Principles of Interactive Graphics

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• Principles of Interactive Graphics
• CMSCD2012
• Dr David England, Room 718,
• ex 2271 d.england_at_livjm.ac.uk
• http//java.cms.livjm.ac.uk/homepage/staff/cmsdeng
  l/Teaching/cmscd2012/
• Web page includes announcements, handouts, web
  links, reading hints, frequently asked questions

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Misc Info

• Misc. functions
• glPostRedisplay() - forces redrawing of the
  screen without waiting for a window event
• glPrint(x,y,string) wraps up the output()
  function and uses the font variable as in
  L\cd2012\bitfont.cpp
• Draws a string starting at x,y

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Todays Lecture 3D concepts

• Coursework 1 Any questions, ask me in the Lab
• Today 3D graphics concepts
• Cues to the dept of a scene
• Perspective
• Hidden line and surface removal
• 3D shapes and surfaces
• Viewpoints
• Lighting
• Shading
• Texture and Materials
• An example program, cube.cpp will demonstrate
  some of these (see code in L\CD2012)

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Perspective

• The idea of perspective - that distant objects
  look proportionally smaller - was discovered by
  artists in the 16th Century
• Perspective is used in 3D computer graphics to
  give an impression of depth and of distance from
  the viewer

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Perspective in OpenGL

• In OpenGL we can specify
• Perspective viewing or
• Orthographic viewing
• The later is useful for architectural models with
  no reduction in size with depth
• glMatrixMode(GL_PROJECTION)
• gluPerspective(
• / field of view in degree / 40.0,
• / aspect ratio / 1.0,
• / Z near / 1.0,
• / Z far / 10.0)
• glMatrixMode(GL_MODELVIEW)

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Hidden Line/Surface Removal

• In 2D graphics we can pretend another object is
  in front of another simply by drawing them in the
  right order
• In 3D graphics we need to determine which pixels
  of a line or surface might be behind another -
  the pixels are sorted along the Z axis (Z
  buffering) .
• glEnable(GL_DEPTH_TEST) will hide hidden pixels

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Lighting

• The human vision system also uses the way light
  reflects off objects to work out their position
  and orientation
• In 3D computer graphics we can set up different
  kinds of lighting and model the way the light is
  reflected off the surfaces of objects
• In OpenGL and other 3D libraries we can create
  different kinds of lights and set their
  properties
• Types Ambient, Diffuse, Spotlight
• Properties Position, orientation, intensity,
  colour
• Properties can be changed in real-time to give
  animation and other special effects
• How is this lecture room illuminated?

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Shading

• Objects can be shaded by calculating the surface
  normal and then calculating the amount of light
  that is reflected.
• Pixel colours are recalculated.
• We can also specify flat shading or smooth
  shading

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Shading .

• In the example program you can rotate the object
  to see the shading change
• The program uses the keys() function to rotate
  the object around the x, y and Z axes
• Looking at the code can you tell how each
  keyboard input rotates the object?

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Viewpoints

• Once we have lit an object and specified the
  perspective we can then position the viewpoint of
  the camera or human viewer
• Viewpoints are specified by
• The x,y,z position of the eye
• Where the centre of the world is
• The orientation of the eye
• For example
• gluLookAt(0.0, 0.0, 5.0, / eye is at (0,0,5) /
  
• 0.0, 0.0, 0.0, / center is at (0,0,0) /
• 0.0, 1.0, 0.) / up is in positive Y
  direction /

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Viewpoints .

• We can specify multiple viewpoints and swap
  between them to get different views of the scene
• Or we could animate the viewpoint to give the
  impression that the user is moving
• We could
• Animate along X and Z to give a walking motion
• Animate along X, Y and Z to give a flying motion
• Animate around an axis to rotate the world
• Animate around an object to rotate the object

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3D shapes and surfaces

• We have seen how 2D shapes can be drawn by
  specifying the vertices or corners of the shape
• We then say how the corners are joined and the
  shape filled in with the argument to glBegin()
• For 3D shapes we specify each vertex as a point
  in 3D space using the x, y and z values
• Additionally GLUT has functions for drawing
• Cube, Torus, Sphere, Cone, Teapot and other 3D
  objects

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Cube.cpp example

• The example program listed in cube.cpp
  (L\cd2012\cube.cpp) draws a red 3D cube rotated
  slightly. It is lit by a diffuse light located in
  the top right of the scene
• The cube is drawn by calls to glVertex3fv() which
  takes an array of points to define each face of
  the cube
• The perspective is then set up
• The eye is then positioned
• The objected is then translated and rotated to
  give a better view
• Finally keys() has been added to show the effect
  of rotation under a light.

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Next Week

• Reading week
• No lecture or lab next week
• Continue with the coursework
• Future weeks
• More on interaction
• Look more at the concepts of 3D graphics