Computer Graphics Spring 2003

1
Computer Graphics (Spring 2003)

• COMS 4160, Lecture 13 OpenGL 2
• Ravi Ramamoorthi

http//www.cs.columbia.edu/cs4160
Many slides courtesy Greg Humphreys
2
State

• OpenGL is a big state machine
• State encapsulates control for operations like
• Lighting
• Shading
• Texture Mapping
• Boolean state settings can be turned on and off
  with glEnable and glDisable
• Anything that can be set can be queried using
  glGet

3
State Management

• glEnable, glDisable, glIsEnabled
• glGet
• Attributes
• glPointSize, glFrontFace, glCullFace,
• Other advanced topics (later)
• Vertex Arrays
• Display Lists
• Curved surfaces

4
Immediate vs. Retained Mode

• Two ways of specifying what is to be drawn
• Immediate Mode
• Primitives are sent to the display as soon as
  they are specified
• Graphics system has no memory of drawn graphics
  primitives
• Retained Mode
• Primitives placed in display lists
• Display lists can be kept on the graphics server
• Can be redisplayed with different graphics state
• Almost always a performance win (if you can get
  away with it)

5
Event Driven Interaction

• OpenGL does not dictate any particular model of
  interaction
• Applications respond to events generated by
  devices (i.e., mice) and window system events
  (i.e., window resized)
• Events are usually placed in a queue awaiting
  action
• Callbacks let you associate a function with a
  particular type of event
• Mouse callback

6
Usual Interactions

• Initialization
• Open / place / resize window
• Expose/resize/hide window
• Mouse
• Button click
• Button release
• Mouse motion
• Mouse drag (motion button)
• Keyboard
• What key was pressed (or released)?
• Where was the mouse?
• Were any modifier keys pressed? (control, alt,
  shift)

7
GLUT

• OpenGL Utility Toolkit
• Written by Mark Kilgard
• Provides basic window system interaction
• Open/close window
• Mouse/keyboard callbacks
• Idle callback for animation
• Menus

8
Typical main Function
int main( int argc, char argv ) glutInit(
argc, argv ) glutInitDisplayMode( GLUT_SINGLE
GLUT_RGB ) glutCreateWindow( A window
) glutDisplayFunc( display ) glutReshapeFunc(
reshape ) glutMouseFunc( mouse
) glutIdleFunc( idle ) glutMainLoop()
9
Viewing (Chapter 3)

• Two parts
• Object positioning (GL_MODELVIEW)
• Projection (GL_PROJECTION)
• Transformation stages (pp 98)
• Perspective, Orthographic transformations
• Camera always at origin, pointing z direction
• Transforms applied to objects
• Equivalent to moving camera by inverse
• More details next

10
Transformations

• Object in world coordinates (modelview)
• glTranslatef(x,y,z) glRotatef(?,x,y,z)
  glScalef(x,y,z)
• Right-multiply current matrix (last is first
  applied)
• Matrix Stacks
• glPushMatrix, glPopMatrix, glLoad
• Useful for Hierarchically defined figures
• gluLookAt (fromv,atv,upv)
• Usually in projection matrix, sometimes in
  modelview
• Concatenate with perspective (orthographic)
  projection

11
Sample Code
glMatrixMode( GL_MODELVIEW ) glLoadIdentity() gl
Translatef( 1, 1, 1 ) glRotatef( 90, 1, 0, 0
) DrawObject()
12
Positioning the Camera

• Use gluLookAt to specify
• Eye location
• Look-at point
• up vector
• gluLookAt( 10, 10, 10, 1, 2, 3, 0, 0, 1 )
• Eye point is (10, 10, 10)
• Look at point is (1,2,3)
• Up vector is (0,0,1)
• This is usually done in the GL_PROJECTION matrix,
  and combined with a perspective matrix

13
Viewing Volumes

• Orthographic projection
• glOrtho( left, right, bottom, top, front, back )
  specifies the boundaries of the parallel viewing
  volume
• Objects are clipped to the specified viewing cube
• Perspective Projection
• glFrustum, gluPerspective
• Clipping volume is a frustum
• Make sure the near and far clipping planes arent
  too far apart
• Make sure the near plane isnt too close to the
  eye

14
Complete Viewing Example
//Projection first glMatrixMode( GL_PROJECTION
) glLoadIdentity() gluPerspective( 60, 1, 1,
100 ) gluLookAt( 10, 10, 10, 1, 2, 3, 0, 0, 1
) //Now object transformations glMatrixMode(
GL_MODELVIEW ) glLoadIdentity() glTranslatef(
1, 1, 1 ) glRotatef( 90, 1, 0, 0 ) DrawObject()
15
Matrix Stacks

• OpenGL has multiple matrix stacks
• glPushMatrix pushes a copy of the top-of-stack
  matrix
• glPopMatrix throws away the top of the stack
• Very useful for hierarchichally defined figures

16
Color (Chapter 4)

• RGBA
• 8 (or whatever number) bits/color channel
• Alpha A important for transparency
• 32 bits, 16 million colors
• Color Index
• Index into color table, small number of colors
  (256)
• Shading model glShadeModel
• GL_FLAT, GL_SMOOTH (Goraud)
• Issues in smooth shading with color index?

17
Simple OpenGL Programs
18
Drawing Primitives

• Specify primitives using glBegin and glEnd
• glBegin( primitive_type )
• specify vertex attributes
• draw vertices
• glEnd()
• primitive_type specifies points, lines,
  triangles, quads, polygons, etc
• GL_POINTS, GL_LINES, GL_TRIANGLES, GL_QUADS,
  GL_POLYGON,

19
Specifying Vertices

• glVertexsizetypevector
• e.g. glVertex3fv
• glVertex2f( 1.0f, 1.0f )
• glVertex3i( 20, 20, 20 )
• float verts4 1.0, 2.0, 3.0, 1.0
• glVertex4fv( verts )

Coordinates are passed in an array Coordinate
types are float 3 coordinates passed (x,y,z)
20
Example A Wireframe Cube
GLfloat vertices3 -1,-1,-1,1,-1,1,
1,1,-1,-1,1,-1, -1,-1,1,1,-1,1, 1,1,1,
-1,1,1 void cube(void) polygon( 1,0,3,2
) polygon( 3,7,6,2 ) polygon( 7,3,0,4 )
polygon( 2,6,5,1 ) polygon( 4,5,6,7 ) polygon(
5,4,0,1 )
21
Example A Wireframe Cube
void polygon( int a, int b, int c, int d
) glColor3f( 1,1,1 ) glBegin( GL_LINE_LOOP
) glVertex3fv( verticesa ) glVertex3fv(
verticesb ) glVertex3fv( verticesc )
glVertex3fv( verticesd ) glEnd()
22
Specifying Vertex Attributes

• Vertex attributes are state settings that are
  usually applied between a glBegin/glEnd pair
• Vertex attributes are set using
• glColor, glNormal, glTexCoord, glEdgeFlag
• Vertex attribute routines take a form similar to
  glVertex
• glNamesizetypevector
• e.g. glColor3us takes 3 unsigned short parameters

23
Pixel Primitives

• Provide a way to manipulate rectangles of pixels
• glDrawPixels, glReadPixels, glCopyPixels move
  pixel rectangles to and from the framebuffer
• glBitmap takes a binary image and renders the
  current color in framebuffer positions
  corresponding to 1s in the image. This might be
  useful for drawing fonts
• glRasterPos defines where the pixels go in the
  framebuffer
• The interaction between glRasterPos and glBitmap
  is subtle and confusing

24
Hidden Surface Removal

• When we draw a fragment, record the z (distance
  to the eye) in the depth buffer
• If the z stored in the depth buffer is greater
  than the z for the fragment about to be drawn,
  draw it
• Otherwise, the fragment is behind something that
  has already been drawn, so throw it away

25
Hidden Surface Removal

• When setting up your window, specify a depth
  buffer
• glutInitDisplayMode( GLUT_DEPTH )
• When clearing, make sure to
• glClear( GL_DEPTH_BUFFER_BIT )
• glEnable( GL_DEPTH_TEST )
• Set the depth test comparison operation
• glDepthFunc( GL_LESS ) (this is the default)

26
Simple Shading
Example shading a sphere with lighting
glShadeModel(GL_FLAT)
glShadeModel(GL_SMOOTH)
OpenGL will interpolate the colors across the
face of a polygon if smooth shading is turned on.