Joints and Body Collisions

For the past few days I've been working on writing and testing an object oriented encapsulation of the open source Chipmunk2D physics library. This compliments my existing SDL and NanoVG bindings for Free Pascal. Below is a short video clip of my program to test precise detection of body collision points as well as the use of spring joints

Video

The program below tests the precise detection of collision points against a class of physics bodies. Receding pink circles are drawn at points where the bodies collide. Dynamic bodies (those that are not anchored) can be moved with the mouse and a green arrow indicates the force being applied to the body being grabbed. All the graphics are drawn in real time using OpenGLESv2 with the extremely fast NanoVG vector graphics library.

Video: Collision Test

Source Code

The following is a listing of the Pascal unit responsible for the test program above. The SDL, NanoVG, and Chipmunk2D code were built by me as static libraries and link inside of programs like this test. I have written well formatted Pascal units handling both the C interface to these projects. Additionally, I am in the process of writing quite sensible object orient encapsulations of each of these units, making it easier to discover and use these libraries in Pascal.

If you are interested in helping me write or test the Pascal encapsulations, message me and we can use Discord to share ideas.

show source     
001unit JointScene;
002 
003{$i options.inc}
004 
005interface
006 
007uses
008  NanoVG, Chipmunk2D, ChipmunkObjs, PhysicsTools;
009 
010{ TJointScene }
011 
012type
013  TJointScene = class(TPhysicsScene)
014  private
015    FHits: array[0..50] of TVect;
016    FHitTimes: array[0..50] of Double;
017    FHitIndex: Integer;
018    procedure HandleHits(arb: cpArbiter);
019  protected
020    procedure Load; override;
021    procedure Render(Width, Height: Integer; const Time: Double); override;
022  end;
023 
024implementation
025 
026const
027  ballType = TCollisionType(1);
028  weightType = TCollisionType(2);
029  boxType = TCollisionType(3);
030 
031procedure TJointScene.HandleHits(arb: cpArbiter);
032var
033  Points: cpContactPointSetStruct;
034begin
035  Points := cpArbiterGetContactPointSet(arb);
036  FHits[FHitIndex] := Points.points[0].pointA;
037  FHitTimes[FHitIndex] := Time;
038  FHitIndex := (FHitIndex + 1) mod (High(FHits) + 1);
039end;
040 
041procedure WeightsCollide(arb: cpArbiter; space: cpSpace; userData: cpDataPointer); cdecl;
042var
043  Scene: TJointScene absolute userData;
044begin
045  if cpArbiterIsFirstContact(arb) <> cpFalse then
046    Scene.HandleHits(arb);
047end;
048 
049procedure TJointScene.Load;
050var
051  Handler: TCollisionHandler;
052  B, C, K: TBody;
053  J: TJoint;
054  M: TFloat;
055  I: Integer;
056begin
057  inherited Load;
058  Space.Gravity := Vect(0, 1000);
059  Space.SleepTimeThreshold := 0.25;
060  Space.IdleSpeedThreshold := 8;
061  Space.Damping := 0.8;
062 
063  Handler := Space.AddCollisionHandler(weightType, weightType);
064  Handler.postSolveFunc := WeightsCollide;
065  Handler.userData := Self;
066 
067  Handler := Space.AddCollisionHandler(weightType, ballType);
068  Handler.postSolveFunc := WeightsCollide;
069  Handler.userData := Self;
070 
071  for I := 0 to 20 do
072  begin
073    C := Space.NewBody;
074    with C.NewCircle(30) do
075    begin
076      Friction := 0.7;
077      Elasticity := 0.75;
078      Density := 2;
079      CollisionType := ballType;
080    end;
081    C.Position := Vect(Random * 1800 + 100, Random * -500 - 200);
082  end;
083 
084  for I := 1 to 7 do
085  begin
086    B := Space.NewBody;
087    with B.NewBox(80, I * 25) do
088    begin
089      Friction := 0.7;
090      Elasticity := 1;
091      Density := 5;
092      CollisionType := weightType;
093    end;
094    B.Position := Vect(I * 200 + 150, 600);
095    K := Space.NewKinematicBody;
096    with K.NewBox(40, 40) do
097    begin
098      Friction := 0;
099      Elasticity := 0;
100      CollisionType := boxType;
101    end;
102    K.Position := Vect(I * 200 + 150, 400);
103    M := cpBodyGetMass(B) / 100000;
104    J := Space.NewDampedSpring(K, B,  Vect(0, 0), Vect(0, -I * 25 / 2), 100, 1000000 * M, 0.1);
105    J.CollideBodies := True;
106  end;
107 
108  GenerateStudioWalls;
109  AllowGrab := True;
110end;
111 
112procedure TJointScene.Render(Width, Height: Integer; const Time: Double);
113const
114  Title = 'Joint & Collision Test'#10 +
115    'This program tests spring joints and precise collision detection of contact points';
116  RingTime = 3;
117var
118  Fade: Double;
119  I, J: Integer;
120begin
121  inherited Render(Width, Height, Time);
122  DrawPhysics;
123  nvgStrokeWidth(Ctx, 3);
124  for I := Low(FHits) to High(FHits) do
125    if Time - FHitTimes[I] < RingTime then
126    begin
127      Fade := 1 - (Time - FHitTimes[I]) / RingTime;
128      Fade := Fade * Fade * Fade * Fade * Fade;
129      for J := 0 to 2 do
130      begin
131        nvgStrokeColor(Ctx, nvgRGBAf(1, 0.5, 1, Fade));
132        nvgBeginPath(Ctx);
133        nvgCircle(Ctx, FHits[I].x, FHits[I].y, 50 * Fade);
134        nvgStroke(Ctx);
135        Fade := Fade * Fade;
136      end;
137    end;
138  nvgResetTransform(Ctx);
139  DrawSceneInfo(Title);
140end;
141 
142end.