Join Brian Bradley for an in-depth discussion in this video A little bit about how light works, part of V-Ray: Control Color Bleed in SketchUp.
- [Instructor] The amazing ability that we have to not only see the world around us, but see it in glorious color can be said at it's most basic level to be handled by two separate by interdependent mechanisms or systems. The first of these being the interaction of light with the matter that makes up the world around us. This physical interaction is, of course, both observable and measurable, and taking the time to gain even just a basic understanding of this object can go a long way towards helping us craft believable, high quality renders in V-Ray.
The second mechanism could, in essence, be said to be you and I ourselves. In a truly amazing way our eyes when combined with the complex working of our brain are designed in such a way as to give us the ability to see or perceive the 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 just a very simplified version of the laws of physics that are at work. As far back as the late 18 and early 1900s, physicists had come to realize that light itself was just one tiny part of a much greater wave spectrum, one that came to be classified as electromagnetic radiation.
During that same time period, Scottish physicist Lord William Kelvin produced what has come to be known as the Kelvin Color Temperature Scale. This measurement system was based on observations he made whilst conducting an experiment that showed how a heated, black body emitter, a solid block of carbon in his case, produced a range of colors that followed a definite and measurable progression. The Kelvin Scale assigns a numeric value in degrees Kelvin to various stages of that progression which obviously starts at black, moves through red, orange, and yellow, onto white, and then finally into the blue part of the visible light spectrum.
In other words, he observed that the carbon block emitted various wavelengths of light based on the amount of heat that it was generating. Indeed, we see exactly the same thing happening within stars or suns. They emit varying wavelengths of light according to the amount of heat being generated at their core, hence the designations that they have such as Red Giant, Blue Dwarf, and so on. Our own sun very helpfully generates light that essentially comes from the white part of the emission spectrum, even though we oftentimes see a variety of yellow and red hues due to the complex interaction of that light with the earth's atmosphere.
To help open up our understanding of how it is that we actually see colored objects in the world around us, we're going to step through the basics of just what 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 typically occur at the point of contact, so light can either be absorbed, reflected, transmitted, or refracted by the surface that it falls on, the truth being that it is oftentimes a complex mix of these and other interactions that actually occur.
The level of complexity involved depending, of course, on the physical makeup of the object upon which the white light is falling. Absorption is a description of what happens when a material holds on to, or absorbs, certain component wavelengths of the light 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, and so what we are left with, quite literally, is an absence of diffused 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, or shiny, reflection 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 those specular properties that we just mentioned. The reflectance value of a given surface is a measurement that is often used to describe the energy, or strength, with which light is being reflected from the object, reflectance typically equating to what we perhaps call the brightness of an object's color.
The third and fourth interactions, transmittance and refraction, measure the ability of some materials, such as glass and plastics, to actually let light pass through rather than just bounce from their surface. Wherever a high level of transmittance occurs, we typically find that there is an absence of surface color, which is why oftentimes completely clear glass can only be seen because of the specular reflection and refractive properties that it also has. 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 water to look as if it has been broken at the water line.
Now, of course, you may be starting to wonder at this point what all of this has to do with controlling color bleed in our V-Ray renders. Well, let's move on to our next video where we can hopefully start to make some of the required connections that will help explain this.
- How light works
- The basics of color bleed
- Controlling reflectance using color values
- Setting up geometry with color bleed in mind
- Adjusting white balance
- Placing problem colors carefully
- Adjusting saturation
- Working with the GI Multiplier control
- Saving GI map files to disk
- Mapping colors
- Using VFB channels