CS 248 OpenGL Help Session

CS 248OpenGL Help Session CS248 Presented by Sean Walker (adapted from Ian Buck’s slides – see ’03 web page) Stanford University Nov. 5, 2004

Overview • What is OpenGL? • Basic primitives and rendering in OpenGL • Transformations and viewing • GLUT and the interaction / display loop • Buffers, Lighting, and Texturing • Other features • Development tips Note: all page references refer to the OpenGL Programming Guide, 4th Edition ver. 1.4(aka “The Red Book”) unless noted otherwise.

What is OpenGL? OpenGL is a software interface to graphics hardware. Application 3-D world Simulation User Interface Graphics Hardware (rasterizer, texturing, lighting, transformations, etc.) OpenGL Geometry, vertices, normals, colors, texture, etc.

OpenGL Basics • Read the Red Book! It’s a great resource and is very readable. • OpenGL is a state machine: polygons are affected by the current color, transformation, drawing mode, etc. • Everything in the OpenGL specification is supported in every implementation

Specifying Object Vertices (Ch.2 p.42, Ch.4 p.171) • Every object is specified by vertices • glVertex3f (2.0, 4.1, 6.0); • glVertex2i (4, 5); • glVertex3fv (vector); • Current color affects any vertices • glColor3f (0.0, 0.5, 1.0); • glColor4ub (0, 128, 255, 0); • glColor3dv (color);

Specifying Object Vertices (Ch.2 p.42) • Vertices are specified only between glBegin(mode) and glEnd(), usually in a counter-clockwise order for polygons. glBegin (GL_TRIANGLES); glVertex2i (0, 0); glVertex2i (2, 0); glVertex2i (1, 1); glEnd();

Primitive Types in glBegin(Ch.2, p.44) • Points GL_POINTS • Lines GL_LINES, GL_LINE_STRIP, GL_LINE_LOOP • Triangles GL_TRIANGLES, GL_TRIANGLE_STRIP, GL_TRIANGLE_FAN • Quads GL_QUADS, GL_QUAD_STRIP • Polygons GL_POLYGON Nate Robins' Tutorial: shapes (show page 45)

Transformations and Viewing(Ch.3) OpenGL has 3 different matrix modes: • GL_MODELVIEW • GL_PROJECTION • GL_TEXTURE Choose the matrix with: glMatrixMode(…);

OpenGL: Modelview matrix • Transforms objects within the scene. glMatrixMode(GL_MODELVIEW); glLoadIdentity(); glTranslatef(10.5, 0, 0); glRotatef(45, 0, 0, 1); DrawCube(); • Remember that the operations are right multiplied, so the transformation just before DrawCube() takes effect first. Tutorial: transformation

OpenGL: Projection Matrix • Sets up a perspective projection. (page 127) • glFrustrum (...); • gluPerspective (fovy, aspect, near, far); • glOrtho (...); • gluLookAt (...); (often applied to modelview matrix)

OpenGL: Projection Matrix Example:glMatrixMode(GL_PROJECTION);glLoadIdentity();gluPerspective(64, (float)windowWidth / (float)windowHeight, 4, 4096); gluLookAt(0.0, 0.0, 2.0, // camera position 0.0, 0.0, 0.0, // target position 0.0, 0.0, 2.0); // up vector Tutorial: projection

GLUT – OpenGL Utility Toolkit (Appendix D) • GLUT is a library that handles system events and windowing across multiple platforms • Includes some nice utilities • We strongly suggest you use it • Find it at: http://graphics.stanford.edu/courses/cs248-04/proj3/index.html

GLUT – Starting Up int main (int argc, char *argv[]) {glutInit(&argc, argv); glutInitDisplayMode (GLUT_DEPTH | GLUT_DOUBLE | GLUT_RGBA); glutInitWindowSize (windowWidth, windowHeight); glutInitWindowPosition (0, 0); glutCreateWindow (“248 Video Game!"); SetStates(); // Initialize rendering states* RegisterCallbacks(); // Set event callbacks* glutMainLoop(); // Start GLUT return 0; } * Your code here

Setting Up Rendering States • OpenGL is a state machine: polygons are affected by the current color, transformation, drawing mode, etc. • Enable and disable features such as lighting, texturing, and alpha blending. • glEnable (GL_LIGHTING); • glDisable (GL_FOG); • Forgetting to enable something is a common source of bugs!

GLUT Event Callbacks • Register functions that are called when certain events occur. • Examples: glutDisplayFunc( Display ); glutKeyboardFunc( Keyboard ); glutReshapeFunc( Reshape ); glutMouseFunc( Mouse ); glutPassiveMotionFunc( PassiveFunc ); glutMotionFunc( MouseDraggedFunc ); glutIdleFunc( Idle );

OpenGL Buffers • Multiple types of buffers • Color buffers (front/back, left/right) • Depth buffer (hidden surface removal) • Stencil buffer (allows masking or stenciling) • Accumulation buffer (antialiasing, depth of field) • Clearing buffers: // Clear to this color when screen is cleared.glClearColor (0.0, 0.0, 0.0, 0.0);// Clear color and depth buffers.glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

OpenGL Double Buffering • Double buffering: • Draw on back buffer while front buffer is being displayed. • When finished drawing, swap the two, and begin work on the new back buffer. • glutSwapBuffers(); • Primary purpose: eliminate flicker

OpenGL: Normals and Lighting • OpenGL can do lighting computations for you • Normal vectors should be of unit length (normalized) in most cases. • Normal vector kept as state – each vertex is assigned the most recently set normal vector ... glNormal3fv (n0); glVertex3fv (v0); glVertex3fv (v1); glVertex3fv (v2); ...

OpenGL: Lighting(Ch.5 p.178) • glEnable (GL_LIGHTING); • OpenGL supports a minimum of 8 lights. glEnable (GL_LIGHT0); ... glEnable (GL_LIGHT7); • Lights have a position, type, and color, among other things (more details in text). • Types of lights are point light, directional light, and spotlight. (page 191) Tutorial: lightposition

OpenGL: Shading • OpenGL supports 2 basic shading models: flat and smooth. glShadeModel(GL_FLAT);glShadeModel(GL_SMOOTH);

OpenGL: Material Properties (Ch.5) • Material properties are associated with each polygon (corresponding light properties) • glMaterial*(GLenum face, GLenum pname, TYPE param); • Some properties (pname),page 206: • GL_AMBIENT: Ambient color of material • GL_DIFFUSE: Diffuse color of material • GL_SPECULAR: Specular component (for highlights) • GL_SHININESS: Specular exponent (intensity of highlight) Tutorial: lightmaterial

OpenGL: Texturing

OpenGL: Texturing • Loading your data • This can come from an image: ppm, tiff • Or create at run time • Final result is always an array • Setting texture state • Creating texture names with “binding”, scaling the image/data, building Mipmaps, setting filters, etc.

OpenGL: Texturing • Mapping the texture to the polygon • specify (s,t) texture coordinates for (x,y,z) polygon vertices • texture coordinates (s,t)are from 0,1: glTexCoord2f(s,t); (x3,y3,z3) (x1,y1,z1) t 0,1 1,1 1,1 + 0,0 1,0 s 0,0 (x0,y0,z0) (x2,y2,z2) Tutorial: Texture pg 403

OpenGL: Advanced Texturing • Advanced texturing techniques • Mipmapping • Multitextures • Automatic texture generation • Let OpenGL determine texture coordinates for you • Environment Mapping • Texture matrix stack • Fragment Shaders • Custom lighting effects

OpenGL: Alpha Blending Ch 6, pg 225 • When enabled, OpenGL uses the alpha channel to blend a new fragment’s color value with a color in the framebuffer • Useful for overlaying textures or other effects ? + = New color Color in framebuffer (r’,g’,b’,a’) (r1,g1,b1,a1) (r0,g0,b0,a0) “source” “destination”

OpenGL: Fog • Simulate atmospheric effects • glFog (): Sets fog parameters • glEnable (GL_FOG); Tutorial: fog

OpenGL: Other Features • Display Lists (ch 7): Speed up your game! • Quadrics (ch 11): Pre-made objects • Also look at GLUT’s objects • Evaluators (ch 12): Bezier curves and surfaces • Selection (ch 13): Clicking on game objects with a mouse

Development • On Windows: • Download the GLUT libraries (linked off the proj3 webpage). • You want to link your project with: opengl32.lib, glut32.lib, and glu32.lib.This is under Project->Settings->Link in MS Visual Studio. • On Linux: • GLUT is already installed on the graphics lab PCs. • In your Makefile, compile with flags: -L/usr/lib -lGL -lGLU –lglut • Headers #include <GL/gl.h> #include <GL/glu.h> #include <GL/glut.h> • Call glutReportErrors() once each display loop for debugging. • This will report any errors that may have occurred during rendering, such as an illegal operation in a glBegin/glEnd pair.

Questions?

CS 248 OpenGL Help Session