Group Nodes

• arealight
• group
• instance
• Transforms
  • scale
  • translate
  • rotate
  • lookat

group

Objects whose component vertices are initially defined in local coordinate systems are appropriately placed and oriented in the World Coordinate System by the application of geometric transformations. A modular way of constructing a complicated scene is to build up a hierarchy by creating multiple instances of primitive objects and appropriately altering the geometric and appearance attributes of each of these instances. Geometric attributes are altered by the application of geometric transformations. Cameras and lights can also be similarly instanced and transformed.

In SLIDE, hierarchy is built up in a recursive manner using groups which are equivalent to compound objects. Groups are collections of instances of primitive objects as well as other groups, lights, and cameras. Groups and objects share the same local identifier name space. The group statement is used to define the scene hierarchy.

An instance_statement can be a geometry, camera, or a light instance. The three different types of instance statements may only occur within a group statement. Each instance includes transformations that place the referenced entity with the coordinate space of the current group. The transformations within an instance statement are applied one at a time in the order specified.

See the SLIDE Flags section for a complete description of the shading_flag and the lod_flag and the Surface Specification section for a description of how to use the surface_id.

arealight

The arealight is a hack stand-in for a real area light provided by more advanced photorealistic renderers, like RenderMan. The arealight is just like a group node; it instances some set of geometry nodes. The difference is that the polygons instanced below an arealight are rendered with emissive shading (they are bright even if they are not lit by other lights) and a representative spot light is rendered at the center of mass of the polygons.

The true use of an arealight is to export the geometry description to a .RIB file and then render with a higher quality renderer. The arealight will correctly translate the geometry to .RIB format.

instance

The instance statement is used to create an instance of geometry; either a primitive object or another group specified by node_id.

The id statement gives the instance a name that can be used to define path names (explained later). Any instance that refers to a group that contains camera or light instances should have an identifier.

See the SLIDE Flags section for a complete description of the shading_flag and the lod_flag and the Surface Specification section for a description of how to use the surface_id.

Each transform_statement defines a linear transformation to be performed on the points that comprise the object or group. These transforms should be applied in the order that they are specified.

For example,

instance fish
  id        red_fish
  surface   shiny_red
  shading   SLF_PHONG
  rotate    (0 1 0) (7)
  translate (100 0 0)
  rotate    (0 0 1) (38)
endinstance

As another example,

group tetra_stack
  surface  green
  instance tetra
    scale     (2 2 2)
    rotate    (0 0 1) (30)
    surface   red
  endinstance
  instance tetra
    scale     (2 2 2)
    translate (0 0 2)
    rotate    (0 0 1) (-30)
  endinstance
endgroup

The default values for an instance statement are:

The TCLinit block generation syntax looks like this:
(note: groupid refers to the group that the instance will be inserted into, and the node_id is the object or group that is being instanced)

slide instance groupid node_id [-surface surfid] [-shading shading_flag] [-lod lod_flag] [-xforms {xform_list}

{
 {translate x y z} |
 {scale x y z} |
 {rotate x y z angle} |
 {lookat -eye {x y z} -target {x y z} -up {x y z} }. . .
}

ie: slide instance gThings oThing1 -surface surfland
      -xforms {{scale 0.5 0.5 1} {rotate 0 0 1 45 } {translate 0.5 0.3 0.2} }

Transformation Statements

• translate
• scale
• rotate
• lookat

Note that transformations for TCL generated objects is dealt with inside "slide instance" commands.

scale

For example

scale (1 2 1)

translate

The translate_triple defines the three components of a translation vector, t = (t_x, t_y, t_z). The operation performed on a point, p = (p_x, p_y, p_z), is p' = (p_x+t_x, p_y+t_y, p_z+t_z),

For example

translate (10 0 0)

rotate

The axis_triple defines the axis of rotation through the origin, v = (v_x, v_y, v_z). The angle of rotation is given in degrees by the angle_float, a.

For example

rotate (0 0 1) (30)

lookat

The lookat transformation defines a rigid body transformation that is specifically designed for orienting a camera to view a scene, although it can be used wherever a transform_statement can appear.

The eye_triple and the target_triple define eye and target as points in 3D. The up_triple defines up as a vector in 3D.

The effect of the lookat transform is to:

1. translate the Object Coordinate System so that the origin of the object lies at the point, eye,
2. and rotate the object so that the point, target, lies on the -z-axis of the object's system and the up vector lies in the yz-plane of the object's system.

lookat
  eye (10 0 0)
  target (0 0 0)
  up  (0 0 1)
endlookat

The default values for a lookat statement define the identity transformation. They are