Rib
Camera Transformations

# disk3.rib
# applying multiple transformations
  
Display "disk3" "framebuffer" "rgb"
Projection "perspective" "fov" 40
Format 320 240 1
  
Translate 0 0 3
Rotate -40  1 0 0 
Rotate -20  0 1 0
WorldBegin
    Color 1 1 0.7
    Polygon "P" [-0.5 0 -0.5  -0.5 0 0.5  0.5 0 0.5  0.5 0 -0.5]
            "st" [0 0  0 1  1 1  1 0]
    Color 1 0 0
    Disk 0 0.25 360
WorldEnd

The purpose of this rib file is to show the effect of applying additional transformations before WorldBegin.

    Translate 0 0 3
    Rotate -40  1 0 0 
    Rotate -20  0 1 0
    WorldBegin

These transformations determine how the 3D scene will be viewed by the camera. For that reason they are know as the view or viewing transformations. Rib files written by Maya (requires Pixar's plugin) use a single transformation command,

    ConcatTransform [ 0.707 -0.331 -0.625  0.0
                      0.0    0.884 -0.469  0.0 
                     -0.707 -0.331 -0.625  0.0 
                      0.0    0.0   44.822  1.0 ]

This statement specifies a transformation matrix of 16 values. Fortunately, the RenderMan standard provides a more human-friendly way of setting the viewing transformation. The Rotate statement has four parameters,

• an angle measured in degrees, followed by
• the xyz coordinates of the axis of rotation.

A good way of understanding the last three values of the command is to consider the one and zero's to be switches ie.

    Rotate -40    1   0   0 
           angle  on  off off

Therefore, this statement specifies a rotation of -40 degrees around the x-axis. The direction of the rotation, clockwise or anti-clockwise, is explained in the tutorial "Rib: Left-hand & Right-hand Coordinate Systems". For the moment, the key points to understand about transformations are that they are,

• applied in reverse order,
• applied to a copy of the current coordinate system,
• applied relative to the the current coordinate system.

# disk3.rib
# applying multiple transformations
  
Display "disk2" "framebuffer" "rgb"
Projection "perspective" "fov" 40
Format 320 240 1

Translate 0 0 3
Rotate -40  1 0 0 
Rotate -20  0 1 0

Two comments about the scene.


Set the camera to give a perspective view with a field of vision of 40 degrees and a frame size of 320 by 240 pixels.


The camera coordinate system is the current (active) system.


The transformations are applied in reverse order.

First, the negative rotation of 20 degrees around the y-axis is applied to a copy of the current coordinate system.

Translate 0 0 3
Rotate -40  1 0 0 
Rotate -20  0 1 0

Next, the negative rotation of 40 degrees around the x-axis is applied to the copy.

Translate 0 0 3
Rotate -40 1 0 0
Rotate -20  0 1 0

Move the transformed coordinate system 3 units along the z-axis of the camera.

WorldBegin

The copy of the coordinate system now becomes the primary, "world", or current coordinate system.

    Color 1 1 0.7
    Polygon "P" [-0.5 0 -0.5  -0.5 0 0.5  
                  0.5 0 0.5  0.5 0 -0.5]
            "st" [0 0  0 1  1 1  1 0]
    Color 1 0 0
    Disk 0 0.25 360

Make yellow the current color.
Insert a 1 x 1 polygon.

Assign the texture coordinates.
Make red the current color.
Insert the disk.

WorldEnd

Conclude the description of the 3D scene.