Implementation of an Outdoor Shooting Game

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Implementation of an Outdoor Shooting Game

• Szirmay-Kalos László
• Budapest University of Technology
• email szirmay_at_iit.bme.hu
• Web http//www.iit.bme.hu/szirmay

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Demo 1 The objective
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Virtual reality
Virtual world
avatar
rendering
interaction
User input
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Tasks in games

• Image synthesis from the point of view of the
  avatar
• Control of the avatar through input devices
  (keyboard, mouse)
• Control of the intelligent virtual objects by
  artificial intelligence algorithms
• Simulation of the physical laws (collisions,
  forces)

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I/O libraries
simulation
Windows GLUT
input
Virtual world
rendering
OpenGL
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I/O management
initialization callback registration
Operating System Windows
main
GLUT
DisplayFunc
KeyboadFunc
SpecialFunc
callbacks
IdleFunc
OpenGL
Graphics hardware
application
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Rendering with OpenGL
Perspective transformation
Virtual world
Viewing transformation
1.
2.
?
628
1325
1325
628
clipping
visibility z-buffer
display
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Geometry definition
glBegin(GL_TRIANGLES) glColor3f( 1, 1, 0 )
glVertex3f( x11, y11, z11 ) glVertex3f( x12,
y12, z12 ) glVertex3f( x13, y13, z13
) glColor3f( 0, 1, 0 ) glVertex3f( x21,
y21, z21 ) glVertex3f( x22, y22, z22
) glVertex3f( x23, y23, z23 ) glEnd()
x12,y12,z12
x13,y13,z13
x11,y11,z11
glColor4f(R,G,B,A)
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Texturing
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Texture mapping
(u2, v2)
x3,y3,z3
(u1, v1)
(u3, v3)
x2,y2,z2
x1,y1,z1
glEnable(GL_TEXTURE_2D) glBindTexture(GL_TEXTURE_
2D, texture_id) // which texture glBegin(GL_TRIAN
GLES) glTexCoord2f(u1, v1) glVertex3f(x1,
y1, z1) glTexCoord2f(u2, v2) glVertex3f(x2,
y2, z2) glTexCoord2f(u3, v3) glVertex3f(x3,
y3, z3) glEnd() glDisable(GL_TEXTURE_2D)
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Games
ControlIt(dt), InteractIt()
AnimateIt(dt), DrawIt()
avatar
rendering
interaction
User input
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Game objects

• ControlIt
• Interacts, thinks and applies his available
  controls (e.g. runs, shoots)
• InteractIt
• Looks at the states of other objects, checks
  collisions
• AnimateIt
• Moves to its new position according to the
  elapsed time
• DrawIt
• Draws itself onto the screen

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Simulation loop (Game loop)
dt
void IdleFunc( ) // idle call back float
old_time time time glutGet(
GLUT_ELAPSED_TIME ) float dt time -
old_time avatar -gt ProcessInput( ) world
-gt Control( dt ) world -gt Animate( dt )
glClearColor(0, 0, 0, 0) glClear(GL_COLOR_BUF
FER_BIT GL_DEPTH_BUFFER_BIT)
avatar -gt SetCameraTransform() world -gt Draw(
) glutSwapBuffers( )
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Data structure of the virtual world

• Game Objects are dynamic (killing)
• Game objects are different (heterogeneous
  collection)
• Parent-child relationships (owner-weapon)

world
avatar
Enemy 2
bullet
explosion
terrain
sky
Enemy 1
Weapon
Join új elem hozzávétele
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Terrain

• Complex geometry
• Height field
• Complex texture
• No Control
• No Animation
• Collision detection,
• Lifts objects

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Terrain geometry
z
z
y
x
x,y
Height function definition Discrete samples
linear interpolation
z height(x,y)
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Discrete samples BW Image
Height map
Triangle mesh
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Reducing the number of triangles Level of detail
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Height-field texturing projecting the top view
y
x
v
u
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Terrain improvement Detail map
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Terrain collision detection
if (height(x,y) gt z) Collision! Walking on
the terrain Position(x, y) (x, y, height(x,y)
legsize)
z
x,y
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Bi-linear Height field interpolation
float Height( float x, float y ) x
wwidth/2 y wlength/2 x x / wwidth w
y y / wlength l int X (int)x, Y
(int)y float h1 height_fieldY w X
wheight float h2 height_fieldY w X1
wheight float h3 height_field(Y1) w
X wheight float h4 height_field(Y1)
w X1 wheight float xd x - X float
yd y - Y float hx1 h1 xd (h2 - h1)
float hx2 h3 xd (h4 - h3) return (hx1
yd (hx2 - hx1))
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Sky

• Image that is textured on something sphere
• Sky geometry dome, sphere
• No Control
• No Animation
• No collision detection

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Definition of surfaces as triangle meshes
Tessellation
1. Find a parametric equation of a
sphere x(u,v) x0 r cos 2?u sin ?v y(u,v)
y0 r sin 2?u sin ?v z(u,v) z0 r cos ?v
u,v ? 0,1 2. Select points in the unit
rectangle
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GLU Quadrics Sphere
GLUquadricObj quadric //
definition quadric gluNewQuadric(
) gluQuadricTexture(quadric, GL_TRUE) //
draw glBindTexture(GL_TEXTURE_2D,
sky_texture_id) gluSphere(quadric, sky_radius,
32, 20)
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Enemy

• Animated geometry
• Defined by keyframes organized as clips
• Stand, run, attack, die,
• mesh animation
• Textures (animated)
• Artificial intelligence
• Collision detection

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Keyframe animation running
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Inbetweening Computation of frames from keyframes
Nonlinear interpolation
linear interpolation
keyframes
t
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What and how to interpolate

• High quality animation
• Newtons laws the second derivative of a motion
  curve is proportional to the force, which acts
  through an elastic mechanism interpolation curve
  is C2
• Even the interpolated frames should meet physical
  constraints Bone animation
• Games
• Linear interpolation
• Interpolate the vertices of the mesh
  Mesh deformation

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Example for bone animation
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Cyclic walk
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Mesh morphing
t 0
Time t
Two neighboring keyframes
Linear interpolation for each vertex
t 1
Current vertex positions
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Running as mesh morphing
position animation position velocity dt
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Motion definition

• Keyframes are organized into clips
• Keyframes are designed off line and stored in a
  file MD2, MD3, etc. file formats
• Typical clips in a game
• Run, stand, attack, die, pain, salute, crouch,
  wave, point, taunt, etc.

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Clips
Stand 40 keyframes
Run 5 keyframes
Salute 11 keyframes
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Motion control
AI machine
AI state
time t
Keyframes stored in an MD2 file
Clip start, stop keyframe
Keyframe animation
Vertex positions of a triangle mesh
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Artificial Intelligence of an Enemy
Dist lt 4 Avatar_angle lt 40
Dont Care
Escape
Dist gt 6
Avatar_angle lt 20
Dist lt 4 Avatar_angle gt 60
Dist lt 1
Chase
Attack
Dist gt 1
Collision with the bullet
Dying
Avatar_angle
Avatar
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Texturing
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Bullet

• Geometry sphere
• Textured
• Not intelligent
• Physical animation

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Physical animation of the bullet
tdt
velocity
t
force, acceleration
acceleration (0, 0, -g) velocity
acceleration dt position velocity dt
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The bullet is flying Animate, Draw
void BulletAnimateIt( float dt )
acceleration Vector(0,0,-g) speed
acceleration dt position speed
dt void BulletDrawIt( ) glPushMatrix(
) glTranslate(position.x, position.y,
position.z) glBindTexture(GL_TEXTURE_2D,
bullet_texture) gluSphere(quadric, 1.0, 16,
10) glPopMatrix( )
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Collision detection between two slow objects
Problem with fast objects
t
given t
t ? t
dist obj1.position - obj2.position min
obj1.BoundingRadius() obj2.BoundingRadius() if
(dist.Length() lt min) Collision!
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Bullet collision detection
BulletInteractIt( GameObject obj ) if
(obj-gtGetType() TERRAIN) Mountain
terrain (Mountain )obj if
(terrain-gtHeight(position.x, position.y)gt
position.z) KillIt()
world -gt Join(new Explosion(position))
if ( obj-gtGetType() AVATAR
obj-gtGetType() ENEMY )
if (bounding spheres overlap)
KillIt() obj -gt KillIt( ) world -gt
Join(new Explosion(position))
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Collision detection with fast objects
ray-tracing
position
rel_velocity velocity - vel2 ray position
rel_velocityt If (ray intersects bounding
sphere first AND tintersect lt dt)
Collision!
vel2
velocity
hit_object world-gtIntersect(position,velocity,t)

world
avatar
ship1
ship2
space
sun
bullet
explosion
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Billboards

• Single semi-transparent texture on an oriented
  quadrilateral

pos
pos
QUAD
X Y Z
x y z
Tmodell
Tview
Tperspective
QUAD
position orientation
camera position camera orientation
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Explosion

• Seemingly complex, random structure
• Similar look from all directions
• Collection of billboards
• Particle system

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Particle Systems
global force field (smoke)
position position velocity
dt velocity velocity acceleration
dt acceleration acceleration force /
weight lifetime age age dt if (age gt
lifetime) Kill() size, dsize size dsize
dt weight, dweight weight dweight
dt color, dcolor color dcolor dt
random initial values
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Explosion settings
Rand
position Rand( center, 0.1 ) lifetime Rand(
2.0, 1.0 ) size 0.001 dsize Rand( 1.0,
0.5 ) / lifetime / 2.0 velocity Rand(
CVector(0, 0, 0), 0.4 ) acceleration Rand(
CVector(0, 0, 0), 0.4 ) color Rand( Color(1,
0.5, 0, 1), Color(0, 0.5, 0, 0) ) dcolor
Color(0, -0.5, 0, -1) / lifetime / 2
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Avatar

• Keyboard controls its behavior
• ProcessInput
• Its position and orientation will be the camera
  position and orientation before rendering
• SetCameraTransform

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Avatar SetCameraTransform
Avatar SetCameraTransform( )
glMatrixMode(GL_MODELVIEW) glLoadIdentity()
gluLookAt(position.x, position.y, position.z,
position.x head.x,
position.y head.y, position.z
head.z, up.x, up.y, up.z)
eye
up 0, 1, 0
lookat
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Keyboard control
KeyboardFunc SpecialFunc
input
IsSpace, IsLeft, IsRight, IsUp, IsDown
IdleFunc GameLoop
virtual world
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Game engine
GLUTWindow
GameObject position, velocity, acceleration Contro
lIt(float dt ) AnimateIt(float dt) InteractIt(
GameObject o) DrawIt( ) IntersectIt(Ray r,
float t)
GameEngine DisplayFunc IdleFunc KeyPress
500 C lines
Texture Load( char fname)
world
Member Control, Animate, Draw Interact,
Intersect, Join
next
Particle
avatar
ParticleSystem Emit(int n)
Avatar ProcessInput() SetCameraTransform()
TexturedObject
BillBoard DrawIt()
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Shooting game
GameEngine
Avatar
TexturedObject
ParticleSystem
Self ProcessInput ControlIt InteractIt
Explosion ControlIt
Enemy DrawIt InteractIt ControlIt
Terrain DrawIt AnimateIt
Sky DrawIt
SpaceGame
350 C lines
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Indoor games?

• No terrain
• Building
• Cannot be represented by a height field
• Textured triangle mesh
• Collision without checking all triangles
• Fast visibility quickly eliminating the
  potentially invisible objects

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Thanks for your audience