Join Brian Bradley for an in-depth discussion in this video A little bit about how light works, part of Mental Ray: Control Color Bleed in 3ds Max.
- [Teacher] The amazing ability that we have to not only see the world around us, but see it in glorious color, can be said, in its most basic form, to be handled by two separate but interdependent mechanisms or systems. The first is the physical interaction of light with the matter that makes up the world around us. This physical interaction is both observable and measurable, and taking the time to gain even just a basic understanding of the subject can go a long way towards helping us craft believable, high-quality renders in the Mental Ray engine.
The second mechanism could in essence be described as you and I ourselves. In a truly amazing way, our eyes, when combined with the complex workings of our brains, give us the ability to see or perceive the complex interaction of light and matter that is continuously occurring all around us. Now of course when dealing with lighting and rendering inside a 3D application, we tend to be working with a very simplified version of the laws of physics. As far back as the late 1800s and early 1900s, physicists had come to realize that light itself was just one tiny part of a much greater wave spectrum that came to be classified as electromagnetic radiation.
During that time period, physicist Lord William Kelvin produced what has come to be known as the Kelvin Colour Temperature Scale. This was a measurement system based on his observations made whilst conducting an experiment that showed how a heated black body emitter, or a solid block of carbon in his case, produced a range of colors that followed a definite and measurable progression. The scale he created assigns a numeric value in degrees Kelvin to various stages of that progression, which obviously starts at black, moves through red, to orange, then to white, and finally into the blue part of the light spectrum.
In other words, the carbon block emitted varying wavelengths of light based on the amount of heat that it was generating. And indeed, exactly the same thing happens with stars or suns; they emit varying wavelengths of light according to the amount of heat being generated at their core, hence the designations red giant, blue dwarf, et cetera. Our own sun, very helpfully, generates light that essentially comes from the white part of the emission spectrum, even though we oftentimes perceive it as being yellow due to its complex interactions with Earth's atmosphere.
To help open up our understanding of just how it is that we actually see colored objects in the world around us, we will step through the basics of just what it is that happens when light, that is white light, falls on or strikes the surface of a real-world object. Although again, what we will present here is a very simplified definition of a very complex set of interactions. For the purpose of a simplified discussion, then, we're going to say that one of four basic interactions between light and matter occur at the point of contact.
So light, or parts of it at least, can either be absorbed, reflected, transmitted, or refracted by the light that falls on it. The truth of course being that oftentimes, it is a complex mix of interactions that actually takes place, with the level of complexity depending of course on the physical properties of the object upon which the light is falling. Absorption is a description of what happens when a material holds onto, or absorbs, certain component wavelengths of the light that is striking it.
In the case of a black or nearly-black object, for instance, pretty much all of the light wavelengths hitting it are being absorbed, so what we're left with, quite literally, is an absence of diffuse light reflection. You will probably have noticed that oftentimes, black objects only have surface shape or a defined outline because the materials from which they are made also contain what we call specular reflective, or shiny, properties as well. Reflection, of course, being pretty much the opposite reaction to absorption, in that here, light wavelengths are reflected off a surface, producing both a visible surface color as well, oftentimes, as specular reflections.
The reflectance value for a given surface is a measurement used to describe the energy or strength with which light is being reflected, a reflectance value typically equating to what we would perhaps call the brightness of an object's color. The third and fourth interactions mentioned, transmittance and refraction, measure the ability that some materials, such as glass and plastics, have to let light pass through rather than bounce from their surface. Wherever a high level of transmittance occurs, we typically find that there is an absence of surface color.
This is oftentimes why materials like completely clear glass can only be seen because of their specular reflection and refractive properties. Refraction, of course, being the bending of light rays as they pass through the volume of an object, the effect that causes a pencil sitting in a glass of clear water to look as if it has been broken somehow. Now of course we may be starting to wonder at this point what all of this has to do with controlling color bleed in our Mental Ray renders. Well let's move on to our next video, where we can hopefully start to make some of the required connections.
- Controlling reflectance
- Understanding how geometry setup affects color bleed
- Choosing color placement carefully
- Using the FG Diffuse Bounce control and Photon Energy setting
- Controlling color bleed via object property controls
- Using the rayswitch map