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In previous versions of POV-Ray, most of the items in the interior
statement were previously part of the finish
statement. Also the halo statement which was once part of the
texture statement has been discontinued
and has been replaced by the media statement
which is part of interior.
You are probably asking WHY? As explained earlier, the
interior contains items which describe the properties of the
interior of the object. This is in contrast to the texture which
describes the surface properties only. However this is not just a
philosophical change. There were serious inconsistencies in the old
model.
The main problem arises when a texture_map
or other patterned texture is used. These features allow you to create
textures that are a blend of two textures and which vary the entire texture
from one point to another. It does its blending by fully evaluating the
apparent color as though only one texture was applied and then fully
reevaluating it with the other texture. The two final results are
blended.
It is totally illogical to have a ray enter an object with one index or refraction and then recalculate with another index. The result is not an average of the two ior values. Similarly it makes no sense to have a ray enter at one ior and exit at a different ior without transitioning between them along the way. POV-Ray only calculates refraction as the ray enters or leaves. It cannot incrementally compute a changing ior through the interior of an object. Real world objects such as optical fibers or no-line bifocal eyeglasses can have variable iors but POV-Ray cannot simulate them.
Similarly the halo calculations were not performed as the
syntax implied. Using a halo in such multi-textured objects did
not vary the halo through the interior of the object. Rather,
it computed two separate halos through the whole object and averaged the
results. The new design for media which replaces
halo makes it possible to have media that varies throughout the
interior of the object according to a pattern but it does so independently of
the surface texture. Because there are other changes in the design of this
feature which make it significantly different, it was not only moved to the
interior but the name was changed.
During our development, someone asked if we will create patterned interiors
or a hypothetical interior_map feature. We will not. That would
defeat the whole purpose of moving these features in the first place. They
cannot be patterned and have logical or self-consistent results.
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