Computer Graphics (Fall 2003) COMS 4160, Lecture 6: OpenGL 2 Ravi Ramamoorthi 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 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.