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3D Transformations

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					3D Graphics
                  Graphics
3D Graphics is about creating 3D scenes and
  viewing them as 2D images – think about how
  you take a picture using a camera
  o 3D Modelling
  o 3D Transformation
  o Viewing
  o Projection
o Graphics Pipeline refers to the steps to render
graphical objects (the overall process of going
  from a 3D scene to a shaded projection on a 2D
  surface
            3D Graphics Pipeline
o Creating objects/scene – 3D modeling
   o Define objects in local coordinate system
   o Transform objects to world coordinate system
   o Determining which part of the objects/scene are visible
o Camera (position, direction, focal length) to define viewing
  volume
   o Transform objects to camera coordinate system
   o Project object to view plane to obtain 2D representation
   o Clip against view volume to determine visible objects
o Determining which pixels should be filled
   o Rasterization – from vectors/polygons to pixels
   o Hidden Surface Removal - determine which object in the view volume
     should appear in image at each pixel
o Determining what color/texture/lighting it is
   o Lighting models and texture mapping
Viewing and Clipping
Projection
           Polygonal Models
• The most common type of model used in 3D –
  store faces of the object as planar polygons
• Each polygonal face may be represented by its
  vertices or edges
• Physical properties of the object may also be
  held as part of the representation, for example
  colour, light, texture
• Representation methods such as this are called
  Boundary Representations or B-reps. This
  may also be used to approximate a curved
  surface, by planar polygonal patches
              3D Transformation

• The translation, scaling and rotation transformations
  used for 2D can be extended to three dimensions
• Using homogeneous coordinates it is possible to
  represent each type of transformation in a matrix
  form and integrate transformations into one matrix
• Homogeneous coordinates also allow for non-affine
  transformations, e.g., perspective projection
• To apply transformations, simply multiply matrices,
  also easier in hardware and software implementation
      Homogeneous Coordinates

• Basis of the homogeneous co-ordinate system is
  the set of n basis vectors and the origin position
• All points and vectors are therefore compactly
  represented using their coordinates:




• 3D Transformations 4 x 4 Homogeneous co-
  ordinates
              Translation
• Translation only applies to points, never
  translate vectors
• Remember: points have homogeneous
  co-ordinate w = 1
    Composing Transformations
• translation
                  
                   1     dx1
                                                         
                                                              1     dx 2
T1  T(dx1,dy1)   1   dy1                               
                                                                1
                                                                        
                                                                    dy 2
                     1               T2  T(dx 2,dy 2)  
                                                            1     
                        1                                         
                                                                   1 
P'' T2  P' T2 [T1 P]  [T2  T1] P,where
          
           1        dx1  dx 2
                            
             1      dy1  dy 2
T2  T1                                   so translations add
               1            
                            
                      1 
Translation
Scale
                           Scale
We would also like to scale points thus we need a homogeneous
transformation for consistency:
                              Scale
all vectors are scaled from the origin:
                           Rotation
all vectors are scaled from the origin:

                             cos          sin         0   0
                              sin        cos           0   0
                 Rotationz                                   
                              0             0            1   0
                                                              
                              0             0            0   1


                              cos         0     sin        0
                              0            1         0       0
                 Rotationy                                   
                              sin        0 cos            0
                                                              
                              0            0         0       1


                            1         0          0           0
                            0    cos           sin        0
                Rotationx                                    
                            0    sin          cos          0
                                                              
                            0         0          0           1
Rotation
Transformation Composition
  Transformation Composition

• Rotation in XY plane by q degrees anti-
  clockwise (round about original point)
  about point P
     Viewing 3D World on 2D plane

• To view 3D, need to project points onto 2D view plane
• Scene or view volume
• Window: projection plane
• Viewport: display plane
• Projection: intersection of projectors w/window
• Center of projection: focal point for all projectors
            3D                    2D Projection
•   We need to transform from a special viewing coordinate system (camera on
    z-axis pointing along the axis) into a projection coordinate
• Type of projection depends on a number of factors:
– location and orientation of the viewing plane (viewport)
– direction of projection (described by a vector)
– projection type:
                     Parallel Projection

•   In parallel projection, the observer position is at an infinite distance, so the
    projection lines are considered parallel
Parallel Projections
               Perspective Projection

•   In a perspective projection, object positions are projected onto the view
    plane along lines which converge at the observer, or Centre of Projection
    (COP)




•   Perspective projection gives realistic views, but does not preserve
    proportions - projections of distant objects are smaller than projections of
    objects of the same size which are closer to the view plane (foreshortening
Perspective Projections
Perspective Projection

				
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posted:2/22/2010
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