Introduction to OpenGL

1
Introduction to OpenGL
2
OpenGL

• A Graphics rendering API introduced in 1992 by
  Silicon Graphics Inc
• Provide the low-level functions to access
  graphics hardware directly
• Cross-platform

3
Related API

• GLU (OpenGL Utility Library)
• Part of OpenGL
• Use the prefix of glu (ex gluLookAt())
• GLUT (OpenGL Utility Toolkit)
• Not officially part of OpenGL
• hide the complexities of differing window system
  APIs.
• Use the prefix of glut (ex glutDisplayFunc())

4
OpenGL Utility Toolkit (GLUT)

• Visual C
• http//www.xmission.com/nate/glut.html
• glut32.dll to WinDir\System,
• glut32.lib to (MSDevDir)\..\..\VC\lib
• glut.h to (MSDevDir)\..\..\VC\include\GL.

5
A Simple Example

• include ltGL/glut.hgt
• void GL_display()
• void GL_reshape(GLsizei w, GLsizei h)
• void main(void)
• glutInitDisplayMode(GLUT_SINGLE GLUT_RGB)
• glutCreateWindow("Sample")
• glutDisplayFunc(GL_display)
• glutReshapeFunc(GL_reshape)
• glutMainLoop()

6
A Simple Example

• void GL_display()
• glClearColor(0.0f, 0.0f, 0.0f, 0.0f)
• glClear(GL_COLOR_BUFFER_BIT)
• glBegin( GL_LINES )
• glColor3f(0.0f, 1.0f, 0.0f)
• glVertex3f(1.0, 1.0, 0.0)
• glVertex3f(-1.0, -1.0, 0.0)
• glColor3f(1.0f, 0.0f, 0.0f)
• glVertex3f(-1.0, 1.0, 0.0)
• glVertex3f(1.0, -1.0, 0.0)
• glEnd()
• glFlush()

7
A Simple Example

• void GL_display()
• glClearColor(0.0f, 0.0f, 0.0f, 0.0f)
• glClear(GL_COLOR_BUFFER_BIT)
• glBegin( GL_LINES )
• glColor3f(0.0f, 1.0f, 0.0f)
• glVertex3f(1.0, 1.0, 0.0)
• glVertex3f(-1.0, -1.0, 0.0)
• glColor3f(1.0f, 0.0f, 0.0f)
• glVertex3f(-1.0, 1.0, 0.0)
• glVertex3f(1.0, -1.0, 0.0)
• glEnd()
• glFlush()

8
A Simple Example

• void GL_display()
• glClearColor(0.0f, 0.0f, 0.0f, 0.0f)
• glClear(GL_COLOR_BUFFER_BIT)
• glBegin( GL_LINES )
• glColor3f(0.0f, 1.0f, 0.0f)
• glVertex3f(1.0, 1.0, 0.0)
• glVertex3f(-1.0, -1.0, 0.0)
• glColor3f(1.0f, 0.0f, 0.0f)
• glVertex3f(-1.0, 1.0, 0.0)
• glVertex3f(1.0, -1.0, 0.0)
• glEnd()
• glFlush()

9
Primitives
10
A Simple Example

• void GL_display()
• glClearColor(0.0f, 0.0f, 0.0f, 0.0f)
• glClear(GL_COLOR_BUFFER_BIT)
• glBegin( GL_LINES )
• glColor3f(0.0f, 1.0f, 0.0f)
• glVertex3f(1.0, 1.0, 0.0)
• glVertex3f(-1.0, -1.0, 0.0)
• glColor3f(1.0f, 0.0f, 0.0f)
• glVertex3f(-1.0, 1.0, 0.0)
• glVertex3f(1.0, -1.0, 0.0)
• glEnd()
• glFlush()

11
OpenGL as a State Machine

• Put a value into various states, then it will
  remain in effect until being changed.
• e.g. glColor()
• Many state variables are enabled or disabled with
  glEnable(), glDisable()
• e.g. glEnable(GL_LIGHT0)

12
A Simple Example

• void GL_display()
• glClearColor(0.0f, 0.0f, 0.0f, 0.0f)
• glClear(GL_COLOR_BUFFER_BIT)
• glBegin( GL_LINES )
• glColor3f(0.0f, 1.0f, 0.0f)
• glVertex3f(1.0, 1.0, 0.0)
• glVertex3f(-1.0, -1.0, 0.0)
• glColor3f(1.0f, 0.0f, 0.0f)
• glVertex3f(-1.0, 1.0, 0.0)
• glVertex3f(1.0, -1.0, 0.0)
• glEnd()
• glFlush()

13
Command Syntax
glVertex3f( 1.0, 1.0, 0.0 )
OpenGL API
Name
The number and types of arguments
14
A Simple Example

• void GL_display()
• glClearColor(0.0f, 0.0f, 0.0f, 0.0f)
• glClear(GL_COLOR_BUFFER_BIT)
• glBegin( GL_LINES )
• glColor3f(0.0f, 1.0f, 0.0f)
• glVertex3f(1.0, 1.0, 0.0)
• glVertex3f(-1.0, -1.0, 0.0)
• glColor3f(1.0f, 0.0f, 0.0f)
• glVertex3f(-1.0, 1.0, 0.0)
• glVertex3f(1.0, -1.0, 0.0)
• glEnd()
• glFlush()

15
A Simple Example

• void GL_reshape(GLsizei w, GLsizei h)
• glViewport(0, 0, w, h)
• glMatrixMode(GL_PROJECTION)
• glLoadIdentity()
• glOrtho(-2.0f, 2.0f, -2.0f, 2.0f, -2.0f, 2.0f)
• glMatrixMode(GL_MODELVIEW)
• glLoadIdentity()

16
A Simple Example

• void GL_reshape(GLsizei w, GLsizei h)
• glViewport(0, 0, w, h)
• glMatrixMode(GL_PROJECTION)
• glLoadIdentity()
• glOrtho(-2.0f, 2.0f, -2.0f, 2.0f, -2.0f, 2.0f)
• glMatrixMode(GL_MODELVIEW)
• glLoadIdentity()

17
A Simple Example

• void GL_reshape(GLsizei w, GLsizei h)
• glViewport(0, 0, w, h)
• glMatrixMode(GL_PROJECTION)
• glLoadIdentity()
• glOrtho(-2.0f, 2.0f, -2.0f, 2.0f, -2.0f, 2.0f)
• glMatrixMode(GL_MODELVIEW)
• glLoadIdentity()

18
Matrix in OpenGL

• There are two matrix stacks.
• ModelView matrix (GL_MODELVIEW)
• Projection matrix (GL_PROJECTION)
• When we call functions of transformation, we
  should change to the appropriate matrix stack
  first.

glMatrixMode(GL_MODELVIEW) //now we are in
modelview matrix stack! //do modelview
transformation here.. glMatrixMode(GL_PROJECTION
) //now we are in projection matrix stack! //do
projection transformation here.
19
ModelView Matrix

• Viewing Transformation
• To positioning and aiming the camera
• Modeling Transformation
• Perform rotate, translate, scale and combinations
  of these transformations to the object.

20
Viewing Transformations

• gluLookAt (eyex, eyey, eyez, atx, aty, atz, upx,
  upy, upz )

21
Modeling Transformations

• glTranslatefd(x, y, z)
• Multiplies current matrix by a matrix that moves
  an object by x,y,z

glTranslatef( 0, 0, -1 )
22
Modeling Transformations

• glRotatefd(angle, x, y, z )
• Multiplies current matrix by a matrix that
  rotates an object in a counterclockwise direction
  about the ray from origin to (x,y,z) with angle
  as the degrees

glRotatef( 45.0, 0, 0, 1)
23
Modeling Transformations

• glScalefd (x, y, z)
• Multiplies current matrix by a matrix that scales
  an object along axes.

glScalef( 2.0, -0.5, 1.0 )
24
Matrix in OpenGL

• Matrix multiplications always apply to the top of
  matrix stack.

Top matrix In the stack
X
Translation matrix
(glTranslatef)
25
Matrix in OpenGL

• The order of transformations is critical.

glTranslatef( 1,0,0 ) glRotatef(45.0, 0,0,1
) drawObject()
glRotatef(45.0, 0,0,1 ) glTranslatef( 1,0,0
) drawObject()
26
Projection Transformations

• Orthographic Projection
• glOrtho( GLdouble left, GLdouble right, GLdouble
  bottom, GLdouble top, GLdouble near, GLdouble far
  )
• gluOrtho2D( GLdouble left, GLdouble right,
  GLdouble bottom, GLdouble top)

27
Projection Transformations

• Perspective Projection
• gluPerspective( GLdouble fovy, GLdouble aspect,
  GLdouble near, GLdouble far )

28
Projection Transformations

• Perspective Projection
• glFrustum( GLdouble left, GLdouble right,
  GLdouble bottom, GLdouble top, GLdouble near,
  GLdouble far )

29
A Simple Example

• void GL_reshape(GLsizei w, GLsizei h)
• glViewport(0, 0, w, h)
• glMatrixMode(GL_PROJECTION)
• glLoadIdentity()
• glOrtho(-2.0f, 2.0f, -2.0f, 2.0f, -2.0f, 2.0f)
• glMatrixMode(GL_MODELVIEW)
• glLoadIdentity()

30
OpenGL Transformations
glTranslatef glRotatef glScalf gluLookAt
gluPerspective gluOrtho2D glFrustum glOrtho
glViewport()
Modelview Matrix
Viewport Transformation
Projection Matrix
Perspective Division
Vertex
X Y Z W
X Y
eye coordinates
clip coordinates
normalized device coordinates
window coordinates
31
Matrix in OpenGL

• Mantain matrix stack
• glPushMatrix() used to save current stack
• glPopMatrix() used to restore previous
  stack

x
glScalef
glPopMatrix()
glPushMatirx()
32
Matrix in OpenGL

• glPushMatrix()
• glTranslatef (-1.0, 0.0, 0.0)
• glRotatef ((GLfloat) shoulder, 0.0, 0.0,
  1.0)
• glTranslatef (1.0, 0.0, 0.0)
• glPushMatrix()
• glScalef (2.0, 0.4, 1.0)
• glutWireCube (1.0)
• glPopMatrix()
• glPushMatrix()
• glTranslatef (1.0, 0.0, 0.0)
• glRotatef ((GLfloat) elbow, 0.0, 0.0, 1.0)
• glTranslatef (1.0, 0.0, 0.0)
• glPushMatrix()
• glScalef (2.0, 0.4, 1.0)
• glutWireCube (1.0)
• glPopMatrix()
• glPopMatrix()
• glPopMatrix()

33
Matrix in OpenGL

• Reference Object Hierarchy
• http//www.gamedev.net/reference/articles/article1
  267.asp

34
References

• OpenGL officially website
• http/www.opengl.org
• NeHe (useful installation guides included)
• http//nehe.gamedev.net/
• Nate Robins (for demonstration)
• http//www.xmission.com/nate/tutors.html
• The Red Book (OpenGL Programming Guide)