Introduction to OpenGL¶

Three stages

Define Objects in World Space

Set Modeling and Viewing Transformations

Render the Scene

How OpenGL Works¶

OpenGL is a state machine
Give orders to set the current state of any one of its internal variables, or to query its current status
The current state won't change until you explicitly specify a new value
Each of the system's state variables has a default value

Sample

Command Syntax
- All command names begin with gl
- Constant names are in all uppercase
- Data types begin with GL
  - GLfloat onevertex[3]
- Most commands end in two characters that determine the data type of expected arguments
  - glVertex3f
glVertex
- glVertex2f( x, y );
- glVertex3f( x, y, z );
- glVertex4f( x, y, z, w );
- glVertex3fv( a ); // with a[0], a[1], a[2]
building objects from vertices: Specify a primitive mode, and enclose a set of vertices in a glBegin / glEnd block
colors
- defined as RGB components from 0.0 to 1.0
- for background
  - glClearColor(0.0,0.0,0.0) //black
  - glClear(GL_COLOR_BUFFER_BIT)
- for objects
  - glColor3f(1.0,1.0,1.0)

Polygon Display Modes
- glPolygonMode( GLenum face, GLenum mode );
  - Faces: GL_FRONT / GL_BACK / GL_FRONT_AND_BACK`
  - Modes: GL_FILL / GL_LINE / GL_POINT
  - By default, both the front and back face are drawn filled
- glFrontFace( GLenum mode );定义多边形的前面方向
  - GL_CCW (default) / GL_CW 默认逆时针
- glCullFace( Glenum mode );指定要剔除（不渲染）的多边形面
  - GL_FRONT / GL_BACK
- You must enable and disable culling with glEnable( GL_CULL_FACE ) or glDisable( GL_CULL_FACE ); 启用或禁用剔除
Drawing Other Objects
- cylinders, cones and more complex surfaces called NURBS
- spheres and cubes
Structure of GLUT-based Programs
- rely on user-defined callback functions

More GLUT

- Additional GLUT fucntions
  - `glutPositionwindow(int x,int y)`
  - `glutReshapewindow(int w,int h)`
- Additional callback functions