OpenGL Functions

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OpenGL Functions

• Yen-Lin Chen

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Outline

• Multiple GLUT Display Windows
• Attributes of Graphics Primitives
• OpenGL Geometric Transformations
• Demo

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Multiple GLUT Display Windows

• GLUT Display-Window Identifier
• Managing Multiple GLUT Display Windows
• GLUT Subwindows

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GLUT Display-Window Identifier
int main (int argc, char argv) glutInit(argc
, argv) glutInitDisplayMode(GLUT_DOUBLE
GLUT_RGB) glutInitWindowSize(250,
250) glutInitWindowPosition(100,
100) glutCreateWindow(argv0) init() glu
tDisplayFunc(display) glutMouseFunc(mouse) gl
utMainLoop() return 0
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GLUT Display-Window Identifier
int main (int argc, char argv) glutInit(argc
, argv) glutInitDisplayMode(GLUT_DOUBLE
GLUT_RGB) glutInitWindowSize(250,
250) glutInitWindowPosition(100, 100) int
windowID glutCreateWindow(argv0) init()
glutDisplayFunc(display) glutMouseFunc(mouse)
glutMainLoop() return 0
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GLUT Display-Window Identifier

• Each window is assigned a display-window
  identifier
• Positive integer start with the value 1
• Delete a display window

WindowID glutCreatWindow (A Display Window)
Display-Window Identifier
glutDestroyWindow (windowID)
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Example
glutInit(argc, argv) glutInitDisplayMode(GLUT_SI
NGLE GLUT_RGB ) glutInitWindowPosition(50,
50) glutInitWindowSize(300, 300) int windowID
glutCreateWindow( "Example") init() glutDispla
yFunc( display ) int windowID2
glutCreateWindow( "Example 2") init() glutDispl
ayFunc( display2 ) glutPositionWindow (50, 400)
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Current GLUT Display Window

• Specify display-window to the current display
  window
• The last display window
• The one you select
• Query the current display window
• ? 0 no display windows or the current display
  window was destroyed

glutSetWindow (windowID)
currentWindowID glutGetWindow()
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Relocating and Resizing a GLUT Display Window

• Reset the screen location
• Reset the size
• Expand to fill the screen

glutPositionWindow (xNewTopLeft, yNewTopleft)
glutReshapeWindow (dwNewWidth, dwNewHeight)
glutFullScreen()

• The aspect ration may change and object may be
  distorted
• glutReshapeFunc (winReshapeFcn) // GLUT routine

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Managing Multiple GLUT Display Windows

• Several routines for manipulating a display window

glutSetWindowTitle(New Window Name)
glutSetWindow (windowID) glutPopWindow () //
in front of all other windows
glutSetWindow (windowID) glutPushWindow ()
// push to the back
glutHideWindow ()
glutShowWindow ()
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GLUT Subwindows

• Within a selected display window
• Can set up any number of second-level display
  windows, called subwindows
• Screen-cursor shape

glutCreatSubWindow (windowID, xBottomLeft,
yBottomLeft, width, height)
GLUT_CURSOR_UP_DOWN GLUT_CURSOR_CYCLE GLUT_CURSOR_
WAIT CLUT_CURSOR_DESTORY
glutSetCursor (shape)
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Result Two Subwindows
? The parent window should also be initiated and
defined the glutDisplayFunc.
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Attributes of Graphics Primitives

• OpenGL State Variables
• OpenGL Color Functions
• Attributes of Primitives
• Fill-Area

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Attributes of Graphics Primitives

• Attribute parameter
• A parameter that affects the way a primitive is
  to be displayed, ex color, size.
• State system / State machine
• A graphics system that maintains a list for the
  current values of attributes

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OpenGL State Variables

• All OpenGL state parameters have default values.
• Changing the attribute settings
• Only affects those primitives that are specified
  after the OpenGL state is changed
• Lines blue or orange, dotted or dashed, and fat
  or thin.

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Color and Gray Scale

• Color information can be stored in two ways
• 1. RGB color codes
• 2. Color table

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Color and Gray Scale

• Gray scale
• Using RGB color functions to set shades of gray
• R value G value B value
• Other color parameters
• Printers ? cyan, magenta, yellow
• Intensity ? light energy radiating in a direction
  over a period of time
• Luminance ? characterize the perceived brightness

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OpenGL Color Functions

• Set color display mode
• 1. OpenGL RGB and RGBA mode
• 2. OpenGL Color-Index Mode

glutInitDisplayMode ( GLUT_SINGLE GLUT_RGB )
GLUT_RGBA
GLUT_INDEX
glColor (colorComponents)
// Select current color
glIndexi(196) glutSetColor (index, red, green,
blue)
// Select current color
// Specify color into a table
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OpenGL Color Functions

• OpenGL color blending
• Combining the color of overlapping objects
• Blending an object with the background
• Color-blending function

glEnable(GL_BLEND)
glDisable(GL_BLEND)
Destination
? Blending methods can be performed only in RGB
or RGBA mode.
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OpenGL Color Functions

• Blending Factors

(SrRsDrRd, SrGsDgGd, SbBsDbBd, SaAsDaAd)
glBlendFunc (sFactor, dFactor)
Default GL_ONE GL_ZERO
(1.0, 1.0, 1.0, 1.0)
(0.0, 0.0, 0.0, 0.0)
Ex. glBlendFunc(GL_SRC_ALPHA,GL_ONE)
Ex implement it by using glColor4f (0.8, 0.2,
0.2, 0.4)
Destination
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Point Attributes

• Attributes
• Color
• Size a square block of pixels
• Function

glPointSize(size)
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Line Attributes

• Attributes
• Color
• Width
• Style
• Solid lines, dashed lines, and dotted lines

glColor()
glLineWidth(width)
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Line Attributes

• Pixel mask
• A pattern of binary digits
• For example, 11111000

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Line Attributes

• OpenGL line-style Function

glLineStippel(repeatFactor, pattern)
How many times each bit is to be repeat
16-bit integer Default 0xFFFF ? Solid
line Lower-order bits are applied first
glEnable(GL_LINE_STIPPLE)
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OpenGL Fill-area

• Filling convex polygons in four steps
• 1. Define a fill pattern
• 2. Invoke the polygon-fill routine
• 3. Activate the polygon-fill feature of OpenGL
• 4. Describe the polygons to be filled

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OpenGL Fill-area

• OpenGL wire-frame methods

glColor3f( 1.0, 0.0, 0.0 ) //red glPolygonMode(
GL_FRONT, GL_FILL ) // GL_FILL, GL_LINE,
GL_POINT glBegin( GL_POLYGON ) //draw solid
polygon glVertex2i( 10, 10 ) glVertex2i(
100, 10 ) glVertex2i( 150, 50 )
glVertex2i( 100, 100 ) glVertex2i( 50, 80 )
glVertex2i( 10, 10 ) glEnd()
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OpenGL Geometric Transformations

• Manipulating the Matrix Stacks
• OpenGL Viewing-Transformation Function
• Orthogonal-projection
• Perspective-projection

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Basic OpenGL Geometric Transformations

• Translation matrix
• Rotation matrix
• Scaling matrix

glTranslate (tx, ty, tz)
glTranslatef (25.0, -10.0, 0.0)
glRotate (theta, vx, vy, vz)
glRotatef (90.0, 0.0, 0.0, 1.0)
glScale (sx, sy, sz)
glScalef (2.0, 3.0, 1.0)
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OpenGL Matrix Operations

• Specify the modelview mode
• Modelview matrix store and combine the geometric
  transformations
• Define a 4 by 4 modelview matrix as the current
  matrix
• A call to a transformation routine
• ? Generate a matrix that is multiplied by the
  current matrix for that mode

glMatrixMode (GL_MODELVIEW) // defaule
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Manipulating the Matrix Stacks

• Modelview and Projection Matrix Stacks
• Useful for constructing hierarchical models

a b c d e f g h i j k l m n o p
a b c d e f g h i j k l m n o p
a b c d e f g h i j k l m n o p
current matrix copy
a b c d e f g h i j k l m n o p
a b c d e f g h i j k l m n o p
Modelview Matrix stack (32 4x4 matrices)
Projection Matrix stack (2 4x4 matrices)

• OpenGL may support more than 32 matrices for
  Modelview
• More than 2 for Projection

? Query the number of matrices
glGetIntegerv(GL_MAX_MODELVIEW_STACK_DEPTH) glGet
Integerv(GL_MAX_PROJECTION_STACK_DEPTH)
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Manipulating the Matrix Stacks

• Draw a car body
• Pushing and popping
• Other

// remember where your are
void glPushMatrix (void)
void glPopMatrix (void)
// go back to where you were
void glLoadIdentity ()
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OpenGL Three-Dimensional Viewing Function

• OpenGL Utility library (GLU) includes functions
• Specifying the three-dimensional viewing
  parameters
• Setting up a symmetric perspective-projection
  transformation
• OpenGL Viewing-Transformation Function
• The viewing matrix is combined with any geometric
  transformations

glMatrixMode (GL_MODELVIEW)
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OpenGL Viewing-Transformation Function

• Specifying viewing parameters

gluLookAt (eyex, eyey, eyez, cenerx, centery,
centerz,
upx, upy, upz)
The origin of the Viewing reference frame
View-up vector
Viewing Reference Position
Default gluLookAt(0.0, 0.0, 0.0, 0.0, 0.0,
-100, 0.0., 1.0,
0.0)
? Projection Reference Point
Viewing-coordinate Origin ? View Plane
Near clipping plane
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OpenGL Viewing-Transformation Function

• Once designate the viewing parameters
• A matrix is formed and concatenated with the
  current matrix
• Before you issue the transformation commands
  (parallel and perspective)

glMatrixMode (GL_PROJECTION) glLoadIdentity()
//affect the projection matrix
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OpenGL Orthogonal-Projection Function

• Set the projection mode
• Orthogonal-projection (Parallel Projection)

glMatrixMode (GL_Projection)
glOrtho (left, right, bottom, top, near, far)
top
left
far
right
Toward the viewpoint
near
bottom
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OpenGL Symmetric Perspective-Projection Function

• OpenGL general perspective-projection function
• Symmetric-perspective
• Oblique-perspective projective

glFrustum( left, right, bottom, top, near, far)
top
left
right
bottom
near
far
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OpenGL Symmetric Perspective-Projection Function

• Advanced
• More intuitive to use

glPerspective( fovy, aspect, near, far)
Field of view angle
Aspect ration (w/h)

• dnear and dfar cant be behind the viewing
  position
• Must be assigned position numerical values

w
fovy the angle between the top and bottom
clipping planes, range 0.0, 180.0
h
fovy
near
far
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OpenGL Viewport and Display Windows

• Viewport Transformation
• The viewport is the rectangle region of the
  window where the image is drawn
• Defining the viewport
• The window system is responsible for opening a
  window on the screen
• By default, the viewport is set to the entire
  pixel rectangle of the window thats open

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OpenGL Viewport and Display Windows

• Defining the viewport

Void glViewport( GLint x, Glint y,
GLsizei width, GLsizei height)
yscreen
Video Screen
gluPerspective ( fovy, 1.0, near,
far) glViewport ( 0, 0, 400, 400)
gluPerspective ( fovy, 1.0, near,
far) glViewport ( 0, 0, 400, 200) // Distorted
Display Window
gluPerspective ( fovy, 2.0, near,
far) glViewport ( 0, 0, 400, 200)
xscreen
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Demo