Introduction to OpenGL
OpenGL is a standard library for 2-D & 3-D drawing
• maps fairly directly to graphics hardware
• state-based architecture
• immediate mode drawing
• doesn’t address windows or input events (we’ll use FLTK)
Why we are using OpenGL
• clean design
• originally by SGI, but enjoys broad cross-platform support
• a free software implementation (www.mesa3d.org)
Decoding OpenGL function names
• general rule: glFunc[234][dfis][v]
• example: glVertex3f(x,y,z) — vertex position as 3 floats
dimension type (e.g., i=int)
v = pass pointer rather than values
Overview of an OpenGL Program
System-dependent initialization
• setup a window on the screen
• bind OpenGL to this window
OpenGL initialization
• setup coordinate system
• setup initial state values
Begin event loop
• wait for system to deliver event (e.g., mouse moved)
• figure out how this will change the scene, if at all
• if redraw event, execute OpenGL commands to draw scene
Event Driven Interactive Programs
This is the model for modern GUI programs
• run some initialization code at startup
• everything after that happens in response to events
Typical events
• window resized
• redraw window
• mouse moved
• button clicked
• key pressed
•…
Most frameworks use callbacks
• procedures bound to specific events
• for instance, call function when a button is pressed
main()
{
InitializeApp();
while( !done ) {
event = WaitForOne();
handle(event);
}
}
OpenGL Uses Immediate Mode Rendering
Execute drawing commands every time window is repainted
• or every time the scene changes (e.g., for animation)
• vs. retained mode where system maintains model of scene
OpenGL does provide some retained mode features
• display lists — capture & store immediate mode stream
• vertex arrays — pass large array of points in one call
Immediate Mode:
void initialize() {}
void on_redraw() {
for(all triangles t)
draw_triangle(t);
}
Retained Mode:
void initialize()
{
load_triangle_list(T);
}
void on_redraw() {}
