article>
<h1>Springs</h1>
<section>
<p>
It is time to investigate what it means to have a deformation in our physics system. By deformation, I mean that the
vertices of our shapes are no longer fixed to one point within the model. Towards this end, we will begin by investigating
springs.
</p>
</section>
<section>
<h2>Undamped Spring Explanation</h2>
<p>
An <b>undamped spring</b> is one that never stops oscillating. Once an undamped spring starts moving back and forth,
it moves back and forth forever.
</p>
<p>
From Newton's second law, we know that the sum of all the forces acting on an object will equal zero. We know that one
of these forces is mass times acceleration, but for a spring we will add another force which uses the <i>spring constant</i>.
This force grows in proportion to the displacment that spring is currently set at. So, if the spring is currently displaced
10m, the effect of this force will be greater than if the spring was at <i>equlibrium</i>, which means that it is displaced by
0m. This should make sense intuitively.
</p>
<p>
One thing to note is that a damped spring has three cases:
<ul>
<li><b>Overdamped</b>: the spring will decompress immediately to the equlibrium position</li>
<li><b>Underdamped</b>: the spring will do one oscillation before returning to the equlibrium position</li>
<li><b>Critically damped: the spring will oscilate, but each oscillation will bring it closer and closer to the equlibrium position</b></li>
</ul>
I will not go into the mathematical details as to why this happens, as the links at the end of this video can very easily tell you. Just know
that they are dependent on your values of <i>c</i> and <i>k</i>, so choose wisely for your use case.
</p>
<p>
The equation is given as such:
<div class="formula">
<math class="formula">
ma + kx = 0
</math>
</div>
where <i>m</i> is the mass, <i>a</i> is acceleration, <i>k</i> is the spring constant, and <i>x</i> is the current displacement
from the rest position.
</p>
<p>
We can define an undamped 1D spring in code like so:
<pre><code><span class="code_keyword">struct</span> Spring {
<span class="code_keyword">float</span> mass = 1.f; <span class="code_comment">// Mass of the weight on the end of the spring</span>
<span class="code_keyword">float</span> k = 4; <span class="code_comment">// Spring Constant, in N / m</span>
<span class="code_keyword">float</span> force = 0.f;
<span class="code_keyword">float</span> velocity = 0.f;
<span class="code_keyword">float</span> position = 0.f;
};
<span class="code_keyword">void</span> updateSpring(Spring* spring, <span class="code_keyword">float</span> dtSeconds) {
spring->force = spring->k * spring->position;
<span class="code_keyword">float</span> acceleration = spring->force / spring->mass; <span class="code_comment">// F = ma</span>
spring->velocity = spring->velocity + acceleration * dtSeconds;
spring->position = spring->position + spring->velocity * dtSeconds;
}
</code></pre>
Note that we are just using Euler Integration here.
</p>
</section>
<section>
<h2>
Undamped Springs Example
</h2>
<p>
<span class='widget_container'>
<label for='undamped_spring_length'>Spring Length (m)</label>
<input type='range' id='undamped_spring_length' min="50" max="300" value="150"/>
<span></span>
</span>
<span class='widget_container'>
<label for='undamped_start_position'>Start Displacement (m)</label>
<input type='range' id='undamped_start_position' min='-150' max='150' value='0'/>
<span></span>
</span>
<span class='widget_container'>
<label for='undamped_spring_constant'>Spring Constant (N / m)</label>
<input type='range' id='undamped_spring_constant' min='0' max='1000.0' value='1.0' step='0.1'/>
<span></span>
</span>
<span class='widget_container'>
<label for='undamped_spring_mass'>Mass (kg)</label>
<input type='range' id='undamped_spring_mass' min='0' max='10.0' value='1.0' step='0.1'/>
<span></span>
</span>
</p>
<div class="opengl_canvas_container">
<canvas id="gl_canvas_undamped" width="800" height="600"></canvas>
<button id="gl_canvas_play_undamped" class="play_button">
Play
</button>
<button id="gl_canvas_stop_undamped" class="stop_button">
Stop
</button>
</div>
</section>
<section>
<h2>Damped Spring Explanation</h2>
<p>
Undamped springs are loads of fun, but if we want to make a softbody physics system, we're going to want
our simulation to stop oscillating at some point. That is where the <b>viscous damping constant</b> (<i>c</i>) force
comes in. The damping force grows in proportion to the velocity. The equation is given as such:
<div class="formula">
<math class="formula">
ma + cv + kx = 0
</math>
</div>
where <i>m</i> is the mass, <i>a</i> is acceleration, <i>c</i> is the damping constant, <i>v</i> is the velocity
<i>k</i> is the spring constant, and <i>x</i> is the current displacement from the rest position.
</p>
<p>
This force produces the most effect when the velocity is largest. Let's try to intuit exactly what this means. When
our spring is extended downward, it will want to move upward to return to its equlibrium position. At this point,
the displacement (<i>x</i>) will be negative and the velocity (<i>v</i>) will be positive. Since the signs are different,
we can see that our spring force will be mitigated by the damping force. Hence, we damp!
</p>
<p>
We can define a damped 1D spring in code like so:
<pre><code><span class="code_keyword">struct</span> Spring {
<span class="code_keyword">float</span> mass = 1.f; <span class="code_comment">// Mass of the weight on the end of the spring</span>
<span class="code_keyword">float</span> k = 4; <span class="code_comment">// Spring Constant, in N / m</span>
<span class="code_keyword">float</span> c = 1; <span class="code_comment">// Viscous damping constant, in N / m/s</span>
<span class="code_keyword">float</span> force = 0.f;
<span class="code_keyword">float</span> velocity = 0.f;
<span class="code_keyword">float</span> position = 0.f;
};
<span class="code_keyword">void</span> updateSpring(Spring* spring, <span class="code_keyword">float</span> dtSeconds) {
spring->force = spring->c * spring->velocity + spring->k * spring->position;
<span class="code_keyword">float</span> acceleration = spring->force / spring->mass; <span class="code_comment">// F = ma</span>
spring->velocity = spring->velocity + acceleration * dtSeconds;
spring->position = spring->position + spring->velocity * dtSeconds;
}
</code></pre>
Note that we are just using Euler Integration here.
</p>
</section>
<section>
<h2>
Damped Springs Example
</h2>
<p>
<span class='widget_container'>
<label for='damped_spring_length'>Spring Length (m)</label>
<input type='range' id='damped_spring_length' min="50" max="300" value="150"/>
<span></span>
</span>
<span class='widget_container'>
<label for='damped_start_position'>Start Displacement (m)</label>
<input type='range' id='damped_start_position' min='-150' max='150' value='0'/>
<span></span>
</span>
<span class='widget_container'>
<label for='damped_spring_constant'>Spring Constant (N / m)</label>
<input type='range' id='damped_spring_constant' min='0' max='1000.0' value='1.0' step='0.1'/>
<span></span>
</span>
<span class='widget_container'>
<label for='damped_viscous_constant'>Viscous Damping Constant (N / m)</label>
<input type='range' id='damped_viscous_constant' min='0' max='100.0' value='1.0' step='0.1'/>
<span></span>
</span>
<span class='widget_container'>
<label for='damped_spring_mass'>Mass (kg)</label>
<input type='range' id='damped_spring_mass' min='0.1' max='10.0' value='1.0' step='0.1'/>
<span></span>
</span>
</p>
<div class="opengl_canvas_container">
<canvas id="gl_canvas_damped" width="800" height="600"></canvas>
<button id="gl_canvas_play_damped" class="play_button">
Play
</button>
<button id="gl_canvas_stop_damped" class="stop_button">
Stop
</button>
</div>
</section>
<footer id='references'>
<h2>References</h2>
<ul>
<li>
<a href='https://www.youtube.com/watch?v=Z52emur7Rko'>Wonderful Undamped Resource</a>
</li>
<li>
<a href='https://www.youtube.com/watch?v=CTd1uVq5-l8'>Wonderful Damped Resource</a>
</li>
<li>
<a href='http://ambrsoft.com/CalcPhysics/Spring/SpringData.htm'>List of Equations for Spring Motion</a>
</li>
</ul>
</footer>
</article>
</main>
</body>
</html>
|
