Specular Reflections
By Neil Blevins
Created On: Feb 4th 2001
Updated On: Dec 8th 2024
Software: Any

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.

Specular Reflections

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

In old school computer graphics, a Specular Reflection was generally broken up into two parts, Highlights and Reflections. This was because calculating Raytraced Reflections were expensive, so you tended to not have Reflections on a surface, and instead you only had a Highlight, which was a simplification of a specular reflection that was easy to calculate (usually a variation of the Blinn or Phong models). Highlights were controlled using Specular controls, such as shininess strength and glossiness. And then as computers got faster in the early 2000s, a second set of controls were added to give your material Raytraced Reflections. That added a second set 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 point light source on your surface (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.



Take a look at the picture above, you see these little highlights on the plastic, not too far off from a standard Blinn Highlight.

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.

Additive vs Default Reflections

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.

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 reflection (the blue of the ball) is not a natural phenomenon, and should be avoided. The right sphere is a more natural specular reflection. More modern materials such as the VrayMtl in vray do energy conservation automatically (and so would look like the right sphere), and so are better to use. Of if your renderer only does additive mode, you can always do energy conservation manually by reducing the brightness of your diffuse if you're going to increase the specular reflection to avoid the over bright sphere as in the example above.

Fresnel and IOR

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...


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, 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.


An IOR of 1.5 on the left, and IOR 5.0 on the right, reflecting a 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 (low glossiness) 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...
Chrome

Knowing this, a recipe for chrome would be:
Here's an example of real chrome reflecting a toothpaste tube. It's almost a perfect reflection. Read more on Chrome in the Chrome material tutorial.


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.

In CG, you usually achieve this by tinting the reflection color. For example, in Vray for 3dsmax, set the Reflection color to a golden yellow.



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, this would look more natural since it would be properly reflecting everything in the environment.

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 plastic, 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 for 3dsmax, 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 most renderers 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 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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