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PA199
Advanced Game Design
Lecture 6
Collision Detection
Dr. Fotis Liarokapis
24th March 2015
Motivation
• Techniques for collision detection depend on
the type of game
• For many games rough approximations are fine
– i.e. Arcade-style games
• For more complex games need to be familiar
with a variety of techniques ranging from
simple to complex
– i.e. 3D games
Rough Approximations Example Collision Detection
• Do objects collide/intersect?
– Static
– Dynamic
• Picking is simple special case of general collision
detection problem
– Check if ray cast from cursor position collides with
any object in scene
– Simple shooting
• Projectile arrives instantly, zero travel time
Collision Detection .
• A better solution
– Projectile and target move over time
– See if collides with object during trajectory
Collision Detection Applications
• Determining if player hit
wall/floor/obstacle and stop them walking
through it
–Terrain following (floor)
–Maze games (walls)
• Determining if projectile has hit target
• Determining if player has hit target
–Punch/kick (desired)
–Car crash (not desired)
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Collision Detection Applications .
• Detecting points at which behavior should
change
– Car in the air returning to the ground
• Cleaning up animation
– Making sure a motion-captured character’s
feet do not pass through the floor
• Simulating motion
– Physics, or cloth, or something else
Simulating Motion
Why it is Hard?
• Complicated for two reasons
– Geometry is typically very complex
• Potentially requiring expensive testing
– Naïve solution is O(n2) time complexity
• Since every object can potentially collide with every
other object
Why it is Hard - Example
Basic Concepts From Simple to Complex
• Boundary check
–Perimeter of world vs. viewpoint or objects
• 2D/3D absolute coordinates for bounds
• Simple point in space for viewpoint/objects
• Set of fixed barriers
–Walls in maze game
• 2D/3D absolute coordinate system
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From Simple to Complex .
• Set of moveable objects
– One object against set of items
• Missile vs. several tanks
– Multiple objects against each other
• Punching game: arms and legs of players
• Room of bouncing balls
Naive General Collision Detection
• For each object i containing polygons p
– Test for intersection with object j containing
polygons q
• For polyhedral objects, test if object i penetrates
surface of j
– Test if vertices of i straddle polygon q of j
• If straddle, then test intersection of polygon q with
polygon p of object i
• Very expensive! O(n2)
Fundamental Design Principles
• Fast simple tests first, eliminate many
potential collisions
– Test bounding volumes before testing
individual triangles
• Exploit locality, eliminate many potential
collisions
– Use cell structures to avoid considering
distant objects
Fundamental Design Principles .
• Use as much information as possible about
geometry
–Spheres have special properties that speed
collision testing
• Exploit coherence between successive
tests
–Things don’t typically change much between
two frames
Example: Player-Wall Collisions
• ‘First person’ games must prevent the player
from walking through walls and other obstacles
• Most general case
– Player and walls are polygonal meshes
• Each frame, player moves along path not known
in advance
– Assume piecewise linear
• Straight steps on each frame
– Assume player’s motion could be fast
Simple Approach
• On each step, do a general mesh-to-mesh
intersection test to find out if the player
intersects the wall
• If they do, refuse to allow the player to move
• Problems with this approach? how can we
improve:
– In response?
– In speed?
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Collision Response
• Frustrating to just stop
– For player motions, often best thing to do is move
player tangentially to obstacle
• Do recursively to ensure all collisions caught
– Find time and place of collision
– Adjust velocity of player
– Repeat with new velocity, start time, start position
(reduced time interval)
• Handling multiple contacts at same time
– Find a direction that is tangential to all contacts
Typical Approaches
Collision Detection Approaches
• Two basic techniques:
– Overlap testing
• Detects whether a collision has already occurred
–Intersection testing
• Predicts whether a collision will occur in the
future
Overlap Testing
• Facts
–Most common technique used in games
–Exhibits more error than intersection testing
• Concept
–For every simulation step, test every pair of
objects to see if they overlap
–Easy for simple volumes like spheres, harder
for polygonal models
Overlap Testing: Useful Results
• Useful results of detected collision
– Time collision took place
– Collision normal vector
Overlap Testing: Collision Time
• Collision time calculated by moving object
back in time until right before collision
– Bisection is an effective technique
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Overlap Testing: Limitations
• Fails with objects that move too fast
– Unlikely to catch time slice during overlap
• Possible solutions
– Design constraint on speed of objects
– Reduce simulation step size
Intersection Testing
• Predict future collisions
• When predicted:
–Move simulation to time of collision
–Resolve collision
–Simulate remaining time step
Intersection Testing: Swept Geometry
• Extrude geometry in direction of movement
• Swept sphere turns into a ‘capsule’ shape
Intersection Testing: Sphere-Sphere
Collision
Intersection Testing: Limitations
• Issue with networked games
– Future predictions rely on exact state of world
at present time
– Due to packet latency, current state not
always coherent
• Assumes constant velocity and zero
acceleration over simulation step
– Has implications for physics model and choice
of integrator
Complexity Issues
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Dealing with Complexity
• Two common issues when dealing with
complexity:
– Complex geometry must be simplified
• Not so easy!
– Reduce number of object pair tests
• Varies depending on the types of objects
Simplified Geometry
• Approximate complex objects with simpler
geometry
– i.e. Ellipsoid shown below
Minkowski Sum
• By taking the Minkowski Sum of two complex
volumes and creating a new volume then
overlap can be found
– By testing if a single point is within the new volume
Minkowski Sum Example
Bounding Volumes
• Bounding volume is a simple geometric
shape
– Completely encapsulates object
– If no collision with bounding volume, no more
testing is required
• Most common bounding volumes is box
– More later on…
Box Bounding Volumes
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Achieving O(n) Time Complexity
• Possible solutions for O(n) time complexity
– Partition space
– Plane sweep algorithm
Partition Space Solution
Plane Sweep Algorithm Solution Terrain Collision Detection
Locate Triangle on Height Field Collision Resolution: Examples
• Two billiard balls strike
– Calculate ball positions at time of impact
– Impart new velocities on balls
– Play “clinking” sound effect
• Rocket slams into wall
– Rocket disappears
– Explosion spawned and explosion sound effect
– Wall charred and area damage inflicted on nearby
characters
• Character walks through wall
– Magical sound effect triggered
– No trajectories or velocities affected
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Collision Resolution Components
• Resolution has three parts:
– Prologue
– Collision
– Epilogue
Prologue Stage
• Collision known to have occurred
• Check if collision should be ignored
• Other events might be triggered
–Sound effects
–Send collision notification messages
Collision Stage
• Place objects at point of impact
• Assign new velocities using either
– Physics
–Some other decision logic
Epilogue Stage
• Propagate post-collision effects
• Possible effects
–Destroy one or both objects
–Play sound effect
–Inflict damage
• Many effects can be done either in the
prologue or epilogue
Resolving Overlap Testing
• Four common stages:
– Extract collision normal
– Extract penetration depth
– Move the two objects apart
– Compute new velocities
Extract Collision Normal
• Find position of objects before impact
• Use two closest points to construct the
collision normal vector
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Extract Collision Normal .
• Sphere collision normal vector
– Difference between centers at point of
collision
Resolving Intersection Testing
• Simpler than resolving overlap testing
–No need to find penetration depth or move
objects apart
• Simply just
–Extract collision normal
–Compute new velocities
Acceleration Techniques Accelerating Collision Detection
• Two kinds of approaches (many others also)
– Collision proxies / bounding volumes hierarchies
– Spatial data structures to localize
• Used for both 2D and 3D
• Accelerates many things, not just collision
detection
– Raytracing
– Culling geometry before using standard
rendering pipeline
Collision Proxies vs Spatial data
Structures
Collision Proxies: Spatial data Structures:
- Object centric - Space centric
- Spatial redundancy - Object redundancy
Collision Proxies vs Spatial data
Structures .
Collision Proxies: Spatial data Structures:
- Object centric - Space centric
- Spatial redundancy - Object redundancy
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Collision Proxies vs Spatial data
Structures ..
Collision Proxies: Spatial data Structures:
- Object centric - Space centric
- Spatial redundancy - Object redundancy
Collision Proxies vs Spatial data
Structures …
Collision Proxies: Spatial data Structures:
- Object centric - Space centric
- Spatial redundancy - Object redundancy
Collision Proxies
• Proxy
– Something that takes place of real object
– Cheaper than general mesh-mesh intersections
• Collision proxy (bounding volume) is piece of
geometry used to represent complex object
for purposes of finding collision
– If proxy collides, object is said to collide
– Collision points mapped back onto original
object
Collision Proxies .
• Good proxy
–Cheap to compute collisions for, tight fit to
the real geometry
• Common proxies
–Sphere, cylinder, box, ellipsoid
• Consider
–Fat player, thin player, rocket, car …
Collision Proxies Example 1 Collision Proxies Example 2
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Collision Proxies Example 3 Trade-off in Choosing Proxies
increasing complexity & tightness of fit
decreasing cost of (overlap tests + proxy update)
AABB OBBSphere Convex Hull6-dop
Trade-off in Choosing Proxies .
• AABB
– Axis aligned bounding box
• OBB
–Oriented bounding box, arbitrary alignment
• k-dops
– Shapes bounded by planes at fixed
orientations
• Discrete orientation
Pair Reduction
• Want proxy for any moving object requiring
collision detection
• Before pair of objects tested in any detail,
quickly test if proxies intersect
• When lots of moving objects, even this quick
bounding sphere test can take too long:
– N2 times if there are N objects
• Reducing this N2 problem is called pair
reduction
– Pair testing isn’t a big issue until N>50 or so…
Spatial Data Structures
• Can only hit something that is close
• Spatial data structures tell you what is
close to object
– Uniform grid, octrees, kd-trees, BSP trees
– Bounding volume hierarchies
• OBB trees
– For player-wall problem, typically use same
spatial data structure as for rendering
• BSP trees most common
Uniform Grids
• Axis-aligned
• Divide space uniformly
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Quadtrees/Octrees
• Axis-aligned
• Subdivide until no
points in cell
KD Trees
• Axis-aligned
• Sub-divide in
alternating
dimensions
BSP Trees
• Binary Space
Partitioning (BSP)
• Planes at arbitrary
orientation
Bounding Volume Hierarchies
OBB Trees
• Oriented bounding box (OBB)
• Applicable to a wide range of problems
BSP Trees Main Idea
• Binary Space Partition (BSP) Tree:
–Partition space with binary tree of
planes
–Fuchs, Kedem and Naylor `80
• Main idea:
–Divide space recursively into half-spaces
by choosing splitting planes that
separate objects in scene
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BSP Trees Methods
• More general, can deal with inseparable
objects
• Automatic, uses as partitions planes
defined by the scene polygons
• Method has two steps:
– Building of the tree independently of
viewpoint
– Traversing the tree from a given viewpoint to
get visibility ordering
BSP Trees Methods .
• First step
–Preprocessing
• Create binary tree of planes
• Second step
–Runtime
• Correctly traversing this tree enumerates
objects from back to front
Creating BSP Trees: Objects Creating BSP Trees: Objects .
Creating BSP Trees: Objects .. Creating BSP Trees: Objects …
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Creating BSP Trees: Objects …. Splitting Objects
• No bunnies were harmed in previous
example
• But what if a splitting plane passes
through an object?
–Split the object; give half to each node
Ouch
Traversing BSP Trees
• Tree creation independent of viewpoint
– Preprocessing step
• Tree traversal uses viewpoint
– Runtime, happens for many different viewpoints
• Each plane divides world into near and far
– For given viewpoint, decide which side is near and which
is far
• Check which side of plane viewpoint is on independently for
each tree vertex
• Tree traversal differs depending on viewpoint!
– Recursive algorithm
• Recurse on far side
• Draw object
• Recurse on near side
Traversing BSP Trees Pseudo Code
• Query: given a viewpoint, produce an ordered
list of (possibly split) objects from back to
front
renderBSP(BSPtree *T)
BSPtree *near, *far;
if (eye on left side of T->plane)
near = T->left; far = T->right;
else
near = T->right; far = T->left;
renderBSP(far);
if (T is a leaf node)
renderObject(T)
renderBSP(near);
BSP Trees: Viewpoint A BSP Trees: Viewpoint A .
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BSP Trees: Viewpoint A ..
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 Decide independently at
each tree vertex
 Not just left or right child!
BSP Trees: Viewpoint A …
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BSP Trees: Viewpoint A ……..
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BSP Trees: Viewpoint B .
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BSP as a Hierarchy of Spaces
• Each node corresponds to
a region of space
–The root is the whole of Rn
–The leaves are
homogeneous regions
BSP Tree Traversal: Polygons
• Split along the plane defined by any
polygon from scene
• Classify all polygons into positive or
negative half-space of the plane
– If a polygon intersects plane, split polygon
into two and classify them both
• Recurse down the negative half-space
• Recurse down the positive half-space
Representation of Polygons
Representation of Polyhedra BSP Trees for Dynamic Scenes
• When an object moves the planes that
represent it must be removed and re-
inserted
• Some systems only insert static geometry
into the BSP tree
• Otherwise must deal with merging and
fixing the BSP cells
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BSP Trees Pos
• Simple, elegant scheme
• Correct version of painter’s algorithm
back-to-front rendering approach
• Popular for video games
BSP Trees Cons
• Slow to construct tree
–O(n log n) to split, sort
• Splitting increases polygon count
–O(n2) worst-case
• Computationally intense preprocessing
stage restricts algorithm to static scenes
BSP Demo
• http://www.symbolcraft.com/graphics/bsp/
BSP Videos
• https://www.youtube.com/watch?v=WAd7vzw
knF0
• https://www.youtube.com/watch?v=jF2a4imSu
vI
• http://www.youtube.com/watch?v=JJjyXRvokE4
Collision Detection Approach Introduction to 3D Breakout
• Most important thing is ball-wall collision
detection
• Can be used in:
–Ball-wall collisions
–Ball-bat collisions
• Apart from some cases
–Ball-Well collisions
• Apart from some cases (similarly to ball-bat)
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Ρ
Σ
Ο
Λ
φ
υ
Calculate Collision With Wall
• We are interested in finding the
– Distance travelled (ΡΣ)
– Collision time (tcollision)
– Final velocity (υfinal)
From the previous diagram:
ΡΣ = ΛΣ – ΛΡ eq. 1
Pythagoras Theorem
• From Pythagoras:
ΟΣ2 = ΟΛ2 + ΛΣ2 →
ΛΣ2 = ΟΣ2 - ΟΛ2 →
ΛΣ= sqrt(ΟΣ2 - ΟΛ2) eq. 2
Calculate Distance Travelled
• Also:
ΛΡ = ΟΡcosφ eq. 3
• So from eq. 1, eq. 2 and eq. 3:
ΡΣ = sqrt(ΟΣ2 - ΟΛ2) - ΟΡcosφ eq. 4
Calculate Distance Travelled .
• But:
sinφ = ΟΛ/ΟΡ →
ΟΛ = ΟΡsinφ
• And:
ΟΛ2 = ΟΡ2sinφ2 eq. 5
Calculate Distance Travelled ..
• From eq. 4 and eq. 5
ΡΣ = sqrt(ΟΣ2 - ΟΡ2sinφ2) - ΟΡcosφ eq. 6
• Also from:
sinφ2 + cosφ2 = 1 → sinφ2 = 1 - cosφ2 eq. 7
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Calculate Distance Travelled …
• From eq. 6 and eq. 7
ΡΣ = sqrt(ΟΣ2 - ΟΡ2 + ΟΡ2cosφ2) - ΟΡcosφ
• Since ΟΡ•υ = (ΟΡ)υ/|υ|cosφ, so the above
equation will become:
ΡΣ = sqrt(ΟΣ2 - ΟΡ2 + (ΟΡυ/|υ|cosφ)2) -
(ΟΡ)υ/|υ|cosφ
Calculate Distance Travelled ….
• From the dot product the previous equation will
become
ΡΣ = sqrt(ΟΣ2 - ΟΡ2 + (ΟΡ•υ/|υ|)2) - ΟΡ•υ/|υ|
eq. 8
• Must take absolute value in case φ > 90
ΡΣ = |(sqrt(ΟΣ2 - ΟΡ2 + (ΟΡ•υ/|υ|)2) - ΟΡ•υ /|υ| |
eq. 9
Calculate Collision Time
• From motion equation:
S = υcollision tcollision
• But S = ΡΣ, so:
ΡΣ = υcollision tcollision →
tcollision = ΡΣ/υcollision eq. 10
Ρ
Σ
Ο
Λ
φ θ
Calculate Final Velocity
θ
θ
|υinitialsinθ|
|υfinalsinθ|
|υinitialcosθ|
υinitial
υfinal
Ο
Σ
Δυcollision
Calculate Final Velocity .
• The change in ball velocity from the collision:
|Δυcollision| = |υfinal - υinitial| eq. 11
• From the above figure:
|Δυcollision| = 2|υinitialcosθ| or eq. 12
|Δυcollision| = 2υinitial•(ΟΣ/|ΟΣ|) eq. 13
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Calculate Final Velocity ..
• But Δυ is anti-parallel to ΟΣ and we want to
make Δυcollision a vector
• From eq. 12 we do:
Δυcollision = -2|υinitialcosθ|(ΟΣ)/|ΟΣ|→
• From eq. 13 we do:
Δυcollision = -2(ΟΣ) (υinitial•ΟΣ)/|ΟΣ|2
2nd Assignment Tips
Some Tips
• Important 3D objects for collision detection in
3D Breakout Assignment
– Invisible ground (optional)
– Ball
– Bat
– Well
Class TBall.h
class TBall
{
public:
double _radius; // defines the radius of the ball
TVector _position; // defines the position of the ball
TVector _velocity; // defines the velocity v0 of the ball
// Constructors
TBall();
TBall(const double& Radius, const TVector& Position, const
TVector& Velocity)
{_radius=Radius; _position=Position; _velocity=Velocity;};
Class TBall.h .
// Selectors
double GetBallRadius() const {return _radius;};
TVector GetBallPosition() const {return _position;};
TVector GetBallVelocity() const {return _velocity;};
void DrawBall(); // Draws the ball
void CalculateVelocity(const TVector& velocity, const double&
seconds); // Assigns the ball a velocity
TVector CalcDistanceTravelled(const double& seconds) const;
// Calculates the distance traveled
void MoveBall(const double& seconds); // Moves the ball
};
Default Constructor for the Ball
TBall::TBall()
{
// Assign default values for the attributes
// of the ball
_radius = 4.0;
_position = TVector(0.0, 0.0, 0.0);
_velocity = TVector(1.0, 0.0, 0.0);
}
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Function to Draw the Ball
void TBall::DrawBall()
{
glPushMatrix();
glTranslatef(_position.X(),
_position.Y(), _position.Z());
glutSolidSphere(_radius, 20, 20);
glPopMatrix();
}
More Functions
• Functions for the TBall Class:
–CalculateVelocity
–CalcDistanceTravelled
–MoveBall
• Function for TDisplayImp
–Idle
• TBat Class
CalculateVelocity Function
void TBall::CalculateVelocity(const TVector&
velocity, const double& seconds)
{
_velocity = velocity;
}
CalcDistanceTravelled Function
TVector TBall::CalcDistanceTravelled(const double&
seconds) const
{
TVector new_velocity, new_position;
new_velocity = _velocity;
new_position = _position +
new_velocity*seconds;
return new_position;
}
MoveBall Function
void TBall::MoveBall(const double& seconds)
{
_position = CalcDistanceTravelled(seconds);
}
Idle Function
void TDisplayImp::idle(void)
{
// Set the time for the simulation
_scene->CalculateSimulations();
glutPostRedisplay();
}
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Class TBat
class TBat
{
public:
TVector _points[16]; // points for the
first bat
TVector _normal[15]; // normal of the ground
public:
// Default constructor
TBat(){};
TBat(double rotation_angle);
Class TBat .
void DrawBat(); // Draws the bats
void MoveBatRight(); // Moves bat on the right
void MoveBatLeft(); // Moves the bat on the right
int BatCollisions(const TBall &ball, const double&
seconds);
int BatCollisionsSides(const TBall &ball, const double&
seconds);
int BatCollisionsEdges(const TBall &ball, const double&
seconds);
Class TBat ..
TVector Bat_Faces_Reflection(TBall
&ball, const double& seconds,
const double& distance);
TVector Bat_Left_Side_Reflections(TBall
&ball, const double& seconds,
const double& parameter);
TVector
Bat_Right_Side_Reflections(TBall &ball,
const double& seconds, const double&
parameter);
Class TBat …
TVector Bat_Edge12_Reflections(TBall &ball,
const double& seconds);
TVector Bat_Edge15_Reflections(TBall &ball,
const double& seconds);
TVector Bat_Edge13_Reflections(TBall &ball,
const double& seconds);
TVector Bat_Edge11_Reflections(TBall &ball,
const double& seconds);
};
TBat Constructor
TBat::TBat(double rotation_angle)
{
TVector initial_vector, upper_vector,
construction_vector;
// Define a vector for the construction of the
ground points of the bats
initial_vector = TVector(1.0, 0.0, 0.0);
// Define a vector for the construction of the
upper points of the bats
upper_vector = TVector(0.0, 10.0, 0.0);
TBat Constructor .
// Define the rotation axis
TVector rotation_axis(0.0,1.0,0.0);
// Define the three rotation matrices for the bats
TMatrix33 bat_construction = TMatrix33(rotation_axis,
rotation_angle);
// Define the vector used for the construction of the bats
construction_vector = bat_construction*initial_vector;
// Define the rotation matrix for the constuction of the bats
TMatrix33 bat_rotation = TMatrix33(rotation_axis, angle);
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TBat Constructor ..
// Construct the 16 points of the bats
_points[0] = construction_vector*bat_radius1;
_points[1] = bat_rotation*_points[0];
_points[2] = bat_rotation*_points[1];
_points[3] = bat_rotation*_points[2];
_points[7] = construction_vector*bat_radius2;
_points[6] = bat_rotation*_points[7];
_points[5] = bat_rotation*_points[6];
_points[4] = bat_rotation*_points[5];
_points[8] = _points[0] + upper_vector;
_points[9] = _points[1] + upper_vector;
_points[10] = _points[2] + upper_vector;
_points[11] = _points[3] + upper_vector;
_points[15] = _points[7] + upper_vector;
_points[14] = _points[6] + upper_vector;
_points[13] = _points[5] + upper_vector;
_points[12] = _points[4] + upper_vector;
}
Drawing Front Side of Bats
glBegin(GL_QUAD_STRIP);
// Front face, normal of first surface
_normal[0] = ((_points[8] - _points[0])*(_points[1] - _points[0])).unit();
glNormal3f(_normal[0].X(), _normal[0].Y(), _normal[0].Z());
// Construct first quad
glVertex3f(_points[0].X(), _points[0].Y(), _points[0].Z());
glVertex3f(_points[8].X(), _points[8].Y(), _points[8].Z());
// Front face, second surface
_normal[1] = ((_points[9] - _points[1])*(_points[2] - _points[1])).unit();
glNormal3f(_normal[1].X(), _normal[1].Y(), _normal[1].Z());
Drawing Front Side of Bats .
// Construct second quad
glVertex3f(_points[1].X(), _points[1].Y(), _points[1].Z());
glVertex3f(_points[9].X(), _points[9].Y(), _points[9].Z());
// Front face, third surface
_normal[2] = ((_points[10] - _points[2])*(_points[3] - _points[2])).unit();
glNormal3f(_normal[2].X(), _normal[2].Y(), _normal[2].Z());
// Construct third quad
glVertex3f(_points[2].X(), _points[2].Y(), _points[2].Z());
glVertex3f(_points[10].X(), _points[10].Y(), _points[10].Z());
glNormal3f(_normal[2].X(), _normal[2].Y(), _normal[2].Z());
// Construct fourth quad
glVertex3f(_points[3].X(), _points[3].Y(), _points[3].Z());
glVertex3f(_points[11].X(), _points[11].Y(), _points[11].Z());
glEnd();
Drawing the Rest of the Bats
• In the same way you will have to draw the:
–Left side of the bat
–Back side of the bat
–Right side of the bat
–Up side of the bat
Bat Collisions
• At least three checks:
– Check for collisions between the ball and the
three bats like ball-wall
– Check for collisions between the ball and the
side of the bats
– Check for collisions between the ball and the
edges of the bats
• Repeat the same procedure for reflections
of the ball after collisions
Calculate the reflection of the ball
after collision
double TBounds::Ball_Reflection(TBall &ball, const
double& seconds)
{
TVector ball_velocity_after_collision,
previous_ball_position, collision_vector, final_velocity;
// Perform calculations for the previous time step
previous_ball_position = ball.GetBallPosition() -
ball.GetBallVelocity()*seconds;
double absBallVelocity =
sqrt(ball.GetBallVelocity().dot(ball.GetBallVelocity()));
3/24/2015
25
Calculate the reflection of the ball
after collision .
// Calculate the Ri*V to calculate the collision
time
double RV =
previous_ball_position.dot(ball.GetBallVelocity()
)/absBallVelocity;
// Absolute RV
double abs_RV = abs(RV);
// Define the initial distance = 100 - 4 = 96
double initial_distance = 100.0 -
ball.GetBallRadius();
Calculate the reflection of the ball
after collision ..
// Calculate the determinant
double Determinant = ((RV*RV) previous_ball_position.dot(previous_ball_position)
+
initial_distance*initial_distance);
// Calculate the collision time
double collision_time = abs(-abs_RV +
sqrt(Determinant))/absBallVelocity;
// Calculate the collision vector (normal vector) from: R = r +
v*t
collision_vector = previous_ball_position +
ball.GetBallVelocity()*collision_time;
// Make the collision vector (normal vector) unit vector
TVector unit_collision_vector = TVector::unit(collision_vector);
Calculate the reflection of the ball
after collision …
// Define velocity by: Vreflected =
(Vinitial*Normal.unit)*Normal.unit
ball_velocity_after_collision =
unit_collision_vector*(ball.GetBallVelocity().dot(unit_col
lision_vector));
// Calculate the velocity of the ball after collision with
the invisible wall
final_velocity = ball.GetBallVelocity() -
ball_velocity_after_collision*2.0;
ball.CalculateVelocity(final_velocity, collision_time);
return collision_time;
}
References
• http://www.cs.wisc.edu/~schenney/courses/c
s679-f2003/lectures/cs679-22.ppt
• http://graphics.ucsd.edu/courses/cse169_w05
/CSE169_17.ppt
Links
• http://en.wikipedia.org/wiki/Bounding_volume
• http://nehe.gamedev.net/data/lessons/lesson.asp?
lesson=30
• http://web.cs.wpi.edu/~matt/courses/cs563/talks/
bsp/bsp.html
• http://www.devmaster.net/articles/bsp-trees/
• http://maven.smith.edu/~mcharley/bsp/createbspt
ree.html
• http://www.cs.unc.edu/~geom/
• http://www.cs.ox.ac.uk/stephen.cameron/distances/
Questions