Programming in OpenGL Ryan Holmes CSE 570 February 12 th, 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_POINTSGL_POLYGON GL_LINESGL_LINE_STRIP GL_LINE_LOOPGL_QUADS GL_QUAD_STRIPGL_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 glMaterial glBegin(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 first Break 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 Project Zero walk-through and skeleton code. Links page The “Red Book” _bookshelves/SGIindex/SGI_Developer_ OpenGL_PG.html _bookshelves/SGIindex/SGI_Developer_ OpenGL_PG.html

Resources II GLUT GLUT Documentation - ation/glut/ ation/glut/.NET Framework -

Questions and Answers