Simple prismatic joint demo. Prismatic joints are odd, it took me awhile to realize how to use them and what they can be used for… Plan on using them in the creation of some Box2D machines in the near futrure…
Check out the swf
QuickBox2D pulley joint demo. This post is really meant to serve as part of the docs so it may seem a bit boring in its simplicity…
Have a look at the swf…
import com.actionsnippet.qbox.*;
import Box2D.Common.Math.*;
[SWF (backgroundColor=0x000000, width=700, height=600, frameRate=60)]
var sim:QuickBox2D = new QuickBox2D(this,{debug:false, frim:true, gravityY:1});
sim.setDefault({fillColor:0x113366, fillAlpha:0.8, lineColor:0x3355AA});
sim.createStageWalls();
var circle:QuickObject = sim.addCircle({x:10, y:10, radius:3});
var head:QuickObject = sim.addCircle({x:5, y:5, radius:0.5});
var torsoA:QuickObject = sim.addBox({x:5, y:6, width:1.5, height:0.8})
var torsoB:QuickObject = sim.addBox({x:5, y:6.5, width:1.25, height:0.8})
var torsoC:QuickObject = sim.addBox({x:5, y:7, width:1.0, height:0.8})
var pelvis:QuickObject = sim.addBox({x:5, y:7.5, width:1.0, height:0.8})
var leftLegA:QuickObject = sim.addBox({x:4.75, y:8.5, width:0.4, height:1.75});
var leftLegB:QuickObject = sim.addBox({x:4.75, y:9.75, width:0.4, height:1.5});
var rightLegA:QuickObject = sim.addBox({x:5.29, y:8.5, width:0.4, height:1.75});
var rightLegB:QuickObject = sim.addBox({x:5.29, y:9.75, width:0.4, height:1.5});
var leftArmA:QuickObject = sim.addBox({x:3.6, y:5.8, width:1.3, height:0.35});
var leftArmB:QuickObject = sim.addBox({x:2.5, y:5.8, width:1.25, height:0.35});
var rightArmA:QuickObject = sim.addBox({x:6.4, y:5.8, width:1.3, height:0.35});
var rightArmB:QuickObject = sim.addBox({x:7.5, y:5.8, width:1.25, height:0.35});
torsoA.body.SetLinearVelocity(new b2Vec2(0, 50));
var anchor:b2Vec2 = new b2Vec2();
function connect(a:QuickObject, b:QuickObject, lower:Number, upper:Number, offX:Number=0, offY:Number = 0):QuickObject{
var min:Number = Math.min(a.y, b.y);
var max:Number = Math.max(a.y, b.y);
anchor.y = min + (max - min) * 0.5 + offY;
min = Math.min(a.x, b.x);
max = Math.max(a.x, b.x);
anchor.x = min + (max - min) * 0.5 + offX;
return sim.addJoint({a:a.body, b:b.body, x1:anchor.x, y1:anchor.y, lowerAngle:lower, upperAngle:upper});
}
sim.setDefault({type:"revolute", collideConnected:false, enableLimit:true, lineColor:0xFFFFFF});
connect(head, torsoA, -.2, .2);
connect(torsoA, torsoB,-.2, .2);
connect(torsoC, torsoB,-.2, .2);
connect(torsoC, pelvis,-.2, .2);
connect(pelvis, leftLegA, -1, 1, 0, -0.5);
connect(leftLegA, leftLegB, -1, 1);
connect(pelvis, rightLegA, -1, 1, 0, -0.5);
connect(rightLegA, rightLegB, -1, 1);
connect(torsoA, leftArmA, -2, 1);
connect(leftArmA, leftArmB, -2, 1);
connect(torsoA, rightArmA, -1, 2)
connect(rightArmA, rightArmB, -1, 2)
sim.start();
sim.mouseDrag();
QuickBox2D alpha 108 is finally finished and different types of joints are now possible. I’ll be auto-posting joint demos over the next few days for each type of Box2D joint. Box2D joints are pretty confusing, I struggled a bit to decide on a good way to handle them in QuickBox2D, any suggestions are welcome.
You’ll need this fla to run this snippet since the graphics are in the library. This snippet shows how to easily use linkage classes as the graphics for your rigid bodies. This was actually one of the first features I implemented in QuickBox2D.
One of the more advanced and useful properties of rigid bodies is the groupIndex. It allows you to specify which rigid bodies collide with one another and which rigid bodies pass through one another. This snippit demo’s the groupIndex property. For more information take a look at what the Box2D manual says.
import com.actionsnippet.qbox.*;
import Box2D.Common.Math.*;
// try altering your frame rate
[SWF(backgroundColor=0x000000, width=700, height=600, frameRate=30)]
// try setting frim to false
var sim:QuickBox2D = new QuickBox2D(this, {iterations:20, timeStep:1 / 60, frim:true});
sim.setDefault({fillColor:0x003366, lineAlpha:0});
sim.createStageWalls();
sim.addBox({x:10, y:18, height:3, density:0});
sim.setDefault({fillColor:0x004466, lineColor:0x2B80F5});
var cVel:b2Vec2 = new b2Vec2();
for (var i:int = 0; i<32; i++){
var c:QuickObject = sim.addCircle({x:13 + i % 8, y:10 + int(i / 8), friction:0.01, radius:0.2 + Math.random()*0.3, angularDamping:1});
cVel.x = Math.random() * 4 - 2;
cVel.y = Math.random() * 4 - 2;
c.body.SetLinearVelocity(cVel);
}
var box:QuickObject = sim.addBox({x:3, y:16, width:2, height: 2});
var boxVel:b2Vec2 = new b2Vec2(15, -25);
sim.start();
// shoot the box from left to right
setTimeout(positionBox, 200);
function positionBox():void{
// reset box position
box.x = 3, box.y = 16;
setTimeout(shootBox, 500);
}
function shootBox():void{
box.body.SetLinearVelocity(boxVel);
box.body.SetAngularVelocity(10);
// shoot the box again
setTimeout(positionBox, 4000);
}
You’ll need to download the latest version of QuickBox2D in order to run this snippet (alpha 107 as of this post). I recommend checking out the demo below first though…
The most notable feature of this release is FRIM (frame rate independent motion). FRIM is used to attempt to keep the speed of the Box2D simulation constant no matter what frame rate the swf is running at. This is good because frame rates will vary from browser to browser and from computer to computer.
Mr.Doob suggested that I implement this feature in QuickBox2D. If you haven’t seen his site I highly recommend checking it out – really great stuff there.
I created a demo to illustrate the FRIM feature… this demo is basically the same as the above snippet with a few extra buttons added to control frame rate and toggle FRIM.
While this new feature seems to be working, I believe there still may be room for improvement. Please let me know if the demo doesn’t work correctly on your machine. As of now, I’ve only tested it on my 2.5 Ghz Intel Core 2 Duo.
Also, any suggestions to improve the clarity of this demo are appreciated… I found it kind of hard to think of a good demo to show FRIM in action. The original demo really didn’t make much sense… so I gave Rich Shupe a call and he helped me come up with the idea for the current demo.
UPDATE: I just updated the demo so that the simulation restarts when you change the frame rate (as per Mr.doob’s suggestion below)
This works with the current version of QuickBox2D…. tomorrow I’ll be uploading the new version of QuickBox2D which supports FRIM (frame rate independent motion) and contains a few additional minor tweaks and fixes.
