Computer Graphics (Fall 2003)

COMS 4160, Lecture 6: OpenGL 2

Ravi Ramamoorthi

http://www.cs.columbia.edu/~cs4160

Many slides courtesy Greg Humphreys

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

State Management

• glEnable, glDisable, glIsEnabled

• glGet

• Attributes– glPointSize, glFrontFace, glCullFace,

• Other advanced topics (later)– Vertex Arrays– Display Lists– Curved surfaces

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)

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

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)

GLUT

• OpenGL Utility Toolkit

• Written by Mark Kilgard

• Provides basic window system interaction:– Open/close window– Mouse/keyboard callbacks– “Idle” callback for animation– Menus

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();

}

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…

Transformations

• Object in world coordinates (modelview)– glTranslatef(x,y,z) ; glRotatef(θ,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

Sample Code

glMatrixMode( GL_MODELVIEW );glLoadIdentity();glTranslatef( 1, 1, 1 );glRotatef( 90, 1, 0, 0 );DrawObject();

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

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 aren’t too far apart

• Make sure the near plane isn’t too close to the eye

Complete Viewing Example

//Projection firstglMatrixMode( GL_PROJECTION );glLoadIdentity();gluPerspective( 60, 1, 1, 100 );gluLookAt( 10, 10, 10, 1, 2, 3, 0, 0, 1 )

//Now object transformationsglMatrixMode( GL_MODELVIEW );glLoadIdentity();glTranslatef( 1, 1, 1 );glRotatef( 90, 1, 0, 0 );DrawObject();

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

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?

Simple OpenGL Programs

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,…

Specifying Vertices

• glVertex{size}{type}{vector}e.g. glVertex3fv

glVertex2f( 1.0f, 1.0f );glVertex3i( 20, 20, 20 );

float verts[4] = { 1.0, 2.0, 3.0, 1.0 };glVertex4fv( verts );

Coordinates are passed in an arrayCoordinate types are float3 coordinates passed (x,y,z)

Example: A Wireframe Cube

GLfloat vertices[][3] = {{-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 );

}

Example: A Wireframe Cube

void polygon( int a, int b, int c, int d ){

glColor3f( 1,1,1 );glBegin( GL_LINE_LOOP ); glVertex3fv( vertices[a] ); glVertex3fv( vertices[b] ); glVertex3fv( vertices[c] ); glVertex3fv( vertices[d] );glEnd();

}

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:

glName{size}{type}{vector}e.g. glColor3us takes 3 unsigned short parameters

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 1’s 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

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

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)

Simple Shading

Example: shading a sphere with lighting

glShadeModel(GL_FLAT) glShadeModel(GL_SMOOTH)

OpenGL will interpolate the colors across the face of a polygonif smooth shading is turned on.