Example 4 : Rigid Body Bridge

Many tutorials are included in Dark Physics covering all kinds of topics. One of them describes how to make a bridge using a collection of rigid bodies and joints. The tutorial guides you through from an empty program to constructing a full working bridge.

` there is a great deal of flexibility in Dark Physics and it allows for all kinds of
` complex structures to be created through the use of joints, in this tutorial
` we will look at how a simple set of boxes can be transformed into a
` bridge structure

phy start
sync on
sync rate 60

load image "stripe5.png",10000
make object sphere 10000,800,48,48
texture object 10000,10000
scale object texture 10000,6,6
set object cull 10000,0
rotate object 10000,0,0,90

load image "stripe6.png", 3

` our next stage is to create our ground object, this object is a simple box that
` then has a rigid body attached to it which then has its dynamic state set to false
make object box 1, 400, 1, 400
color object 1, rgb ( 100, 0, 0 )
phy make rigid body static box 1
ghost object on 1, 0
texture object 1, 3

` when the bridge has been created it will need to be attached to something, it cannot
` simply be positioned in empty space so we need to think about what each end of the bridge
` can be attached to, we can create 2 box objects and position these on top of the ground
` and by doing this have 2 locations upon which we can attach the bridge, the code for
` this is fairly straight forward, the first block of code creates a box with an ID number
` of 2 and size of 60, 50, 400, it is then positioned on the left side of the ground object
` and a rigid body box created for it
make object box 2, 60, 50, 400
position object 2, -170, 25, 0
phy make rigid body static box 2
texture object 2, 3

` the second block of code is similar except this time an ID number of 3 is used and the
` object is positioned on the right side of the ground as shown by the screenshot
make object box 3, 60, 50, 400
position object 3, 170, 25, 0
phy make rigid body static box 3
texture object 3, 3

` construct the support chains for the bridge
CreateSupportChain( 4, 27, 1, -137, 50, 50 )
CreateSupportChain( 28, 51, 50, -137, 50, -50 )

` the next block of code creates the beams between the support chains and also hooks
` everything together with hinge joints
JointID = 100

x# = -131
y# = 50
z# = 0

a = 4
b = 28

` run through objects
for i = 52 to 75
   ` create a beam and position it
   make object box i, 2, 2, 100
   position object i, x#, y#, z#

   ` create a rigid body box for it
   phy make rigid body dynamic box i

   color object i, rgb ( rnd ( 255 ), rnd ( 255 ), rnd ( 255 ) )
   set object specular i, rgb ( rnd ( 255 ), rnd ( 255 ), rnd ( 255 ) )
   set object specular power i, 255
   set object ambient i,0

   ` create a revolute joint between the beam and its closest support chain block on the far side
   phy make revolute joint JointID, i, a,  1, 0, 0, object position x ( a ), object position y ( a ), object position z ( a )
   JointID = JointID + 1

   ` create a revolute joint between the beam and its closest support chain block on the near side
   phy make revolute joint JointID, i, b,  1, 0, 0, object position x ( b ), object position y ( b ), object position z ( b )
   JointID = JointID + 1

   ` increment the values
   a = a + 1
   b = b + 1
   x# = x# + 12
next i

` create a heavy sphere to roll across the bridge
make object sphere 100, 50
position object 100, 175, 90, 0

phy make rigid body dynamic sphere 100

color object 100, rgb ( rnd ( 255 ), rnd ( 255 ), rnd ( 255 ) )
set object specular 100, rgb ( rnd ( 255 ), rnd ( 255 ), rnd ( 255 ) )
set object specular power 100, 255
set object ambient 100,0

` position and rotate the camera
position camera -120, 135, -161
rotate camera 35, 34, 0

make light 1
set directional light 1, -5,-5,5

load image "logo.png", 100000
sprite 1, 0, 600 - 60, 100000

` now our main loop
do
   ` display information
   set cursor 0, 0
   print "Bridge construction example, press space to move the sphere"

   rotate object 10000, 0, object angle y( 10000 ) + 0.3, 0

   ` when the space key is pressed apply a velocity to the sphere
   if spacekey ( )
      phy set rigid body linear velocity 100, -20, 10, 0
   endif

   phy update
   sync
loop


function CreateSupportChain( StartID, EndID, JointID, x#, y#, z# )
   ` this function will create a support chain for the bridge from the specified
   ` starting point

   ` create the chain of boxes
   for i = StartID to EndID
      make object box i, 10, 2, 2
      position object i, x#, y#, z#
      phy make rigid body dynamic box i

      color object i, rgb ( rnd ( 255 ), rnd ( 255 ), rnd ( 255 ) )
      set object specular i, rgb ( rnd ( 255 ), rnd ( 255 ), rnd ( 255 ) )
      set object specular power i, 255
      set object ambient i,0

      x# = x# + 12
   next i

   ` set up ball joints between them
   for i = StartID to EndID - 1
      phy make sphere joint JointID, i, i + 1, object position x ( i ), object position y ( i ), object position z ( i )
      JointID = JointID + 1
   next i

   ` ball joints going in other direction
   for i = StartID + 1 to EndID
      phy make sphere joint JointID, i, i - 1, object position x ( i ), object position y ( i ), object position z ( i )
      JointID = JointID + 1
   next i

   ` further ball joints
   phy make sphere joint JointID, 2, StartID, object position x ( StartID ), object position y ( StartID ), object position z ( StartID )
   JointID = JointID + 1

   phy make sphere joint JointID, 3, EndID - 1, object position x ( EndID - 1 ), object position y ( EndID - 1 ), object position z ( EndID - 1 )
   JointID = JointID + 1
endfunction


function strapline(mode,s$)
   if mode=0
      ink 0x66FFFFFF,0 : set text font "Tahoma" : set text size 32
   else
      center text screen width()/2,20,s$
   endif
endfunction

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