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16.90.5.47 - structure (SceneScript element)

 <structure

This term should be used before every group of objects that are conceptually one item; the group should then be followed by the  </structure element. The  <structure language element pairs tell the ray trace engine that each differing object (rect, triangle, sphere) between them is a member of the same physical object. This is used in the refraction of light and calculation of translucency, and so it is particularly important to use this with "solid" items (such as the gems we provide as an example.)

To explain a little more, when you make a complex object that is to have the appearance and lighting behaviour of being solid, you construct it out of the relatively simple primitive objects the tracer can render: Triangles, spheres and rectangles. In the case of our faceted gem examples, the objects are made up of a fairly large number of triangles, placed and sized precisely where the faceted surfaces of a gemstone would be.

Because refraction takes place when light transits from an objct of one IOR (index of refraction) to an object (or non-object) with a differing IOR, the tracer must know when light is entering and leaving a specific multi-primitive object in order to correctly adjust the light rays. In the case of these gems, light can enter the object through a facet and it can exit the object though a facet; in both instances, the IOR is known to change, so the light ray is bent (refracted) appropriately.

It is entirely possible for an object to have internal structures of the same (or different) IOR and/or translucency; if that is the case, the renderer knows what to do based on which structure the objects belong to.

With all that in mind, the following shows how to use the  <structure and  </structure language elements:

 <structure series of geometric items that make up one physical object  </structure  <structure series of geometric items that make up another physical object  </structure

Remember: If you don't want light to bend, make sure you set the IOR of your objects to the same value as that of the atmosphere!

See also: The  </structure language element

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