• Announcements

    • khawk

      Download the Game Design and Indie Game Marketing Freebook   07/19/17

      GameDev.net and CRC Press have teamed up to bring a free ebook of content curated from top titles published by CRC Press. The freebook, Practices of Game Design & Indie Game Marketing, includes chapters from The Art of Game Design: A Book of Lenses, A Practical Guide to Indie Game Marketing, and An Architectural Approach to Level Design. The GameDev.net FreeBook is relevant to game designers, developers, and those interested in learning more about the challenges in game development. We know game development can be a tough discipline and business, so we picked several chapters from CRC Press titles that we thought would be of interest to you, the GameDev.net audience, in your journey to design, develop, and market your next game. The free ebook is available through CRC Press by clicking here. The Curated Books The Art of Game Design: A Book of Lenses, Second Edition, by Jesse Schell Presents 100+ sets of questions, or different lenses, for viewing a game’s design, encompassing diverse fields such as psychology, architecture, music, film, software engineering, theme park design, mathematics, anthropology, and more. Written by one of the world's top game designers, this book describes the deepest and most fundamental principles of game design, demonstrating how tactics used in board, card, and athletic games also work in video games. It provides practical instruction on creating world-class games that will be played again and again. View it here. A Practical Guide to Indie Game Marketing, by Joel Dreskin Marketing is an essential but too frequently overlooked or minimized component of the release plan for indie games. A Practical Guide to Indie Game Marketing provides you with the tools needed to build visibility and sell your indie games. With special focus on those developers with small budgets and limited staff and resources, this book is packed with tangible recommendations and techniques that you can put to use immediately. As a seasoned professional of the indie game arena, author Joel Dreskin gives you insight into practical, real-world experiences of marketing numerous successful games and also provides stories of the failures. View it here. An Architectural Approach to Level Design This is one of the first books to integrate architectural and spatial design theory with the field of level design. The book presents architectural techniques and theories for level designers to use in their own work. It connects architecture and level design in different ways that address the practical elements of how designers construct space and the experiential elements of how and why humans interact with this space. Throughout the text, readers learn skills for spatial layout, evoking emotion through gamespaces, and creating better levels through architectural theory. View it here. Learn more and download the ebook by clicking here. Did you know? GameDev.net and CRC Press also recently teamed up to bring GDNet+ Members up to a 20% discount on all CRC Press books. Learn more about this and other benefits here.
Sign in to follow this  
Followers 0

How to code a swinging movement

7 posts in this topic

Hi guys!

I have to built a small prototype of a concept my team came up with.
I'm having some issues on letting to spider swing it's web.

So how can I code a swinging movement of a spider with a web attached to a fixed point?
It has to swing a few times, before losing it's velocity and then hanging still.

This may sound a bit confusing, but this "drawing" should make it clear.

Many thanks in advance!

(This would have been better in the Math & Physics forum...) Edited by MattProductions

Share this post

Link to post
Share on other sites
This is basically a pendulum:

If you look at:


you'll see that the angle(in radians) between the spider and the anchor point at a given time is:

angle = initialAngle * cos(sqrt(gravity/length)*time)

That formula is reasonably accurate as long as the initial angle is below 1 radian. (around 57 degrees)

the position of the spider is then x=cos(angle), y=sin(angle) and you can simply reduce the initial angle in that calculation to reduce the amplitude of the swing. (Not physically accurate but it should work well enough for your situation).

another option is to use a proper physics model but it is quite a bit more complicated.

Share this post

Link to post
Share on other sites
[font=arial,helvetica,sans-serif]If you want to slowly reduce the peak height, you could always do something like intialAngle *= friction, to decrease the max height geometrically.[/font]

Share this post

Link to post
Share on other sites
An even better solution would be to use a 2d physics engine. Then, you would just attach an object (the spider) to a constraint, which would be attached to the static object as the fixed point. Give the spider some values (mass, friction, and starting location), apply gravity, and watch the engine do all the work for you.

I personally use chipmunk-physics, but box2d is a popular one as well (google them).

Here's a video of the chipmunk-physics engine at work, You'd be using the most basic one, the pin joint (used 1st):

[url="http://www.youtube.com/watch?v=ZgJJZTS0aMM&feature=player_embedded#!"]http://www.youtube.com/watch?v=ZgJJZTS0aMM&feature=player_embedded#![/url] Edited by BeerNutts

Share this post

Link to post
Share on other sites
You can write a simple verlet integrator, which will allow you to do many more fun things easily (this is the home-made version of using a 2D Physics engine).

[code]#include <iostream>
#include <cmath>

float const degrees = std::atan(1.0f)/45.0f;

struct Vector2D {
float x, y;
Vector2D(float x, float y) : x(x), y(y) {

std::ostream &operator<<(std::ostream &os, Vector2D v) {
return os << '(' << v.x << ',' << v.y << ')';

Vector2D operator+(Vector2D v, Vector2D w) {
return Vector2D(v.x+w.x, v.y+w.y);

Vector2D operator-(Vector2D v, Vector2D w) {
return Vector2D(v.x-w.x, v.y-w.y);

Vector2D operator*(Vector2D v, float s) {
return Vector2D(v.x*s, v.y*s);

float length(Vector2D v) {
return std::sqrt(v.x*v.x+v.y*v.y);

int main() {
float const dt = 0.01f; // (s) (100 Hz simulation)
float const dt_squared = dt*dt;
float length_of_string = 0.1f; // (m)
float gravity = 9.81f; // (m/s^2)
float friction = 1.5f; // (s^-1), I believe, but you better ask a Physicist if you care ;)
float friction_coef = std::exp(-friction*dt);
float alpha = 15.0f*degrees;
Vector2D position(length_of_string*std::sin(alpha), -length_of_string*std::cos(alpha));
Vector2D previous_position = position;
Vector2D acceleration(0.0f, -gravity);

// For 10 seconds
for (float t = 0; t < 10.0; t += dt) {
// Verlet integration
Vector2D next_position = position + (position - previous_position) * friction_coef + acceleration * dt_squared;

// Constrain the point to be at the desired length from the origin
next_position = next_position * (length_of_string / length(next_position));

// Update
previous_position = position;
position = next_position;

// Output
std::cout << t << ' ' << position << '\n';

[/code] Edited by alvaro

Share this post

Link to post
Share on other sites
I used SimonForsman's solution.
It was the easiest one the implement (just for a prototype).


Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  
Followers 0