  // Chassis
begin
shape := Tb2PolygonShape.Create;
shape.SetAsBox(2.5, 1.0);
sd := Tb2FixtureDef.Create;
sd.density := 1.0;
sd.shape := shape;
sd.filter.groupIndex := -1;
bd := Tb2BodyDef.Create;
bd.bodyType := b2_dynamicBody;
{$IFDEF OP_OVERLOAD}
bd.position := pivot + m_offset;
{$ELSE}
bd.position := Add(pivot, m_offset);
{$ENDIF}
m_chassis := m_world.CreateBody(bd);
m_chassis.CreateFixture(sd);
end;
begin
cshape := Tb2CircleShape.Create;
cshape.m_radius := 1.6;
sd := Tb2FixtureDef.Create;
sd.density := 1.0;
sd.shape := cshape;
sd.filter.groupIndex := -1;
bd := Tb2BodyDef.Create;
bd.bodyType := b2_dynamicBody;
{$IFDEF OP_OVERLOAD}
bd.position := pivot + m_offset;
{$ELSE}
bd.position := Add(pivot, m_offset);
{$ENDIF}
m_wheel := m_world.CreateBody(bd);
m_wheel.CreateFixture(sd);
end;
begin
jd := Tb2RevoluteJointDef.Create;
{$IFDEF OP_OVERLOAD}
jd.Initialize(m_wheel, m_chassis, pivot + m_offset);
{$ELSE}
jd.Initialize(m_wheel, m_chassis, Add(pivot, m_offset));
{$ENDIF}
jd.collideConnected := False;
jd.motorSpeed := m_motorSpeed;
jd.maxMotorTorque := 400.0;
jd.enableMotor := m_motorOn;
m_motorJoint := Tb2RevoluteJoint(m_world.CreateJoint(jd));
end;
{$IFDEF OP_OVERLOAD}
wheelAnchor := pivot + MakeVector(0.0, -0.8);
{$ELSE}
wheelAnchor := Add(pivot, MakeVector(0.0, -0.8));
{$ENDIF}
CreateLeg(-1.0, wheelAnchor);
CreateLeg(1.0, wheelAnchor);
m_wheel.SetTransform(m_wheel.GetPosition, 120.0 * Pi / 180.0);
CreateLeg(-1.0, wheelAnchor);
CreateLeg(1.0, wheelAnchor);
m_wheel.SetTransform(m_wheel.GetPosition, -120.0 * Pi / 180.0);
CreateLeg(-1.0, wheelAnchor);
CreateLeg(1.0, wheelAnchor);
As you can see at the bottom it uses a function called CreateLeg to create each of the six legs. Here is the source code for the CreateLeg procedure:
procedure TJansenWalker.CreateLeg(s: PhysicsFloat; const wheelAnchor: TVector2);
var
p1, p2, p3, p4, p5, p6: TVector2;
vertices: array[0..2] of TVector2;
poly1, poly2: Tb2PolygonShape;
fd1, fd2: Tb2FixtureDef;
bd1, bd2: Tb2BodyDef;
body1, body2: Tb2Body;
djd: Tb2DistanceJointDef;
rjd: Tb2RevoluteJointDef;
begin
p1 := MakeVector(5.4 * s, -6.1);
p2 := MakeVector(7.2 * s, -1.2);
p3 := MakeVector(4.3 * s, -1.9);
p4 := MakeVector(3.1 * s, 0.8);
p5 := MakeVector(6.0 * s, 1.5);
p6 := MakeVector(2.5 * s, 3.7);
fd1 := Tb2FixtureDef.Create;
fd2 := Tb2FixtureDef.Create;
fd1.filter.groupIndex := -1;
fd2.filter.groupIndex := -1;
fd1.density := 1.0;
fd2.density := 1.0;
poly1 := Tb2PolygonShape.Create;
poly2 := Tb2PolygonShape.Create;
if s > 0.0 then
begin
vertices[0] := p1;
vertices[1] := p2;
vertices[2] := p3;
poly1.SetVertices(@vertices[0], 3);
vertices[0] := b2Vec2_zero;
{$IFDEF OP_OVERLOAD}
vertices[1] := p5 - p4;
vertices[2] := p6 - p4;
{$ELSE}
vertices[1] := Subtract(p5, p4);
vertices[2] := Subtract(p6, p4);
{$ENDIF}
poly2.SetVertices(@vertices[0], 3);
end
else
begin
vertices[0] := p1;
vertices[1] := p3;
vertices[2] := p2;
poly1.SetVertices(@vertices[0], 3);
vertices[0] := b2Vec2_zero;
{$IFDEF OP_OVERLOAD}
vertices[1] := p6 - p4;
vertices[2] := p5 - p4;
{$ELSE}
vertices[1] := Subtract(p6, p4);
vertices[2] := Subtract(p5, p4);
{$ENDIF}
poly2.SetVertices(@vertices[0], 3);
end;
fd1.shape := poly1;
fd2.shape := poly2;
bd1 := Tb2BodyDef.Create;
bd1.bodyType := b2_dynamicBody;
bd1.position := m_offset;
bd2 := Tb2BodyDef.Create;
bd2.bodyType := b2_dynamicBody;
{$IFDEF OP_OVERLOAD}
bd2.position := p4 + m_offset;
{$ELSE}
bd2.position := Add(p4, m_offset);
{$ENDIF}
bd1.angularDamping := 10.0;
bd2.angularDamping := 10.0;
body1 := m_world.CreateBody(bd1);
body2 := m_world.CreateBody(bd2);
body1.CreateFixture(fd1);
body2.CreateFixture(fd2);
djd := Tb2DistanceJointDef.Create;
// Using a soft distance constraint can reduce some jitter.
// It also makes the structure seem a bit more fluid by
// acting like a suspension system.
djd.dampingRatio := 0.5;
djd.frequencyHz := 10.0;
{$IFDEF OP_OVERLOAD}
djd.Initialize(body1, body2, p2 + m_offset, p5 + m_offset);
{$ELSE}
djd.Initialize(body1, body2, Add(p2, m_offset), Add(p5, m_offset));
{$ENDIF}
m_world.CreateJoint(djd, False);
{$IFDEF OP_OVERLOAD}
djd.Initialize(body1, body2, p3 + m_offset, p4 + m_offset);
{$ELSE}
djd.Initialize(body1, body2, Add(p3, m_offset), Add(p4, m_offset));
{$ENDIF}
m_world.CreateJoint(djd, False);
{$IFDEF OP_OVERLOAD}
djd.Initialize(body1, m_wheel, p3 + m_offset, wheelAnchor + m_offset);
{$ELSE}
djd.Initialize(body1, m_wheel, Add(p3, m_offset), Add(wheelAnchor, m_offset));
{$ENDIF}
m_world.CreateJoint(djd, False);
{$IFDEF OP_OVERLOAD}
djd.Initialize(body2, m_wheel, p6 + m_offset, wheelAnchor + m_offset);
{$ELSE}
djd.Initialize(body2, m_wheel, Add(p6, m_offset), Add(wheelAnchor, m_offset));
{$ENDIF}
m_world.CreateJoint(djd);
rjd := Tb2RevoluteJointDef.Create;
{$IFDEF OP_OVERLOAD}
rjd.Initialize(body2, m_chassis, p4 + m_offset);
{$ELSE}
rjd.Initialize(body2, m_chassis, Add(p4, m_offset));
{$ENDIF}
m_world.CreateJoint(rjd);
end;
Download the Box2d Firemonkey physics engine and demos to check out the Theo Jansen Walker.
Or check out the full source code for the Theo Jansen Walker test web demo on Github.
