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, 4 th

(aka “The Red Book”) unless noted otherwise.

Edition ver. 1.4

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What is OpenGL?

OpenGL is a software interface to graphics hardware

.

Application 3-D world Simulation User Interface OpenGL Geometry, vertices, normals, colors, texture, etc.

Graphics Hardware (rasterizer, texturing, lighting, transformations, etc.) 3

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

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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);

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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();

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Primitive Types in glBegin

(Ch.2, p.44)

• • • • •

Points Lines GL_POINTS 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

(show page 45) Nate Robins' Tutorial: shapes 7

Transformations and Viewing

(Ch.3) OpenGL has 3 different matrix modes:

•

GL_MODELVIEW

• •

GL_PROJECTION GL_TEXTURE Choose the matrix with: glMatrixMode(…);

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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 9

OpenGL: Projection Matrix

•

Sets up a perspective projection.

(page 127)

•

glFrustrum (...);

•

gluPerspective (fovy, aspect, near, far);

•

glOrtho (...);

•

gluLookAt (...);

(often applied to modelview matrix) 10

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 11

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

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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(); return 0; // Start GLUT * Your code here

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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!

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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 );

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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);

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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

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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); ...

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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 19

OpenGL: Shading

•

OpenGL supports 2 basic shading models: flat and smooth.

glShadeModel(GL_FLAT); glShadeModel(GL_SMOOTH);

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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 21

OpenGL: Texturing

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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.

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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);

t (x3,y3,z3) 0,1 (x1,y1,z1) 1,1 1,1 + 0,0 s 0,0 (x0,y0,z0) 1,0 (x2,y2,z2) Tutorial: Texture pg 403 24

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 25

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 New color (r1,g1,b1,a1) “source” + Color in framebuffer (r0,g0,b0,a0) “destination” = ?

(r’,g’,b’,a’) • Useful for overlaying textures or other effects 26

OpenGL: Fog

Simulate atmospheric effects

•

glFog (): Sets fog parameters

•

glEnable (GL_FOG);

Tutorial: fog 27

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

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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 #include #include

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.

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Questions?

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