Computer Graphics Conceptual Model

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Computer GraphicsConceptual Model
API
Output Devices
Application Program
Application Model
Graphics System
Input Devices
Function Calls or Protocol
Data
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What Is OpenGL?

• Graphics rendering API
• high-quality color images composed of geometric
  and image primitives
• window system independent
• operating system independent
• close to hardware
• leads to discussions of algorithms and
  implementation

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OpenGL and the Windowing System

• OpenGL is concerned only with rendering
• Window system independent
• No input functions
• OpenGL must interact with underlying OS and
  windowing system
• Need minimal interface which may be system
  dependent
• Done through additional libraries AGL, GLX, WGL

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GLU and GLUT

• GLU (OpenGL Utility Library)
• part of OpenGL
• NURBS, tessellators, quadric shapes, etc.
• GLUT (OpenGL Utility Toolkit)
• portable windowing API
• not officially part of OpenGL

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OpenGL and Related APIs
application program
OpenGL Motif widget or similar
GLUT
GLX, AGLor WGL
GLU
GL
X, Win32, Mac O/S
software and/or hardware
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OpenGL as a Renderer

• Geometric primitives
• points, lines and polygons
• Image Primitives
• images and bitmaps
• separate pipeline for images and geometry
• linked through texture mapping
• Rendering depends on state
• colors, materials, light sources, etc.

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Components of a Graphics API

• Primitive functions
• What to draw (points, polygons, pixels, text)
• Primitive attributes
• How to draw it (color, pattern, style)
• Synthetic camera
• Viewing functions (position, orientation, lens)
• Transformation functions
• Rotate, scale, translate objects (models)
• Input functions
• Handle interactivity (keyboards, mouse)
• Control functions
• Communicate with window system
• Initialization, error handling

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Point and Line Segment Primitives
GL_POINTS
GL_LINES
GL_LINE_STRIP
GL_LINE_LOOP
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Polygon Primitives
GL_POINTS
GL_POLYGON
P2
P1
P3
P0
P4
P7
P5
P6
GL_TRIANGLES
GL_QUADS
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Attributes for lines and polygons
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Filled Objects
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Different Attributes for Polygon
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Example of Stroke text
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Example of Raster text
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Display of Stoke text
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Display of Raster text
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OpenGL Primitive Syntax
glBegin ( type ) glVertex ( . . .
) . . . . glVertex ( . . .
) glEnd ( )
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Polygons
OpenGL only supports rendering for simple, convex
and flat polygon
1. Closed 2. Has an interior
Simple Well defined interior
Complex
Simple No pair of edges of a polygon cross each
other
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Polygons Defining the Interior
Simple Well defined interior
Complex
Exercise Design a test to determine whether a 2D
polygon is SIMPLE. Extend your test for 3D
polygon.
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Polygons Convexity
Convex
Non-Convex
P1
P2
Definition extensible to 3D.
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Setting Color Attribute in OpenGLRGB Mode
void glColor3b s i d f ub ud ui(TYPE r, TYPE
g, TYPE b) glColor3f(0.0, 0.0, 0.0)
/black/ glColor3f(1.0, 0.0, 0.0)
/red/ glColor3f(0.0, 1.0, 0.0)
/green/ glColor3f(0.0, 0.0, 1.0)
/blue/ glColor3f(1.0, 1.0, 0.0)
/yellow/ glColor3f(0.0, 1.0, 1.0)
/cyan/ glColor3f(1.0, 0.0, 1.0)
/magenta/ glColor3f(1.0, 1.0, 1.0) /white/
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Specifying a ColorColor Index Mode
void glIndexs i f d(TYPE c) OpenGL does not
provide intrinsic method of entering a colormap.
This is left to the underlying windowing system.
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Other Simple OpenGL Attributes

• glClearColor(1.0, 1.0, 1.0, 0.0)
• Sets background color to white
• Fourth argument is transparency 0.0 is opaque
• Sets a state variable
• glPointSize(2.0)
• Sets point size to be 2 pixels wide
• Note that this is not a device-independent
  attribute
• Sets a state variable
• glLinewidth (2.0)
• glLineStipple(Glint factor, Glushort pattern)

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Synthetic Camera
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Synthetic Camera Projection Geometry
Y
X
x,y,z
d
Z
COP
(0,0,-L)
x x ( d/(z L) ) y y( d/(z L) ) z d
- L
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Clipping
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Orthographic Projection
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Orthographic Projection
void glOrtho(Gldouble left, Gldouble right,
Gldouble bottom, Gldouble top, Gldouble near,
Gldouble far) void glOrtho2D(Gldouble left,
Gldouble right Gldouble bottom, Gldouble top)
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Transformation Matrices in OpenGL
Matrix Mode
3D Model Vertices
2D
3D
Vertices
Modelview
Projection
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Setting Viewing Matrix in GLA Simple Case
glMatrixMode(GL_PROJECTION) Sets the switch so
that loaded matrix goes into the projection
stack. glLoadIdentity() Pushes an identity
matrix onto the stack glOrtho2D(GLdouble left,
Gldouble right, Gldouble bottom,
Gldouble top) Sets the current view to an
orthographic projection with view volume bounded
by x left, x right, y bottom, y top, z
-1.0 and z 1.0.
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Viewport Transformation
MyWindow
w
Clipping Window
h
x
y
void glViewport(Glint x, GLint y, GLsizei w,
Glsizei h)
Default viewport corresponds to entire window
drawable area.
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GL Library Organization (under X Windows)
GLU
GL
OpenGl application program
Frame buffer
GLUT
Xlib, Xtk
GLX
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GLUT Basics

• Application Structure
• Configure and open window
• Initialize OpenGL state
• Register input callback functions
• render
• resize
• input keyboard, mouse, etc.
• Enter event processing loop

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Simple GLUT Window Management Functions
glutInit(int argc, char argv) Initializes a
window session. glutCreate Window(char
name) Creates a window with title
name. glutInitDisplayMode(GLUT_SINGLEGLUT_RGB)
Sets the display mode to single buffered and RGB
color. glutInitWindowSize (GLsizei h, GLsizei
w) Sets initial window size to h x
w. glutInitWindowPosition(x,y) Sets initial
window position to (x, y).
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Sample Program
void main( int argc, char argv ) int mode
GLUT_RGBGLUT_DOUBLE glutInitDisplayMode(
mode ) glutCreateWindow( argv0 ) init()
glutDisplayFunc( display ) glutReshapeFunc(
resize ) glutKeyboardFunc( key )
glutIdleFunc( idle ) glutMainLoop()
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OpenGL Initialization

• Set up whatever state youre going to use
• void init( void )
• glClearColor( 0.0, 0.0, 0.0, 1.0 )
• glClearDepth( 1.0 )
• glEnable( GL_LIGHT0 )
• glEnable( GL_LIGHTING )
• glEnable( GL_DEPTH_TEST )

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GLUT Callback Functions

• Routine to call when something happens
• window resize or redraw
• user input
• animation
• Register callbacks with GLUT
• glutDisplayFunc( display )
• glutIdleFunc( idle )
• glutKeyboardFunc( keyboard )

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Rendering Callback

• Do all of your drawing here
• glutDisplayFunc( display )
• void display( void )
• glClear( GL_COLOR_BUFFER_BIT )
• glBegin( GL_TRIANGLE_STRIP )
• glVertex3fv( v0 )
• glVertex3fv( v1 )
• glVertex3fv( v2 )
• glVertex3fv( v3 )
• glEnd()
• glutSwapBuffers()

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Idle Callbacks

• Use for animation and continuous update
• glutIdleFunc( idle )
• void idle( void )
• t dt
• glutPostRedisplay()

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Form of Simplestglut/OpenGL program
include ltglut.hgt / glut.h includes gl.h and
glu.h / void init (void) / Here the
initialization of the drawing context. / void
display (void) /This function contains all of
the draw/redraw commands
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Form of Simplestglut/OpenGL program (slide 2)
void reshape (int w, int h) / What to do
whenever the window is resized. Usually includes
resetting the viewport / int main (int argc,
char argv) / create window / init
() glutDisplayFunc(display) / register display
/ glutReshapeFunc(reshape) / register reshape
/ glutMainLoop() return 0
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Sierpinski Gasket Source Code Slide 1
include ltGL/glut.hgt void myinit(void) /
attributes / glClearColor(1.0, 1.0, 1.0,
0.0) / white background / glColor3f(1.0,
0.0, 0.0) / draw in red / / set up viewing
/ / 500 x 500 window with origin lower left
/ glMatrixMode(GL_PROJECTION)
glLoadIdentity() gluOrtho2D(0.0, 500.0, 0.0,
500.0) glMatrixMode(GL_MODELVIEW)
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Sierpinski Gasket Source Code Slide 2
void display( void ) / define a point data
type / typedef GLfloat point22
point2 vertices30.0,0.0,250.0,500.0,500.0
,0.0 / A triangle / int i, j, k
long random() / standard random number
generator / point2 p 100.0,100.0 / An
srbitrary initial point /
glClear(GL_COLOR_BUFFER_BIT) /clear the window
/
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Sierpinski Gasket Source Code Slide 3
/ computes and plots 5000 new points /
for( k0 klt5000 k) jrandom()3
/ pick a vertex at random / / Compute
point halfway between vertex and old point /
p0 (p0verticesj0)/2.0
p1 (p1verticesj1)/2.0 / plot
new point / glBegin(GL_POINTS)
glVertex2fv(p) glEnd()
glFlush() / clear buffers /
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Sierpinski Gasket Source Code Slide 4
void main(int argc, char argv) / Standard
GLUT initialization / glutInit(argc,argv)
glutInitDisplayMode (GLUT_SINGLE
GLUT_RGB) / default, not needed /
glutInitWindowSize(500,500) / 500 x
500 pixel window / glutInitWindowPosition(0,
0) / place window top left on display
/ glutCreateWindow("Sierpinski Gasket")
/ window title / glutDisplayFunc(disp
lay) / display callback invoked when
window opened / myinit() / set
attributes / glutMainLoop() / enter
event loop /