Computer Graphics Spring 2003 COMS 4160 Lecture 13

Computer Graphics (Spring 2003) COMS 4160, Lecture 13: Open. GL 2 Ravi Ramamoorthi http: //www. cs. columbia. edu/~cs 4160 Many slides courtesy Greg Humphreys

State • Open. GL 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 gl. Enable and gl. Disable • Anything that can be set can be queried using gl. Get

State Management • gl. Enable, gl. Disable, gl. Is. Enabled • gl. Get • Attributes – gl. Point. Size, gl. Front. Face, gl. Cull. Face, • 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 • Open. GL 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 • Open. GL 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[] ) { glut. Init( &argc, argv ); glut. Init. Display. Mode( GLUT_SINGLE | GLUT_RGB ); glut. Create. Window( “A window” ); glut. Display. Func( display ); glut. Reshape. Func( reshape ); glut. Mouse. Func( mouse ); glut. Idle. Func( idle ); glut. Main. Loop(); }

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) – gl. Translatef(x, y, z) ; gl. Rotatef(θ, x, y, z) ; gl. Scalef(x, y, z) – Right-multiply current matrix (last is first applied) • Matrix Stacks – gl. Push. Matrix, gl. Pop. Matrix, gl. Load – Useful for Hierarchically defined figures • glu. Look. At (fromv, atv, upv) – Usually in projection matrix, sometimes in modelview – Concatenate with perspective (orthographic) projection

Sample Code gl. Matrix. Mode( GL_MODELVIEW ); gl. Load. Identity(); gl. Translatef( 1, 1, 1 ); gl. Rotatef( 90, 1, 0, 0 ); Draw. Object();

Positioning the Camera • Use glu. Look. At to specify: – Eye location – “Look-at point” – “up” vector • glu. Look. At( 10, 10, 1, 2, 3, 0, 0, 1 ) – Eye point is (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 – gl. Ortho( left, right, bottom, top, front, back ) specifies the boundaries of the parallel viewing volume – Objects are clipped to the specified viewing cube • Perspective Projection – gl. Frustum, glu. Perspective – 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 gl. Matrix. Mode( GL_PROJECTION ); gl. Load. Identity(); glu. Perspective( 60, 1, 1, 100 ); glu. Look. At( 10, 10, 1, 2, 3, 0, 0, 1 ) //Now object transformations gl. Matrix. Mode( GL_MODELVIEW ); gl. Load. Identity(); gl. Translatef( 1, 1, 1 ); gl. Rotatef( 90, 1, 0, 0 ); Draw. Object();

Matrix Stacks • Open. GL has multiple matrix “stacks” • gl. Push. Matrix pushes a copy of the top-ofstack matrix • gl. Pop. Matrix 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 gl. Shade. Model – GL_FLAT, GL_SMOOTH (Goraud) – Issues in smooth shading with color index?

Simple Open. GL Programs

Drawing Primitives • Specify primitives using gl. Begin and gl. End: gl. Begin( primitive_type ); …specify vertex attributes …draw vertices gl. End(); • primitive_type specifies points, lines, triangles, quads, polygons, etc… • GL_POINTS, GL_LINES, GL_TRIANGLES, GL_QUADS, GL_POLYGON, …

Specifying Vertices • gl. Vertex{size}{type}{vector} e. g. gl. Vertex 3 fv Coordinates are passed in an array Coordinate types are float 3 coordinates passed (x, y, z) gl. Vertex 2 f( 1. 0 f, 1. 0 f ); gl. Vertex 3 i( 20, 20 ); float verts[4] = { 1. 0, 2. 0, 3. 0, 1. 0 }; gl. Vertex 4 fv( verts );

Example: A Wireframe Cube GLfloat vertices[][3] = { {-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 { gl. Color 3 f( 1, 1, 1 ); gl. Begin( GL_LINE_LOOP ); gl. Vertex 3 fv( vertices[a] gl. Vertex 3 fv( vertices[b] gl. Vertex 3 fv( vertices[c] gl. Vertex 3 fv( vertices[d] gl. End(); } c, int d ) ); );

Specifying Vertex Attributes • Vertex attributes are state settings that are usually applied between a gl. Begin/gl. End pair • Vertex attributes are set using: gl. Color, gl. Normal, gl. Tex. Coord, gl. Edge. Flag • Vertex attribute routines take a form similar to gl. Vertex: gl. Name{size}{type}{vector} e. g. gl. Color 3 us takes 3 unsigned short parameters

Pixel Primitives • Provide a way to manipulate rectangles of pixels • gl. Draw. Pixels, gl. Read. Pixels, gl. Copy. Pixels move pixel rectangles to and from the framebuffer • gl. Bitmap 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 • gl. Raster. Pos defines where the pixels go in the framebuffer • The interaction between gl. Raster. Pos and gl. Bitmap 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: glut. Init. Display. Mode( GLUT_DEPTH ); • When clearing, make sure to: gl. Clear( GL_DEPTH_BUFFER_BIT ); • gl. Enable( GL_DEPTH_TEST ); • Set the depth test comparison operation: gl. Depth. Func( GL_LESS ); (this is the default)

Simple Shading Example: shading a sphere with lighting gl. Shade. Model(GL_FLAT) gl. Shade. Model(GL_SMOOTH) Open. GL will interpolate the colors across the face of a polygon if smooth shading is turned on.