26,758

Views

Beginner

Level

7

Comments

Working with Physics

by Jon (Updated on 2014-01-26)


Contents

  • What is Box2D?
  • The Physics Page
    • General Tab
    • Heaviness Tab
    • Material Tab
    • Damping Tab
    • Advanced Tab
  • Lightweight Actors (No Physics)
  • Gotchas
  • The Future

 

What is Box2D?

Stencyl incorporates an industry-standard physics engine (Box2D) into all its games. What's the benefit? You get realistic and accurate collisions for free, and Actors generally behave like real-world objects.

How realistic? Try out the following demo.

 

Note: The rest of this article walks through the Physics Page interface. If you're skimming or are a first timer, read through the following sections and skip the others for now. They are the most important!

- General Tab
- Lightweight Actors
- Gotchas

 

General Tab


What kind of Actor Type?

Determines whether the Actor can move or not.

Cannot Move: The actor cannot move or be moved, thus, becoming stationary.
Cannot Be Pushed: The actor can move but cannot be moved by another actor.
Normal: The Actor can move on its own and be moved by other Actors.

Note: A "Cannot Be Pushed" actor will not collide with tiles. In Box2D speak, it's known as a Kinematic body. A clever way to simulate "Cannot Be Pushed" without its sideeffects is to assign a very high mass to an Actor.


Can Rotate?

Determines whether or not your actor can rotate. Rotation happens either naturally through physics, setting the turning speed or twisting it with some force.

Note: Setting the direction of an actor can still be done, regardless of this setting. Tweening also ignores this setting.


Affected by Gravity?

Gravity is a constant force that affects all bodies within a scene.

Gravity can be set to *any* direction and magnitude (strength) via the Physics page of a Scene. Gravity does not always point down.

You can set an Actor to obey or ignore gravity.

Note: The apparent "strength" of gravity is affected by an Actor's mass. The higher the Actor's mass, the slower it will fall. This is not how gravity works in real life but is a simple approximation by Box2D to simulate it.


Heaviness Tab


Mass

How much your actor "weighs." The "heavier" you make an actor, the harder it will be for other objects to move it.

Tip: Setting a very high mass can roughly simulate infinite mass / cannot be pushed, without the side-effects of that mode.


Angular Mass

Angular mass determines an actors resistance to rotational forces. As such, the higher the number, the slower an actor will rotate when subjected to rotational forces (twisting).

Note: Angular mass is only relevant if the Actor is set to "Can Rotate"


Material Tab


Preset Materials

Preset Materials are preset values that use different combinations of Friction and Bounciness to simulate the material listed.

  Friction Bounciness
Wood 0.5 0.17
Metal 0.13 0.17
Stone 0.75 0.05
Plastic 0.38 0.09
Rubber 0.75 0.95
Dead Weight 1 0
Super Ball 1 0.95


Friction

Friction determines the "roughness" of the Actor's surface. A high friction will cause an Actor to slow down quicker when sliding. A low, or zero, friction will cause the Actor to glide far or never slow down at all.

In real-life high friction surfaces would include dirt and sandpaper. Low-friction surfaces could include ice and glass.

Demo Note: In this demo, press left/right and lift your key off. Observe that one actor stops moving quickly (high friction) and the other keeps moving, as if he were on ice.

 

Bounciness

Setting bounciness to a value of 1.0 means an actor will bounce back to the same height it fell from, whereas a value of 0.0 means it won’t bounce at all.

Demo Note: In this demo, the balls bounce differently depending on their bounciness. The one with bounciness = 1 bounces back to the same height.

 

Note: Friction and Bounciness multiply the values of the two colliding Actors to come up with the effective Friction or Bounciness. For example, if two actors have a friction of 0.5, their effective friction becomes 0.25.

This works out well for actors that have a friction of bounciness of 0 - as you'd expect, that negates everything and causes an effective friction/bounciness value of 0 too.

Terrain/Tiles have friction and bounciness of values of 1, so they have no net effect whichever way.


Damping Tab

Damping is a difficult topic to visualize. It's a form of resistance against motion or rotation that varies depending on how quickly the Actor is moving or rotating.

 

Linear Damping

Linear Damping is like wind or air resistance. It grows stronger as your Actor moves more quickly to the point where the Damping is so strong that the Actor is unable to move any faster. This is known as Terminal Velocity.


Angular Damping

Angular Damping is the same idea, except applied to rotation. It puts an effective cap on how fast an Actor can rotate and smooths out things if you find your Actor to rotate too quickly when subjected to twisting (angular forces).


Advanced Tab


Disable Physics

Let you opt this Actor out of physics. We talk about this further in the next section.

Note: Collisions for this Actor will also cease to function when you do this.

 

Auto-Scale Collision Bounds

Choosing “Yes” for this option will automatically re-size your Actor’s collision bounds when you resize the Actor using the "scale to" Tweening block.

Choosing “No” means the collision bounds will stay the same regardless of what happens to the Actor.

Demo Note: Notice that the actor on the right remains at the same collision size.

 

Enable Continuous Collisions

Enable this setting to prevent this Actor, if it moves quickly enough, from penetrating through thin surfaces, like shown below. This setting is useful for small and quick actors, such as bullets, arrows and other weapons.

We've got an entire article devoted to this topic.

 

Can be paused?

This setting lets you opt an Actor in or out of the Pausing system.

We've got an entire article devoted to Pausing.

 

Lightweight Actors

Lightweight Actors are actors that opt out of the physics system.

The Lightweight Actors feature works well for games that involve many actors or require more precise motion and collision detection than Box2D allows. For example...

  • Shoot 'Em ups
  • Puzzle Games
  • Pixel Perfect Platformers (like N)
  • Point and Click Adventures

Although we're exploring the idea of an engine in which physics can be turned entirely off, the Lightweight physics feature lets you selectively opt an actor out of physics. Even in regular games, this can prove to be useful for...

  • User Interface Elements
  • Particles
  • HUDs

Bear in mind that when you turn off physics, the actor no longer collides with anything, can no longer be pushed and is generally barred from any physics-related features. Setting position and velocity will still work.

Going with lightweight actors can be a game-saving choice if done right, but you need to understand its (current) limitations.

 

Gotchas

Snagging

Sometimes, an actor will get caught in the corner of a box and be unable to move further. In a similar scenario, a jumping actor who jumps right next to a wall versus a little bit off the wall will find his jump to be shorter.

The reasons behind this are quite technical, but the workarounds are less so.

The easiest workaround is to redo the actor's collision shape as...

  • A circle.
     
  • A box with its corners chopped off slightly or some other means to avoid the sharp 90 degree corners. We did this for a Mario-like platformer and effectivley made an octagon.

 

My actors don't collide!

Several legitimate reasons for this.

  • The two actors are set to "Cannot Move", "Cannot Be Pushed" or a combination of the two. This is just the way Box2D works and can catch you off guard.
     
  • Did you make sure the actors aren't set as Sensors?


     
  • Did you make sure that the actors are from a pair of Groups that are to collide with other?

 

The game crashes when my actor gets squished between two rotating objects.

This is rare, but it's is a flaw in the current version of Box2D. To work around it, try to detect this scenario ahead of time and move the actor out of the way. (In Stencyl 3, this problem appears to have gone away due to the rewrite.)


Summary

  • Through Box2D, Stencyl brings realistic physics to all games.
  • Opt an Actor out of physics to improve performance for games with many actors or no need for physics.
  • Box2D is powerful, but it can sometimes present challenges when you want your game to NOT operate like the real world.

 

Challenge: Platforms!

Create a simple game with moving platforms inside of it, like you've find in any Mario game. It's trickier than it sounds at first!

What are some of the considerations?

  • How do you prevent the platform from shifting when something steps on it?
  • How will the character stay on the platform after stepping on it?
  • If the platform is moving up and down, what happens if it moves too quickly downwards?
  • How will you move the platforms? Setting position? Velocities? Forces?
  • How do you ensure that the platform starts and ends at the same positions?

Start with a platform that moves up and down. Then, try one that moves left and right.

Disclaimer: All articles are geared towards Stencyl 3.0 and above. Use comments to provide feedback and point out issues with the article (typo, wrong info, etc.). If you're seeking help for your game, please ask a question on the forums. Thanks!

7 Comments

trwilson3
I been trying to recreate a Balloon shooter but it is based on an older version of Stencyl, and therefore it dose not have the same menus as the current version notably the missing messages tool bar. I have spent hours trying to find an example or tutorial on making the balloons float. I am stumped on how to implement floating, how can I do this?

Respectfully
Ted

0 6 months, 2 weeks ago
Donis
Does the friction applied to an actor? You do not understand how I can make an ice surface with low friction for example
0 9 months, 1 day ago
MellowPandaGaming
I used the Wander behaviour for an AI character, and set it that they start by walking, but nothing happens, what must I do?
0 12 months, 1 day ago
Valeriana
thx! it was very helpfull! it make me unterstand what i was doing wrong

0 1 year, 7 months ago
snortch
The mini demos are very helpful. I don't think I would have been able to understand these options without them.
0 1 year, 8 months ago
72master
thanks whoever put this up I finally got my random walking AI behaviour
0 1 year, 9 months ago
Jon
Updated with some new demos to illustrate some of the concepts.
1 2 years, 1 week ago

Sign In to Comment