Reflections and Highlights
By Neil Blevins
Created On: Feb 4th 2001
Updated On: Aug 15th 2016

This tutorial will discuss Reflections and Highlights 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 as my example software, the theory applies to any commercially available 3d app and renderer.

If you are a 3dsmax user, a few notes on renderers and materials: 3dsmax has a number of available renderers, including the Scanline (the original renderer that came bundled with max), mentalray, and then there's a number of 3rd party renderers such as Vray. In the Scanline renderer, your main two materials are the "Standard Material", which is an older material that doesn't work particularly well with raytracing, and also the "Raytrace Material" that gives you simple raytrace functionality inside the scanline. Vray's "VrayMtl" and mentalray's "Arch And Design" materials are more modern materials that have far more raytracing controls. While you should use any renderer you want, I'd recommend using Vray or mentalray as they have access to more modern features such as Global Illumination and Glossy Raytrace effects.

The 3 Kinds Of Reflected Light

So there are three basic types of reflected light in CG, Ambient, Diffuse and Specular. These names are short for Ambient Reflection, Diffuse Reflection and Specular Reflection.

Ambient Light, and Why To Turn It Off

What is ambient? Well, before the popular use of Global Illumination, ambient was a cheat to give a little boost to the brightness of your objects in the shadow areas, basically it was trying to mimic fill light. But sadly, it applies itself uniformly over the entire surface, so if you have an ambient color to your material, or have an ambient light in your scene, you will never be able to achieve pure black in your render. While this may be fine for some sorts of stylization, in general, this makes your scenes look very unrealistic and flat, so my recommendation is always turn ambient colors to black and never add an ambient light to your scene.


Ambient And Diffuse Reflection

Diffuse Reflections

What is a Diffuse Reflection? Well, when light hits a surface, part of the surface will scatter light back in a uniform manner, and that's a Diffuse Reflection. For most non-metal materials, it is the primary color of your object. A popular shader for creating a diffuse reflection is called a Lambert Shader.


Just Diffuse Reflection, no Ambient

Specular Reflections

What are Specular Reflections? When light hits a surface, part of the surface will scatter light back in a much more focused manner. 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. A popular shader for creating a specular reflection is Blinn or Phong.


Diffuse and Specular Reflections, no Ambient

In old school computer graphics, a Specular Reflections is generally broken up into two parts, Highlights and Reflections. This terminology is annoying, because when someone says "Reflection", they generally mean "Specular Reflections", even though there's Diffuse and Ambient Reflections as well. In the early days of CG, because calculating Raytraced Reflections were expensive, you tended to not have Reflections on a surface, and instead you only had a Highlight, which was a cheat. These days, most renderers do Reflections, and you don't need to use the highlight cheat anymore.

The Truth About Highlights

So in the early days of CG, your Specular Reflections was Highlights only. You generally controlled the Highlight on your material using Specular controls, such as shininess strength and glossiness. And then as computers got faster, a second set of controls were added to give your material Raytraced Reflections. So now we had two sets of controls, one for your Highlight, and one for your Reflection. But in the real world, there is no difference between a Reflection and a Highlight. What we call a Highlight is just a Reflection of a light source on your surface. And in those early days, all light sources were what is known as a point source: the light is sent from an infinitely small point to the objects it illuminates. Real lights don't work this way, light is emitted from an area, such as your light bulb, and as such, real Highlights come in all sorts of shapes and sizes, most of which are not round (the Highlight produced by a point light source is generally round).



A real world Highlight, not quite round (more oval), but not too far off from the standard Blinn Highlight.

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

...of our light source overhead.

So while cg used to be all about Highlights and point light sources (because calculating the real thing would be too slow), for the most part these days, it's now about true Reflections and area lights. So you turn on the Specularity of your material, and you will get both a Highlight (Reflection of point lights) AND a Reflection (Reflection of all the objects in your scene, including area lights). No more two separate controls, one control that controls both equally.

Additive vs Default Reflections

Different materials in your cg software provide either Additive reflections, proper Energy Conservation based reflections, or both. For example, the Raytrace Material in 3dsmax has two reflection modes in extended parameters. These two modes are Additive and Default. The 3dsmax Standard Material uses Additive and cannot be changed, the Raytrace Material gives you the option, and defaults to Default. What's the difference? The difference is that Default follows the laws of Energy Conservation, and Additive does not.

For more info on Energy Conservation, please read Energy Conservation In Shaders.

Look at this image...

The material to the left is a 3dsmax Scanline Renderer Raytrace Material using Additive mode, the material on the right uses Default mode. So based on what we learned about Energy Conservation, the left sphere that mixes a strong Specular Reflection with a high amount of Diffuse illumination (the blue of the ball) is not a natural phenomenon. This is a good reason (IMO) to not use the Standard Material, and instead use a more advanced material like the Raytrace Material in 3dsmax's Scanline Renderer, the Arch & Design Material in mentalray, or the VrayMtl in vray, as they follow Energy Conservation. Or if you do want to use the Standard Material, remember to reduce the brightness of your diffuse if you're going to increase the specular reflection to avoid the over bright sphere in the example above.

Fresnel and IOR

Now that we've done away with the Specular Highlight cheat, and replaced the cheat with a true Reflection of your scene and light sources, we can move on to how these Reflections appear on different materials. 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...


All surfaces have some amount of fresnel, even ones that may seem very diffuse and not specular. At a very grazing angle, you'll see the fresnel reflection kick in. Even stuff like cardboard (which is pretty rough) has fresnel...


Different renderers have different ways to do Fresnel. In 3dsmax's Scanline Renderer, it's simulated using the falloff map in the Reflection slot of the Raytrace material. Place a falloff map in your reflection slot, and set it to fresnel. Uncheck override IOR, and return to the top level of your material, there's an IOR setting there. In Vray, Fresnel is a checkbox on the material itself, with an IOR ready for setting.

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.


An IOR of 1.5 on the left, and IOR 5.0 on the right, reflecting the checkerboard environment.

Glossy Specular Reflections

Depending on how rough your surface is, your specular reflections may also be glossy. A Glossy Specular Reflection is like a perfect mirror, you see a sharp copy of the environment on the surface. A Rough Specular Reflection will have a blurry reflection, as the reflection is disturbed by the roughness of your surface.



For more info on Glossy Specular Reflections, please read Brushed Metal Material.

Metals vs Non Metals

So the main difference between metals and non metals are...
These two categories of materials are frequently called Dielectrics (non metal materials that don't conduct electricity, like plastic) and Conductives (materials that do conduct electricity, like metals).

3dsmax Scanline's Metal Shader

3dsmax's Standard Material comes with a shader called "metal", and if you play with it, you'll notice it seems to follow the laws of energy conservation, in that the more specular highlight, the less the diffuse color affects the surface, and the more diffuse the object is, the less the specular highlight. Unfortunately, this shader is overly simplistic, and does not actually follow the real science behind the Diffuse / Specular relationship. That is one of the reasons I'd suggest avoiding the metal shader, a little knowledge and good observation on your part will allow you to simulate this Diffuse / Specular relationship without being stifled by a shader type that is inaccurate. Read more about it in my Chrome Material lesson. The way to achieve a more accurate and flexible shader is by simply changing the shader to blinn or phong, and then changing the Diffuse color to almost black. That way, a high IOR will apply a bright clear reflection, and reflect almost no Diffuse light since your Diffuse color is almost black. The advantage of this method is that if you need to add a little Diffuse color to your object to get it to light properly, you can, and without reducing the reflective qualities of the surface. Just remember that the shinier the metal, generally the closer your diffuse color should be to black, and vice versa, and observe real world examples to help you find the right balance.

Following these rules means ideal materials can be made quite simply. For example, chrome is just a Raytrace Material with fresnel in the reflection slot, an IOR of 10-20, a black ambient and diffuse color, and remember, no specular highlight (since specular is now a reflection of your environment). Worn chrome can be simulated by applying a bump map to this material.


Real Chrome, almost a perfect reflection of the environment at all viewing angles (with just a tiny bit of diffuse showing through.).

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.

Again, how do we do this in our 3dsmax materials? This is achieved by tinting the color of your reflection. And the way you do this depends on what material you're using. In the Standard and Raytraced Materials, the reflection color in the falloff map is changed to a golden color. In Vray, the Reflect color should be a golden color.

By doing so, the colors of the balls being reflected change to approx. the same colors we saw in our real world photograph.

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, and it were a completely raytraced image, this would look natural since it would be properly reflecting everything in the environment. However, if you're integrating this material into a scene with no raytracing (but possibly a reflected environment map) you may wish to place the falloff map inside of a second falloff map set to shadow light, with the fresnel falloff map placed in the light part of your shadow/light falloff, and the color black in the shadow slot. This will stop the material from glowing where it receives no illumination from a standard max light source in scenes where you're mixing reflections and more traditional point light sources.

But again, in general, I recommend using only area lights and real raytraced reflections nowdays, then you don't have to worry about these older cheats.

How Glossiness Affects How Reflective An Objects Is

How rough the surface is can affect its reflectivity. Check out the following photos of a dielectric, Picture 1 is a series of red balls that are reflective and with high glossiness (low roughness).



The IOR of the surface is approx. 1.3, notice how the sky reflection is stronger at the edge and less in the middle, this is a perfect example of the fresnel edge effect mentioned earlier.

Now I took sandpaper and made one of the balls rough. Notice how the reflection of the light source is now blurry.



Now lets bring it outside, notice how we can still see a fresnel effect on the surface, with the edge going brighter than the center.









However, note the colors I picked from this photo. On the glossy ball, you get a bright fresnel edge. On the rough ball, the fresnel edge is still bright, but not as bright as the glossy ball.









This darkening is caused by a number of effects.

First to recap, the rougher the surface...
The amount of dimming of the reflection is actually a pretty complex phenomena that is affected by the following factors....
Here's a similar red ball setup in vray, with 3 spheres, one with a Glossiness of 1, one with a Glossiness of 0.7 and one with a Glossiness of 0.3. Notice how the amount of brightness at the edge to remains stable.



To achieve more realistic results, the reflection should in fact dim. But by how much and in what spots is a complex issue, it's not as simple as the whole reflection getting darker equally, but its also not about changing the IOR of the surface, because that remains the same even when the surface is rougher.

Hopefully in the near future we'll have better equations built into the standard vray package that take care of all the various things that cause the reflection to dim in a more realistic way. Or you can play with some rough fresnel approximations, such as this one by Rens Heeren. But check your renderer of choice and see what options it currently has.

If you're just going to deal with this manually, making the reflection slightly darker as it gets rougher is probably close enough for rock and roll.

Reference

Hopefully you find something in here that can help you better understand how 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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