CS 248 OpenGL 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.

OpenGL is a software interface to graphics hardware. 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 Clearing 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 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 Setting texture state 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 0,0 s (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 Headers On Windows: On Linux: 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?