Programming in OpenGL

Ryan HolmesCSE 570February 12th, 2003

Overview

What does OpenGL include? What doesn’t OpenGL include? How do you get started with OpenGL? Tips and Tricks Resources Questions and Answers

OpenGL Is

A low-level 3D graphics API A separate piece of code you access

through an API An interface to hardware Primitive-based

A state machine

A low-level 3D graphics API

Separate code Opengl32.dll on Windows Vendors package their own version of

this library with the graphics card Windows 2000 supports a software-

only version of OpenGL 1.1 out of the box

A low-level 3D graphics API

An interface to hardware The library knows how to interface

with the card drivers to get the hardware to handle the graphics.

Anything not done in hardware is done in software

A low-level 3D graphics API

Primitive-based Objects consist of points, line-

segments, and polygons OpenGL is not aware of any

connections between primitives Exception

The GLU libraries include quadric and NURBS “objects” that encapsulate primitives for you

A state machine

Functions are global and change the state of the OpenGL environment State can be pushed onto stacks and popped back off OpenGL properties remain as you set them until you set them again

OpenGL Is Not

A modeling language Compiled directly into your code 3D object-oriented

Getting Started - Syntax

OpenGL core functions are prefixed with gl OpenGL utility functions are prefixed with glu OpenGL typedef defined types are prefixed with GL OpenGL constants are all caps and prefixed with GL_

Getting Started - Example::glBegin(GL_LINE_LOOP);

::glVertex3d(0, 0, 0);

::glVertex3d(x, 0, 0);

::glVertex3d(x, y, 0);

::glVertex3d(0, y, 0);

::glEnd();

::glBegin(GL_LINE_LOOP);

::glVertex3d(0, 0, z);

::glVertex3d(x, 0, z);

::glVertex3d(x, y, z);

::glVertex3d(0, y, z);

::glEnd();

::glBegin(GL_LINES);

::glVertex3d(0, 0, 0);

::glVertex3d(0, 0, z);

::glVertex3d(x, 0, 0);

::glVertex3d(x, 0, z);

::glVertex3d(x, y, 0);

::glVertex3d(x, y, z);

::glVertex3d(0, y, 0);

::glVertex3d(0, y, z);

::glEnd();

Getting Started - Interface

OpenGL has no direct interface functionality Mouse, keyboard and window management handled by separate interface Windows wgl (“wiggle”) functions

Environment Choice

MFC Strong interface support (Dialogs,

menus, mouse handling) Steep learning curve

GLUT Portable Easier learning curve

MFC Program Organization

Document/View architecture Document – Encapsulate Data View – Display Data from Document

OnDraw() Called every time the window needs

to be displayed. Indirectly called by you with the Invalidate() call.

GLUT Program Organization

Callback architecture – Register functions to be called when events happen. glutCreateWindow() – Setup glutDisplayFunc() – Registers your “OnDraw” equivalent glutMainLoop() – Handles redraw decisions

Creating Primitives

glBegin(type) and glEnd() All primitives begin as vertices GL_POINTS GL_POLYGON GL_LINES GL_LINE_STRIP GL_LINE_LOOP GL_QUADS GL_QUAD_STRIP GL_TRIANGLES GL_TRIANGLE_FAN GL_TRIANGLE_STRIP

Vertex Data

x,y,z coordinates Color (If lighting is disabled) Materials (If lighting is enabled) Normal (If lighting is enabled) Other data (e.g. texture coordinates)

Vertex data is part of the current state!

Shading

OpenGL supports flat shading (GL_FLAT) and smooth (GL_SMOOTH) (Gouraud) shading only – glShadeModel(type) Shading is vertex-centric, not face-centric Lighting disabled – glColor Lighting enabled – glMaterial and normals must be specified properly

Flat Shading Sequence

// Set material with glMaterial glNormal3d(0, 0, 1.0); glBegin(GL_TRIANGLES); glVertex3d(0,0,0); glVertex3d(2, 0, 0); glVertex3d(1, 1, 0);glEnd();

Smooth Shading Sequence

// Set material with glMaterialglBegin(GL_TRIANGLES); glNormal3d(0, 0, 1.0); glVertex3d(0,0,0); glNormal3d(0, 0, 1.0); glVertex3d(2, 0, 0); glNormal3d(0, 0, 1.0); glVertex3d(1, 1, 0);glEnd();

Typical Rendering Sequence

Clear the frame buffer and depth buffer – glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)

Specify all primitives in the scene glBegin(type) glVertex()… glEnd()

Swap the buffers – windowing system specific

Tips and Tricks I

Plan firstBreak things into pieces Put only what is necessary into OnDraw Choose your “default” state and initialize it during program start-up. Make sure that every function returns the state to that default before it exits

Tips and Tricks II

Read the Red Book Appendices contain a wealth of

speed-up and optimization information

Learn the debugging tools. When a graphics program doesn’t display anything, it can be difficult to figure out what’s wrong

Fundamental Principle of Graphics Programming

Graphics programming is 30% understanding what data you need and 70% keeping track of where that data is and updating it.

Design your data structures accordingly.

Resources I

http://www.eas.asu.edu/~cse470/ Project Zero walk-through and

skeleton code. Links page

The “Red Book” http://biology.ncsa.uiuc.edu/library/SGI_bookshelves/SGIindex/SGI_Developer_OpenGL_PG.html

Resources II

GLUT - http://www.xmission.com/~nate/glut.html

GLUT Documentation - http://www.opengl.org/developers/documentation/glut/

.NET Framework - http://csgl.sourceforge.net/

Questions and Answers