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Recently i posted a simple car made with joints without too much explanations... so What is a joint? This video shows joints in action. At the end, the stand alone executable and the complete project. The following text and figure is from the "ode-latest-userguide", figures are also form the JV-ODE documentation: In real life a joint is something like a hinge, that is used to connect two objects. It is a relationship that is enforced between two bodies so that they can only have certain positions and orientations relative to each other. This relationship is called a constraint, the words joint and constraint are often used interchangeably. The figure shows three different constraint types. The first is a ball and socket joint that constraints the “ball” of one body to be in the same location as the “socket” of another body. The second is a hinge joint that constraints the two parts of the hinge to be in the same location and to line up along the hinge axle. The third is a slider joint that constraints the “piston” and “socket” to line up, and additionally constraints the two bodies to have the same orientation. Each time the integrator takes a step all the joints are allowed to apply constraint forces to the bodies they affect. These forces are calculated such that the bodies move in such a way to preserve all the joint relationships. Each joint has a number of parameters controlling its geometry. An example is the position of the balland- socket point for a ball-and-socket joint. The functions to set joint parameters all take global coordinates, not body-relative coordinates. A consequence of this is that the rigid bodies that a joint connects must be positioned correctly before the joint is attached. Figure: types of joints The "Integrator step" is the part of the physics engine that does all the calculations. Based on the current state, force applied, constrain parameters, collisions etc... recalculates the next position and rotation of every affected physical obejct. Leadwerks then, does the render of the objects acordingly to it's new positions, or that is what is supposed to do... (Josh should correct me if I'm wrong). A joint normally is placed between two objects, one is called Parent and the other Child. The child should be NULL, and in this case the joint is between one body and the world or scene. In the case of a door, you need a Door and a Frame, the Frame should be the Parent body, the Door the child body. In real life you have to decide where the Hinge (or more than one) will be placed and use some screws to attach both parts of the hinge to the frame and the door. In software, things are practically the same In other words, we have to have 3 things clear: Who is the parent and where it should be positioned and oriented. Who is the child and where it should be poitioned and oriented. Which type of joint you need, where it should be positioned and oriented. Normally we create and place the two bodies first and then, at joint creation we say where the joint will be and how it should be oriented, who the parent is and who child is, if any. After that, joints normally has some other properties that should be adjusted, used, controlled, limited, enabled... etc, depending on the joint type. Joints have two "limits": If the joint is a Hinge, the limits are the Start Angle and End Angle "the door should turn". If the joint is a Slider, the limits are the Start Position and End Position the piston should extend. If the joint is a Ball, the limits are the cone angle and twist or torsion... keep reading Joints (hinge and slider) should have a "motor" if not, as a normal door, we need external forces to "open the door", if we place a door on a frame attached with hinges and the door has no lock at all, then the wind should open/close it, or some body that collides with the door should move it. As we use a hinge, the door do not fall, do not take an upward motion, the motion is well known and constrainded tho the clasic movement we all know. But if, for instance we have a "motorized door" then the open/close operation is started in some way (press of a button?) and a motor does the job. Normally it stops automatically, because there are end-run switches or the like installed and working for us. A sofware motorized joint is exactly that, we set where we want the joint go (setTargetAngle), and start a motor (enableMotor). Ball Joint A ball is used when we need a "cone" movement freedom, think that you have a rope attached at the roof of your room and you hang a box on it. If the rope is made of a rigid material, the box should made a pendulum movement not so large and probably the box orientations should not change too much. How much we let the rope to move is dictated by the first limit and how much the object attached should change orientation (twist/torsion) is dictated by the second limit. A rope is a nice looking example, all you have to do is place N cubes (shape is not important) and place ball joints in the middle of each pair. You may have the first box joined to the world (attached to a fixed 3d position) and then a link of bodies and ball joints: Joint N has body N as parent and body N+1 as child Leadwerks came with a simple ball.lua script: function Script:Start() local pos = self.entity:GetPosition(true) self.joint = Joint:Ball(pos.x, pos.y, pos.z, self.entity, self.entity:GetParent()) self.joint:SetFriction(10) self.entity:SetDamping(0,0) end If you have a chain of parented objetcts, then you may set this script and you have a rope build with a few clicks of your mouse on the editor. Procedure: Create some boxes, set prop collision and a mass not zero (1 perhaps), place then in a vertical row somewhat separated. In the editor drag the second box to the first, the third to the second building a hierachy (parent/child relationship) Select all (one by one with ctrl click on each box) and then apply the ball script. Voila! You have a rope with no limits: cone angle is the maximum the engine lets you to be and the same for torsion. Collide the rope with some object or, place the boxes horizontally (instead of vetically) and let gravity do it's job. I made another ball script: ballEnhaced: Script.parent = nil--Entity "Parent" Script.child = nil--Entity "Child" Script.useLimits=false--bool "Use limits" Script.coneAngle = 45--Float "Cone Angle" Script.torsion = 45--Float "Torsion" Script.hide = true--bool "Hide entity" function Script:Start() local pos = self.entity:GetPosition(true) if self.parent~=nil then self.joint = Joint:Ball(pos.x, pos.y, pos.z, self.parent, self.child) self.joint:SetLimits(self.coneAngle, self.torsion) if self.useLimits then self.joint:EnableLimits() System:Print("limits:"..self.coneAngle..", "..self.torsion) end if self.hide then self.entity:Hide() end end end if using this script, you have to create a pivot or box (collision type: none, and no mass) and tell the script ho the parent is and ho the child is, if no child, then the parent body will be attached to the environement, fixed at this 3d point. In the map included in this blog there are two ropes, one made with the leadwerks ball.lua and one with the ballEnhaced.lua script, in this script you may tell how to constrain the ball cone anlgle and trosion. Look the video, and you will see the difference on how the cubes behave when hitted by the car. A rope suspension bridge should be made with both ends attached to the world and a double chain of joints... doing two unions between each part of the brige. It's nice so see the car passing over it! The attached video shows at first the car passing over the bridge... the mass of the car is very little, if not, the joints get broken.. as always there are some other things to consider. Hinje Joint As I used a hinge for some explanation, i guess there is not to much to say here, I use some hinges for doors in the map attached and also used them for the steer and wheels of the car. One door have a motor and the other not, so the second one moves as the car collides it. I wrote two scripts: hingeEnhaced.lua Script.parent = nil --entity "parent" Script.child = nil --entity "hild" Script.hide = true--bool "Hide entity" Script.pin = Vec3(0,0,1) --Vec3 "Hinge Pin" Script.limitsenabled=false--bool "Enable limits" Script.limits = Vec2(-45,45) --Vec2 "Limits" Script.friction = 0--float "Friction" function Script:Start() local pos = self.entity:GetPosition(true) if self.parent~=nil then self.joint = Joint:Hinge(pos.x, pos.y, pos.z, self.pin.x, self.pin.y, self.pin.z, self.parent, self.child) if self.limitsenabled then self.joint:EnableLimits() end self.joint:SetLimits(self.limits.x,self.limits.y) self.joint:SetFriction(self.friction) self.entity:SetDamping(0,0) if self.hide then self.entity:Hide() end end end As in the case of a ball joint, with this script you may set parent, child and limits hingeMotorized.lua Script.parent = nil --entity "parent" Script.child = nil --entity "hild" Script.hide = true--bool "Hide entity" Script.pin = Vec3(0,0,1) --Vec3 "Hinge Pin" Script.limitsenabled=false--bool "Enable limits" Script.limits = Vec2(-45,45) --Vec2 "Limits" Script.tOpenClose = 15 --Int "Open/Close time" Script.movespeed = 60 --Int "Speed" Script.startTime = 0 Script.action = 1 --1 open, 0 close function Script:Start() local pos = self.entity:GetPosition(true) if self.parent~=nil then self.joint = Joint:Hinge(pos.x, pos.y, pos.z, self.pin.x, self.pin.y, self.pin.z, self.parent, self.child) if self.limitsenabled then self.joint:EnableLimits() end self.joint:SetLimits(self.limits.x,self.limits.y) self.joint:SetTargetAngle(self.limits.x) self.joint:EnableMotor() self.startTime = Time:GetCurrent() self.joint:SetMotorSpeed(self.movespeed) if self.hide then self.entity:Hide() end end end function Script:UpdatePhysics() local time_ = Time:GetCurrent() if time_ - self.startTime > self.tOpenClose*1000 then self.action = 1-self.action --toggle action if self.action == 1 then self.joint:SetTargetAngle(self.limits.x) else self.joint:SetTargetAngle(self.limits.y) end self.startTime = Time:GetCurrent() end end This scripts creates the joint, then set a target angle, enables the motor and apply a velocity, so the "door" starts going to the first limit. After the tOpenClose time, the variable action is toggled and based on it the TargetAngle will be first limit or second limit, in this way, the door continously open and then close after tOpenClose time Slider Joint For a slider joint, in this map i wrote one script: sliderMotorized.lua Script.child = nil --entity "hild" Script.hide = true--bool "Hide entity" Script.pin = Vec3(0,0,1) --Vec3 "Hinge Pin" ScrScript.parent = nil --entity "parent" ipt.limitsenabled=false--bool "Enable limits" Script.limits = Vec2(-45,45) --Vec2 "Limits" Script.tOpenClose = 15 --Int "Open/Close time" Script.movespeed = 60 --Int "Speed" Script.startTime = 0 Script.action = 1 --1 open, 0 close function Script:Start() local pos = self.entity:GetPosition(true) if self.parent~=nil then self.joint = Joint:Slider(pos.x, pos.y, pos.z, self.pin.x, self.pin.y, self.pin.z, self.parent, self.child) if self.limitsenabled then self.joint:EnableLimits() end self.joint:SetLimits(self.limits.x,self.limits.y) self.joint:SetTargetAngle(self.limits.x) self.joint:EnableMotor() self.startTime = Time:GetCurrent() self.joint:SetMotorSpeed(self.movespeed) if self.hide then self.entity:Hide() end end end function Script:UpdatePhysics() local time_ = Time:GetCurrent() if time_ - self.startTime > self.tOpenClose*1000 then self.action = 1-self.action --toggle action if self.action == 1 then self.joint:SetTargetAngle(self.limits.x) else self.joint:SetTargetAngle(self.limits.y) end self.startTime = Time:GetCurrent() end end This script is 99% the same as the hingeMotorized.lua, the only difference is that a slider joint is created instead of a hinge joint. Note that the limis are not angles, they are offsets from the initial position of the plataform. Stand alone executable: jointsworldDistro.zip Project: jointsworldProject.zip Enjoy Juan

Here there is a way of making a simple car based on hinges This is not a tutorial, is just simply a stating point for the ones that want/like to play arround physics and hinges... I included the entire project and the distribution executable to test the scripts so, you have as I say a starting point and just that, hpe it helps someone This is the editor view and the initial placement of the parts needed Basically I made 3 scripts physicsProperties.lua wheel.lua steer.lua First you have to create and place the car body or chassis, 4 wheels and 6 auxiliary pivots/ or any brush you like (a cube is fine) for the hinges 4 of the auxiliary entities are for the wheels hinges and 2 for the wheels steer. Place the wheel hinge and steer centered with the wheel. After that you may set some script parameters: Wheel scritp: Basically, the position of the entity script holder is used to create a hinge between the "parent" and the "child" you choose (in the picture above: between the auxiliary entity SteerFL and the WheelFL) If vel control is checked then, a motor is enabled for that hinge and keys Up/Down are used to increase/decrease speed If vel control is not cheched, no motor is enabled and the wheel is free to run Be carefull whit the Hinge Pin, which dictates the axis over which the wheel will rotate, in this case I used X axis, but if you use other pieces direction/alignement you should adjust this values. Steer script: The steer hinge is used to turn the wheel to handle car heading, so the pin is the Y axis Limits and Motor are needed to control the steer Limits is for how much the steer will turn right/left using the default keys left/right arrow When you press left or ritght key, the right limit will be set as the hinge angle and the hinge will try to reach this angle at the "steer speed", the same, but whit the left limit happen if you press the left key. physicsProperties just let you adjust the friction of the wheels and or the floor Script.sfrict=0--float "static friction" Script.kfrict=0--float "kinetic friction" function Script:Start() System:Print("phy properties start") self.entity:SetFriction(self.sfrict, self.kfrict) end so simple, and in the editor it looks: Here is a hand drawing of how scripts, objects, parent/child are connected Here is the wheel script Script.currspeed = 0 Script.parent = nil--Entity "Parent" Script.child = nil--Entity "Child" Script.pin = Vec3(0,0,1) --Vec3 "Hinge Pin" Script.motorspeed=500--float "Motor speed" Script.velcontrolled=false--bool "velControl" function Script:Start() System:Print("wheel start") self.entity:Hide() local pos = self.entity:GetPosition(false) --true for global if self.child ~= nil then if self.parent ~= nil then self.joint = Joint:Hinge(pos.x, pos.y, pos.z, self.pin.x, self.pin.y, self.pin.z, self.child, self.parent) else Debug:Error("no parent assigned") end else Debug:Error("no child assigned") end end function Script:setMotorSpeed(speed) if self.velcontrolled then System:Print("setMotorSpeed: "..speed) self.currspeed = speed if speed~=0 then self.joint:EnableMotor() end self.joint:SetMotorSpeed(self.currspeed) end end function Script:UpdateWorld() if self.motorspeed>0 then self.joint:SetAngle(self.joint:GetAngle()+100) else self.joint:SetAngle(self.joint:GetAngle()-100) end if App.window:KeyDown(Key.Space) then self:setMotorSpeed(0) end if self.velcontrolled then if App.window:KeyDown(Key.Up) then self.currspeed = self.currspeed + 10 if self.currspeed>self.motorspeed then self.currspeed=self.motorspeed end if self.currspeed == 10 then self.joint:EnableMotor() end self.joint:SetMotorSpeed(self.currspeed) end if App.window:KeyDown(Key.Down) then self.currspeed = self.currspeed - 10 if self.currspeed<-self.motorspeed then self.currspeed=-self.motorspeed end self.joint:SetMotorSpeed(self.currspeed) end end end Here is the steer scritp Script.parent = nil--Entity "Parent" Script.child = nil--Entity "Child" Script.pin = Vec3(0,1,0) --Vec3 "Hinge Pin" Script.useLimits=false--bool "use limits" Script.limits = Vec2(-45,45) --Vec2 "Limits" Script.useMotor=flase--bool "use motor" Script.motorspeed=50--float "Steer speed" function Script:Start() System:Print("steer start") if self.child == nil then Debug:Error("No child assigned.") end if self.parent == nil then Debug:Error("No parent assigned.") end self.entity:Hide() local pos = self.entity:GetPosition() self.joint = Joint:Hinge(pos.x, pos.y, pos.z, self.pin.x, self.pin.y, self.pin.z, self.child, self.parent) if self.useLimits then self.joint:EnableLimits() self.joint:SetLimits(self.limits.x,self.limits.y) end if self.useMotor then self.joint:SetMotorSpeed(self.motorspeed) self.joint:EnableMotor() end end function Script:UpdateWorld() local direction=0 if App.window:KeyDown(Key.Left) then direction=self.limits.x end if App.window:KeyDown(Key.Right) then direction=self.limits.y end self.joint:SetAngle(direction) end here the distro: simpleCarDistro.zip here the project: simpleCar.zip and here a video... enjoy Juan