This tutorial will discuss Specular Reflections (sometimes referred to as Highlights as well) in the real world, and how they're simulated in your CG scenes. It will also touch on the topics of fresnel and IOR, rough vs smooth reflections, and metallic and non metallic surfaces.

While I'll be using 3dsmax to illustrate the examples, what I'm talking about is meant to be non software specific, and should work the same in any commercially available 3d app and renderer.

The most common material you'll make a CG object is a combination of Diffuse Reflections and Specular Reflections. So what are Specular Reflections? When light hits a surface, part of the surface will scatter light back in a much more focused manner (compared to the diffuse). Some great examples are your reflection in a mirror, or the glint of the sun on a metallic surface, or the shine on a plastic surface.

The shader ball below shows both Diffuse and Specular Reflections, the diffuse provides the red color, and the specular reflection provides the bright highlight.

The Truth About Highlights

We get closer, and we see the Highlight isn't an oval shape, but a Reflection...

...of our light source overhead.

These days, most renderers do Reflections quick and easy, and we've replaced point lights with area lights, so you don't need to use the highlight cheat anymore. No more two separate controls, just change the Specular Reflectivity of your surface and the software controls everything for you.

Different materials in your cg software provide either Additive reflections or proper Energy Conservation based reflections. What's the difference? In additive, the specular reflection is added to your diffuse color. With Energy conservation, it places the specular reflection over the diffuse reflection. That way, if you have a really bright specular reflection, your diffuse reflection will get darker automatically. For more info on Energy Conservation, please read Energy Conservation In Shaders.

So now that most renderers give us true specular reflections, how do we control the strength of these reflections? This brings us to the concept of Fresnel and IOR. Materials are assigned an IOR based on how they reflect light (IOR stands for Index Of Refraction, but even non refractive objects such as metals have an IOR, usually referred to as a "complex IOR". A complex IOR measures a slightly different property then a regular IOR, but it deals with the same stuff, light reflecting or refracting off / through a surface). As a surface starts facing away from the viewer, it reflects more then a surface that is directly facing a viewer.

Here's a reflective glass surface. Notice how it's less reflective when I look directly at it from the front vs looking at it from a side angle. The greater the angle the more it reflects.

On round surface like a sphere (as opposed to a flat surface like the glass), this means the edges of the sphere will reflect more Specular light then the front of the sphere. This phenomenon is called Fresnel.

Here's another example, as we see more of the leaf's side angle, it reflects more of the sun...

Different renderers have different ways to do Fresnel, most have a checkbox or do fresnel automatically, and then provide an IOR value to change.

Different types of real world materials have different IORs, some good example are air is 1.0, glass has an IOR of about 1.5, plastic between 1.1 and 2.0, gloss materials such as porcelain or car paint 3-5, super shiny metals like chrome a value of 10-20. The IOR controls the proportion between how much the faces pointing away from your reflect vs the faces pointing towards you. For example, a lower IOR means that the away faces will reflect a lot, and the faces facing your reflect very little, and a higher IOR will make just about all faces of your object reflect the scene the same amount.

Metals vs Non Metals

• Non metals (Dielectrics)
• non metal materials that don't conduct electricity, like plastic
• tend to have a lower IOR (hence, more diffuse reflection and less specular reflection)
  
• the color of the reflection is the same as the thing its reflecting
• Metals (Conductives)
• materials that do conduct electricity, like metals
• tend to have a higher IOR (hence, more specular and less diffuse)
  
• can have reflections that are tinted by the color of the metal.

• IOR of 10-20
• a black diffuse color
• high specular reflection that's very glossy (not blurry)
  

Colored Metals like Gold

Another thing to consider is colored metals (like gold). As mentioned above, metals can tint the color of their Specular Reflections. So since gold is more yellowish in color, its Specular Reflections take on a yellowish tint.

Here's an example, a set of keys and a blue pen. Notice how the grey key reflects the true color of the pen top, and the golden key tints the highlight and pen color the key color. Also note the reflection of the keys in the pen top, which is plastic. Although on first glance it may seem like the reflections are tinted blue, it's actually just the blue diffuse showing through the weaker reflection (remember, as stated above, non metals don't tint their reflection). The pen top probably has an IOR of somewhere between 2 and 3, the keys are closer to 10, but the reflections are broken up a bit since the keys are all scratched up.

Here's another example, 4 colored pencils reflected in colored metal. Notice how all the colors are tinted gold, with the white pencil appears the most tinted.

Note that since the ball is in a completely white environment, and its IOR is so high (ie, it reflects pretty uniformly over the entire surface of the sphere) the sphere is almost entirely gold. If you replaced the white environment with say a room, this would look more natural since it would be properly reflecting everything in the environment.

• The blurrier the specular reflection
• The dimmer the specular reflection
• The fresnel edge effect is still present

• Attenuation: At the 90 degree edge of the object, tiny pieces of the rough surface are no longer pointed at 90 degrees to the camera, and so you get dimming
• Energy Conservation: Because the reflection is blurred (stretched out), the reflection at a given point is darker to retain energy conservation. The rough ball is spreading that energy over a larger surface, so you have less energy in a specific spot, like spreading the same amount of butter over a larger piece of bread, the butter will be thinner.
• Self Shadowing: The tiny pieces of the rough surface shadow each other, causing some apparent dimming.
• Multi-Scattering: Light rays enter the surface of your object and then bounce back out at different angles, which actually causes your reflection to lighten again slightly https://www.youtube.com/watch?v=JtBTffVVa-c&feature=youtu.be
  

Reference

Hopefully you find something in here that can help you better understand how Specular Reflections in the real world work, and how to simulate them in 3d. For my parting words, I can't recommend enough good reference for whatever you're doing, in preparing for this tutorial, I really had no idea for example how the reflections on a colored surface should look. Finding the golden object, taking some good reference photos, this told me how things should look, and after playing in 3d, I was able to figure out how to simulate that look in 3d. Just playing in 3d without reference till I got something that I thought looked right would have taken me a lot longer, and probably would have returned incorrect results, trust your eyes, research before you pick up that mouse and start texturing. It'll save you a lot of work.

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