The keywords halo and atmosphere have been totally eliminated with no backwards compatability of any kind provided. They have been replaced by a new feature called media. At the scene level, media acts as atmospheric media for fog, haze, dust, etc. On objects, media is not part of texture like halo was. Object media is now part of a new feature called interior. Media is not just a rename for halo. It is a new model with some similar features of halo. BECAUSE POV-Ray 3.1 DISCONTINUES SOME 3.0 FEATURES YOU MAY WISH TO KEEP 3.0 TO RENDER OLDER SCENES.
Any pattern type (bozo, wood, dents, etc.) may be used as a density function for media.
New patterns spherical, cylindrical, planar, and boxed added for pigment, normal, texture, and density.
New wave types cubic_wave and poly_wave Float have been added.
New object modifier interior{...}. Interior contains information about the interior of the object which was formerly contained in the finish and halo parts of a texture. Interior items are no longer part of the texture. Instead, they attach directly to the objects. The finish items moved are ior, caustic, fade_power, and fade_depth. The refraction keyword is no longer necessary. Any ior other than 1.0 turns on refraction. These 5 finish keywords which are now part of interior will still work in finish but will generate warnings. Some obscure texture_map statements with varying ior will not work.
Added reflection_exponent Float to finish to give more realistic reflection of very bright objects.