## physics engine: impulse

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### physics engine: impulse

Here is some code to add impulse (acceleration) to an object . Note, the longer you hold down the key, the longer the force is applied to the object. Therefore, the acceleration will be greater. Its like turning rockets on. The object will not stop until acted on by an equal but opposite impulse. In other words, if you turn on the forward rockets for 10 seconds you will need to turn on the back rockets with one burst for 10 seconds or two bursts at 5 seconds / burst to cancel out the impulses.
Impulse = Force X time = mass X change in velocity

As long as the rockets are on, the velocity with increase. Turn the rockets off and the velocity with remain the same, but not decrease. Note: To get the position, I had to use
Code: Select all
`self.cubeBoy.getY(self.plnp)`

which is the position of the cube relative to a point light I attached to the render node at 0,0,0.

Using
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`self.cubeBoy.getPos()`

always returned (0,0,0)

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`from direct.directbase.DirectStart import * from pandac.PandaModules import * from direct.showbase.DirectObject import DirectObjectfrom direct.task.Task import Taskfrom direct.gui.OnscreenText import OnscreenTextimport sys, math, random# Function to put title on the screen.def addTitle(text):    return OnscreenText(text=text, style=1, fg=(1,1,1,1),            pos=(1.0,0.8), align=TextNode.ARight, scale = .08)# Function to put instructions on the screen.def addInstructions(pos, msg):    return OnscreenText(text=msg, style=1, fg=(1,1,1,1),            pos=(-1.23, pos), align=TextNode.ALeft, scale = .07)            # Function to put output on the screen.def addOutput(pos, msg):    return OnscreenText(text=msg, style=1, fg=(1,1,1,1),            pos=(-1.1, pos), align=TextNode.ALeft, scale = .06)            # Function to put information to the screen.def addInfo (msg):    return OnscreenText(text=msg, style=1, fg=(1,1,1,1),            pos=(0, -0.8), align=TextNode.ACenter, scale = .06)class World (DirectObject):    def __init__(self):        base.win.setClearColor(Vec4(0,0,0,1))        base.setBackgroundColor(0,0,0,1)        # Post the instructions        self.title = addTitle("Impulse")        self.inst1 = addInstructions(0.85, "[ESC]: Quit")        self.inst2 = addInstructions(0.75, "[W]:   Applies +Y force")        self.inst3 = addInstructions(0.65, "[E]:   Applies -Y force")        self.inst4 = addInfo ("The force is applied as long as the key is held down, i.e., the rockets are on and the impulse is increasing")        self.output1 = addOutput(0.30, "")        self.output2 = addOutput(0.20, "")        self.output3 = addOutput(0.10, "")        self.output4 = addOutput(0, "")                                #ambient light        self.alight = AmbientLight('alight')        self.alight.setColor(VBase4(0.5,0.5, 0.5, 1))        self.alnp = render.attachNewNode(self.alight)        render.setLight(self.alnp)                #light used as a reference point for 0,0,0        self.plight = PointLight('plight')        self.plight.setColor(VBase4(1, 1, 1, 1))        self.plight.setAttenuation(Point3(0,0.2, 0))        self.plnp = render.attachNewNode(self.plight)        render.setLight(self.plnp)        #object to apply a force to        self.cubeBoy = loader.loadModel('1cube')                #'camera'        base.oobe()        base.disableMouse()        self.camDist =20        self.camHeight = 10        base.camera.setPos(0,-self.camDist,self.camHeight)        base.camera.lookAt(self.cubeBoy)        base.camera.reparentTo(self.cubeBoy)                   #initialize variables        self.Yforce = 40        self.thisPImpulse = 0        self.totalPImpulse = 0        self.thisNImpulse = 0        self.totalNImpulse = 0        self.totalImpulse = 0        self.currentImpulse = 0        self.timeForceApplied = 0                #setup a force along Y axis        self.setupForce()                #keys to add positive and negative impulse        #        #as long as the key is held down, the force is applied         #i.e., the rockets are on        #impulse = force X time         #if you increase the amount of time the force is applied         #you increase the impulse, which is an increase in acceleration                self.accept("w", self.PYimpulse, [1])        self.accept("w-up", self.PYimpulse, [-1])        self.accept("e", self.NYimpulse, [1])        self.accept("e-up", self.NYimpulse, [-1])        self.accept("escape", sys.exit)        #task to update output         taskMgr.add(self.updateT, "updateT")                    def updateT(self,task):        self.output1 ['text'] = "Cube Position:           %0.2f" % self.cubeBoy.getY(self.plnp)        self.output2 ['text'] = "Last applied impulse:    %0.2f" % self.currentImpulse        self.output3 ['text'] = "Force of 40 applied for: %0.4f seconds " % self.timeForceApplied        self.output4 ['text'] = "Total impulse:           %0.2f" % self.totalImpulse        return Task.cont        def PYimpulse(self, arg):        if arg ==1:            self.pull = LinearVectorForce(0, self.Yforce, 0)            self.fn.addForce(self.pull)            self.an.getPhysical(0).addLinearForce(self.pull)                        self.startTime = globalClock.getFrameTime()        else:            self.fn.removeForce(self.pull)            self.an.getPhysical(0).removeLinearForce(self.pull)                        self.endTime = globalClock.getFrameTime()            self.timeForceApplied = self.endTime - self.startTime              self.thisPImpulse = self.timeForceApplied * self.Yforce             self.currentImpulse = self.thisPImpulse            self.totalPImpulse+=self.thisPImpulse #ever applied            self.totalImpulse = self.totalPImpulse + self.totalNImpulse #current net impulse                                    def NYimpulse(self, arg):        if arg ==1:            self.pull = LinearVectorForce(0, -self.Yforce, 0)            self.fn.addForce(self.pull)            self.an.getPhysical(0).addLinearForce(self.pull)                        self.startTime = globalClock.getFrameTime()        else:            self.fn.removeForce(self.pull)            self.an.getPhysical(0).removeLinearForce(self.pull)                        self.endTime = globalClock.getFrameTime()            self.timeForceApplied = self.endTime - self.startTime            self.startTime =0                        self.thisNImpulse = self.timeForceApplied * -self.Yforce             self.currentImpulse = self.thisNImpulse            self.totalNImpulse+=self.thisNImpulse            self.totalImpulse = self.totalPImpulse + self.totalNImpulse            def setupForce(self):         # The physical object, that is, the object which is acted upon by         # the physics system, needs an ActorNode to define the coordinate         # system in which it is moving, and to manifest the object's         # actions.          self.an = ActorNode('actor node')         self.anp = render.attachNewNode(self.an)         # A model is parented to the ActorNode just so we can see what         # it's doing.                 self.cubeBoy.reparentTo(self.anp)             #light to highlight object        self.plight1 = PointLight('plight1')        self.plight1.setColor(VBase4(1, 1, 1, 1))        self.plight1.setAttenuation(Point3(0,0.2, 0))        self.plnp1 = self.cubeBoy.attachNewNode(self.plight1)        self.plnp1.setPos(0,0,1)        render.setLight(self.plnp1)        # We need to attach it to a PhysicsManager to do the actual work         # of moving it around.          base.physicsMgr.attachPhysicalNode(self.an)         # This method must be called to enable the task that computes the         # PhysicsManager every frame.          base.enableParticles()         # Now apply a force to the ActorNode.  The Force needs a         # ForceNode, to define the coordinate system in which it is         # applied.         self.fn = ForceNode('pull')         self.fnp = render.attachNewNode(self.fn)         self.pull = LinearVectorForce(0, 0, 0)         self.fn.addForce(self.pull)         # Here we make it a local force that affects only this one         # ActorNode.  We could make it a global force instead by adding it         # to base.physicsMgr.         self.an.getPhysical(0).addLinearForce(self.pull)    w=World()run()`
BrianInSuwon

Posts: 98
Joined: Wed Jun 18, 2008 1:25 am

Very nice, thanks.
koryrok

Posts: 23
Joined: Sat May 16, 2009 7:35 am

nice snip thanks for share it - it would be perfect adding a counterforce attenuation (friction) so that would be applied on any kind of vehicle anyhow is a good starting point
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astelix

Posts: 866
Joined: Mon Mar 27, 2006 4:36 pm
Location: Milano, ITA

Thanks for the example!

I got a couple of errors, one was presumable because the models which come with panda have changed (box is clearly too small but it works) and the other due to late initialization of the particle system. Here is a diff (simple fix but maybe someone will find it useful ):

***************
*** 56,66 ****
self.plight.setAttenuation(Point3(0,0.2, 0))
self.plnp = render.attachNewNode(self.plight)
render.setLight(self.plnp)

#object to apply a force to
! self.cubeBoy = loader.loadModel('1cube')

#'camera'
base.oobe()
base.disableMouse()
self.camDist =20
--- 56,66 ----
self.plight.setAttenuation(Point3(0,0.2, 0))
self.plnp = render.attachNewNode(self.plight)
render.setLight(self.plnp)

#object to apply a force to
! self.cubeBoy = loader.loadModel('box')

#'camera'
base.oobe()
base.disableMouse()
self.camDist =20
***************
*** 151,160 ****
--- 151,164 ----
self.totalImpulse = self.totalPImpulse + self.totalNImpulse

def setupForce(self):
+ # This method must be called to enable the task that computes the
+ # PhysicsManager every frame.
+ base.enableParticles()
+
# The physical object, that is, the object which is acted upon by
# the physics system, needs an ActorNode to define the coordinate
# system in which it is moving, and to manifest the object's
# actions.
self.an = ActorNode('actor node')
***************
*** 175,188 ****

# We need to attach it to a PhysicsManager to do the actual work
# of moving it around.
base.physicsMgr.attachPhysicalNode(self.an)

- # This method must be called to enable the task that computes the
- # PhysicsManager every frame.
- base.enableParticles()
-
# Now apply a force to the ActorNode. The Force needs a
# ForceNode, to define the coordinate system in which it is
# applied.
self.fn = ForceNode('pull')
self.fnp = render.attachNewNode(self.fn)
--- 179,188 ----
shazoom

Posts: 18
Joined: Mon Feb 08, 2010 8:29 pm

good stuff, thanks for sharing. ran it directly with the above diffs and all works fine, except for the little camera that follows the cube, but i can manage that
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radu

Posts: 153
Joined: Fri Nov 06, 2009 11:09 am
Location: Bucharest

how do you apply the patch?
ambyra

Posts: 130
Joined: Sat Sep 20, 2008 5:47 pm

You could probably use some sort of software that can use the diff file posted above, but I did manually. You can see there where the lines that need replacement or adding are and there are (+) and (-) signs where you need to add or cut stuff from the original file.

Anyway, since you probably need the final code, here it is, in case you want to skip patching:

Code: Select all
`from direct.directbase.DirectStart import * from pandac.PandaModules import * from direct.showbase.DirectObject import DirectObject from direct.task.Task import Task from direct.gui.OnscreenText import OnscreenText import sys, math, random # Function to put title on the screen. def addTitle(text):     return OnscreenText(text=text, style=1, fg=(1,1,1,1),             pos=(1.0,0.8), align=TextNode.ARight, scale = .08) # Function to put instructions on the screen. def addInstructions(pos, msg):     return OnscreenText(text=msg, style=1, fg=(1,1,1,1),             pos=(-1.23, pos), align=TextNode.ALeft, scale = .07)             # Function to put output on the screen. def addOutput(pos, msg):     return OnscreenText(text=msg, style=1, fg=(1,1,1,1),             pos=(-1.1, pos), align=TextNode.ALeft, scale = .06)             # Function to put information to the screen. def addInfo (msg):     return OnscreenText(text=msg, style=1, fg=(1,1,1,1),             pos=(0, -0.8), align=TextNode.ACenter, scale = .06) class World (DirectObject):     def __init__(self):         base.win.setClearColor(Vec4(0,0,0,1))         base.setBackgroundColor(0,0,0,1)         # Post the instructions         self.title = addTitle("Impulse")         self.inst1 = addInstructions(0.85, "[ESC]: Quit")         self.inst2 = addInstructions(0.75, "[W]:   Applies +Y force")         self.inst3 = addInstructions(0.65, "[E]:   Applies -Y force")         self.inst4 = addInfo ("The force is applied as long as the key is held down, i.e., the rockets are on and the impulse is increasing")         self.output1 = addOutput(0.30, "")         self.output2 = addOutput(0.20, "")         self.output3 = addOutput(0.10, "")         self.output4 = addOutput(0, "")                                 #ambient light         self.alight = AmbientLight('alight')         self.alight.setColor(VBase4(0.5,0.5, 0.5, 1))         self.alnp = render.attachNewNode(self.alight)         render.setLight(self.alnp)                 #light used as a reference point for 0,0,0         self.plight = PointLight('plight')         self.plight.setColor(VBase4(1, 1, 1, 1))         self.plight.setAttenuation(Point3(0,0.2, 0))         self.plnp = render.attachNewNode(self.plight)         render.setLight(self.plnp)         #object to apply a force to         self.cubeBoy = loader.loadModel('box')         #'camera'         base.oobe()         base.disableMouse()         self.camDist =20         self.camHeight = 10         base.camera.setPos(0,-self.camDist,self.camHeight)         base.camera.lookAt(self.cubeBoy)         base.camera.reparentTo(self.cubeBoy)                     #initialize variables         self.Yforce = 40         self.thisPImpulse = 0         self.totalPImpulse = 0         self.thisNImpulse = 0         self.totalNImpulse = 0         self.totalImpulse = 0         self.currentImpulse = 0         self.timeForceApplied = 0                 #setup a force along Y axis         self.setupForce()                 #keys to add positive and negative impulse         #         #as long as the key is held down, the force is applied         #i.e., the rockets are on         #impulse = force X time         #if you increase the amount of time the force is applied         #you increase the impulse, which is an increase in acceleration                 self.accept("w", self.PYimpulse, [1])         self.accept("w-up", self.PYimpulse, [-1])         self.accept("e", self.NYimpulse, [1])         self.accept("e-up", self.NYimpulse, [-1])         self.accept("escape", sys.exit)         #task to update output         taskMgr.add(self.updateT, "updateT")                     def updateT(self,task):         self.output1 ['text'] = "Cube Position:           %0.2f" % self.cubeBoy.getY(self.plnp)         self.output2 ['text'] = "Last applied impulse:    %0.2f" % self.currentImpulse        self.output3 ['text'] = "Force of 40 applied for: %0.4f seconds " % self.timeForceApplied         self.output4 ['text'] = "Total impulse:           %0.2f" % self.totalImpulse         return Task.cont         def PYimpulse(self, arg):         if arg ==1:             self.pull = LinearVectorForce(0, self.Yforce, 0)             self.fn.addForce(self.pull)             self.an.getPhysical(0).addLinearForce(self.pull)                         self.startTime = globalClock.getFrameTime()         else:             self.fn.removeForce(self.pull)             self.an.getPhysical(0).removeLinearForce(self.pull)                         self.endTime = globalClock.getFrameTime()             self.timeForceApplied = self.endTime - self.startTime               self.thisPImpulse = self.timeForceApplied * self.Yforce             self.currentImpulse = self.thisPImpulse             self.totalPImpulse+=self.thisPImpulse #ever applied             self.totalImpulse = self.totalPImpulse + self.totalNImpulse #current net impulse                                     def NYimpulse(self, arg):         if arg ==1:             self.pull = LinearVectorForce(0, -self.Yforce, 0)             self.fn.addForce(self.pull)             self.an.getPhysical(0).addLinearForce(self.pull)                         self.startTime = globalClock.getFrameTime()         else:             self.fn.removeForce(self.pull)             self.an.getPhysical(0).removeLinearForce(self.pull)                         self.endTime = globalClock.getFrameTime()             self.timeForceApplied = self.endTime - self.startTime             self.startTime =0                         self.thisNImpulse = self.timeForceApplied * -self.Yforce             self.currentImpulse = self.thisNImpulse             self.totalNImpulse+=self.thisNImpulse             self.totalImpulse = self.totalPImpulse + self.totalNImpulse             def setupForce(self):         # This method must be called to enable the task that computes the         # PhysicsManager every frame.         base.enableParticles()         # The physical object, that is, the object which is acted upon by         # the physics system, needs an ActorNode to define the coordinate         # system in which it is moving, and to manifest the object's         # actions.          self.an = ActorNode('actor node')         self.anp = render.attachNewNode(self.an)         # A model is parented to the ActorNode just so we can see what         # it's doing.                 self.cubeBoy.reparentTo(self.anp)             #light to highlight object         self.plight1 = PointLight('plight1')         self.plight1.setColor(VBase4(1, 1, 1, 1))         self.plight1.setAttenuation(Point3(0,0.2, 0))         self.plnp1 = self.cubeBoy.attachNewNode(self.plight1)         self.plnp1.setPos(0,0,1)         render.setLight(self.plnp1)         # We need to attach it to a PhysicsManager to do the actual work         # of moving it around.          base.physicsMgr.attachPhysicalNode(self.an)         # Now apply a force to the ActorNode.  The Force needs a         # ForceNode, to define the coordinate system in which it is         # applied.         self.fn = ForceNode('pull')         self.fnp = render.attachNewNode(self.fn)         self.pull = LinearVectorForce(0, 0, 0)         self.fn.addForce(self.pull)         # Here we make it a local force that affects only this one         # ActorNode.  We could make it a global force instead by adding it         # to base.physicsMgr.         self.an.getPhysical(0).addLinearForce(self.pull)     w=World() run() `
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radu

Posts: 153
Joined: Fri Nov 06, 2009 11:09 am
Location: Bucharest

### Thank you!

I've been searching for examples of addLinearForce(), and couldn't find any. The official doc pages are woefully skimpy.

Thank you for providing a working example!
huttarl

Posts: 4
Joined: Fri Jan 21, 2011 4:29 pm

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