Translucency vs Refraction

Comments

• CypherFOX Posts: 3,401

  March 2016 edited March 2016

  Greetings,

  I vaguely got the idea someplace that translucency maps were useful on their own for subtle internal veins, both mammalian and in plants...

  Just hoping that triggers someone else's memory.

  --  Morgan

   

  Post edited by CypherFOX on March 2016
• Peter Wade Posts: 1,666

  March 2016

  Well in the real world translucency is light that passes through an object but is scattered or diffused so it doesn't have a specific direction. Refraction is where the direction of travel of the light is bent but it still keeps travelling in one direction. So enough tranlucency would make refraction invisible since the light would be going in all directions. The one situation I can think of where refraction might still be visible is total internal reflection, where the angle of the light is such that refraction stops it from leaving a substrance with a higher refractive index.

  Of course than doesn't mean that these things are done in the same way by whatever render engine you are using.
• Rashad Carter Posts: 1,830

  March 2016

  Peter Wade said:

  Well in the real world translucency is light that passes through an object but is scattered or diffused so it doesn't have a specific direction. Refraction is where the direction of travel of the light is bent but it still keeps travelling in one direction. So enough tranlucency would make refraction invisible since the light would be going in all directions. The one situation I can think of where refraction might still be visible is total internal reflection, where the angle of the light is such that refraction stops it from leaving a substrance with a higher refractive index.

  Of course than doesn't mean that these things are done in the same way by whatever render engine you are using.

  I think this is generally correct. Basic transparency allows light in as we well know, what happens from there is where the distinction lies. The rays of light that enter an object may be redirected as they pass through the object, and will refract in a single direction such that a coherent beam of light will remain coherent traveling in a single direction at all times within the medium determined by the refractive index of the material. The key with basic refraction alone is that in cases where refraction is the primary visible effect most often the entire object behaves as a single refracting surface; thus the beam only changes direction once during its transit and can remain a single coherent beam (even phase can be retained) upon exit from the medium. Translucency however, is usually the result of a medium interaction of some sort, that has multiple mini-refractions caused by multiple influences (such as floating particles), facing in multiple directions, thus a single beam will exit the object as fully dispersed and "diffuse." This is generally considered "scattering." So seawater, which has lots of stuff floating in it, might sometimes behave more like translucency than transparency, depending on the conditions of the shot. For example; If shot from within the water medium itself like a diver, seawater can seem quite translucent when visiblity is low due to sand upkick or coral spawning. Shot from above the water however, there is a phase change between the atmosphere and the water surface, thus affecting the bending of the rays differently than if one were under the water surface taking the same shot.

  There could also be the issue of electromagnetic conductors vs insulators. Electrons and photons tend to behave similarly. If electrons cannot transit through a given medium efficiently, then photons probably won't either, and vice versa. Photons and electrons both transit well in purified water. Transparent metals, aka transparent Conductors, due to their efficient transport of electrons, I'm assuming will likely refract incoming rays of light along a single angle, creating caustic artifacts. Pure, clean water (having lots of metalic hydrogen atoms is this way which is probably why we get nice caustic effects from water surfaces. Insulators by contrast, are likely to be translucent only because the medium is not pure but indeed occupied by all different types of non-metal chemical components.  Thus things like human flesh are generally considered insulators, and thus require translucency effects for accurate rendering. You would not expect to see caustic effects from human tissues except perhaps the cornea or other purposefully transparent tissues.

  There are obviously exceptions to the ideal I painted above, such as diamonds and most other gaseous non-metalic solids. While photons transit diamonds and other gem stones well, the nature of the covalent carbon bonds within diamonds means there are no free electrons upon which to conduct an electrical current upon, so diamonds are not conductors but are considered insulators even though they are perfectly transparent. This is true for most gem stones created by thermal heat and pressure of gasses.

  So in conclusion, the more purified a medium may be the more likely you only need transparency and refraction alone. But if there's more complexity to the medium, then you probably need translucency and scattering and all of that.
• Oso3D Posts: 15,085

  March 2016

  This was in reference to marble statues, so looks like I definitely need to go back to translucence, possibly with refracting top/thin coats.