OpenGL: The Open Graphics Language

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OpenGL The Open Graphics Language

• Technology and Historical Overview

By Ricardo Veguilla
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OpenGL The Open Graphics Language

• De facto Application Programming Interface (API)
  for cross-platform development of 3D graphics
  applications.
• Implementations available for all major Operating
  Systems and hardware platforms.
• Support for hardware accelerated 3D rendering.
• Scalable, high-level, easy to use, well
  documented.

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History of OpenGL

• Originally released by SGI in the early 90s.
• Descendant of IRIX GL.
• Previous 3D graphics APIs were generally platform
  dependant.
• Born out of market pressure for a cross-platform
  3D API during the late 80s.

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OpenGL - Primitive types
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Transformation Matrices

• OpenGL provide 3 transformation matrix stacks
• Perspective Matrix Used for viewing
  transformations equivalent to positioning and
  aiming a camera.
• Modeling Matrix Used for modeling
  transformations equivalent to positioning and
  orienting the model to be drawn.
• Texture Matrix Used for texture transformations
  equivalent to positioning and orienting the
  texture to be drawn over a polygon.

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Transformation Matrix Selection

• Using glMatrixMode(GLenum mode) function with one
  of the following argument
• GL_MODELVIEW
• GL_PROJECTION
• GL_TEXTURE.
• All subsequent transformation commands affect the
  specified matrix.

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Transformation functions

• glLoadIdentity()
• glTranslate(TYPE x, TYPE y, TYPE z)
• glRotate(TYPE angle, TYPE x, TYPE y, TYPE z)
• glScale(TYPE x, TYPE y, TYPE z)
• glPushMatrix()
• glPopMatrix()

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glLoadIdentity

• glLoadIdentity()
• Loads the identity matrix into the current
  transformation matrix.
• Used to reset the current transformation matrix
  before performing a transformation.

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Translatoin

• glTranslate(TYPE x, TYPE y, TYPE z)
• Multiplies the current transformation matrix by a
  matrix that moves an object (the local coordinate
  system) by the given x, y, and z values.

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Rotation

• glRotate(TYPE angle, TYPE x, TYPE y, TYPE z)
• Multiplies the current transformation matrix by a
  matrix that rotates an object (or the local
  coordinate system) in a counter clockwise
  direction about the ray from the origin through
  the point (x, y, z). The angle parameter
  specifies the angle of rotation in degrees.

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Scaling

• glScale(TYPE x, TYPE y, TYPE z)
• Multiplies the current transformation matrix by a
  matrix that stretches, shrinks, or reflects and
  object (or the local coordinate system) along the
  axes. Each x, y, and z coordinate of every point
  in the object is multiplied by the corresponding
  argument x, y, or z.

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Controlling the tranformation matrix stacks

• glPushMatrix()
• Pushed the current transformation matrix into the
  stack.
• glPopMatrix()
• Loads the matrix at the top of the stack into the
  current transformation matrix.

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OpenGL - Code Example

• // Set the viewport size
• glViewport(0, 0, width, height)
• // Clear the window
• glClear(GL_COLOR_BUFFER_BIT)
• // Set the drawing color
• glColor3f(1.0, 1.0, 1.0)
• // Start primitive type definition
• glBegin(GL_POLYGON)
• // Specify verticies
• glVertex2f(-0.5, -0.5)
• glVertex2f(-0.5, 0.5)
• glVertex2f(0.5, 0.5)
• glVertex2f(0.5, -0.5)
• // End primitive type definition
• glEnd()
• // Flush the buffer to force drawing of all
  objects thus far
• glFlush()

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Development Considerations

• OpenGL API designed only for drawing images.
• Auxiliary visual toolkits are required for
  developing OpenGL applications for modern
  windowed desktop environments.
• Potential cross-platform options
• GLUT, SDL, GTK

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Auxiliary Toolkits

• GLUT Specifically designed for developing OpenGL
  demo applications.
• SDL (Simple DirectMedia Layer) Library for
  multimedia and game development.
• GTK General purpose toolkit for creating
  graphical user interfaces with OpenGL extensions
  available.

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References

• OpenGL - The Industry Standard for High
  Performance Graphics
• http//www.opengl.org/
• Wikipidia OpenGL
• http//en.wikipedia.org/wiki/OpenGL