Computer Graphics (Spring 2003)

Computer Graphics (Spring 2003) COMS 4160, Lecture 13: 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 usingglGet

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 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();

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 array Coordinate types are float 3 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 polygon if smooth shading is turned on.

Computer Graphics (Spring 2003)