TurboTips: V-Ray Material, Part 2: Reflection

Calvin Bryson 3D Modeling, Artists, TurboTips, V-Ray Material Leave a Comment

In this series of Turbo Tips, we’re giving you an in-depth guide to regular V-Ray Material. We’ll cover the theory behind many of the features of the material and give you specific examples of settings and tricks to use. While the example images are from 3ds Max, the same concepts and settings can be used in V-Ray for Maya. The information covered here is generally useful in V-Ray for C4D, but the specific fields and values may be different.

 Last week, we talked about the Diffuse tab.  This week, we’ll be moving on to:

Reflection

After Diffuse, Reflection is the second most important component of the VrayMtl. It also features a lot more options than Diffuse.

Perhaps it can be easier to understand how the Reflection works if you imagine it as a layer on top of the Diffuse. At 100% strength [255;255;255], it shows pure reflection of the environment, lights, etc. Use a darker color and the Diffuse will start to show through. Drop it down to pure black and only the Diffuse is visible. (It’s not entirely as simple as that, but that’s the general idea on how Diffuse and Reflection interact.)

 

Adding Realism with Texture

The Reflections (just like most other maps in V-Ray) can be defined by using a color, a map, or a texture. The same principle of Diffuse map applies– if it’s not a shiny, slick studio render, there are bound to be some imperfections in the reflection amount. Also, it’s best to use a Map or a Texture instead of a simple color.

In general, it may be best to keep the Reflection value in the range from 1~230 for realistic results.

 

Always Turn on Fresnel

So, why does our material look so artificial?

The problem is that the light reflects equally at all angles. Real world objects have different strengths of reflections, depending on the viewing angle relative to your line of sight. In general, the lower the angle, the stronger the reflection becomes. Even when some materials initially appear to be non-reflective, they will reflect quite a bit when they nearly parallel to the direction you’re looking at.

Let’s look at some examples…

Notice how the reflection gets stronger as the floor goes further from the camera (or the closer it approaches the edge of the bowling ball). The smaller the viewing angle, the stronger the reflection. If you look directly at something (90°), the reflection is much weaker than if you look at it with a small angle.

To imitate this effect in Vray, you can use the Fresnel Reflections option.

In general, it is a good idea to use Fresnel for every material you create. The difference between chrome and concrete lies in the Fresnel IOR value. This value determines exactly how this reflection falloff occurs. To access it, turn off the L button. (Note: the default value of 1.6 is only good for glass and maybe some plastics.)

Below is a general guide on which values to use for which materials.

  • water: 1.33

  • plastic: 1.4-2.4

  • glass: 1.5-1.8

  • diamond: 2.4

  • compound materials (wood, stone, concrete, etc.): 3-6

  • metals: 18-100 (Though you should rarely exceed 40)

 

Reflection Glossiness

 

 

This parameter controls how glossy our material looks. The higher the value, the higher the glossiness. A perfectly polished surface would have a glossiness of 1 (default value). Since nothing is ever perfect, we wouldn’t go higher than 0.99

Decreasing glossiness makes the reflections blurrier. The effect is somewhat similar to taking a fine sandpaper to our shader and roughing the surface up. This comes with a cost, though: the more blurry your reflections get, the harder it is for V-Ray to calculate them; thus, the result is noisier and the render time increases. For very rough surfaces, we wouldn’t go lower than 0.35

This gives us a useable range of 0.35-0.99.

 

Unlinking Specular & Reflection Glossiness

By default, Reflection glossiness and Highlight glossiness are locked together. However, there are times that it can be useful to unlink these and use a slightly lower Highlight gloss. The effect is that you still have sharp reflections with some slight glow around them. Many real world objects show this kind of behaviour:

This little cheat helps us simulate that look without increasing the render time. There are no rules on how much to lower the Highlight gloss, so use your eyes to make the judgement– although, generally, a difference of 1.0 to 1.5 works well.

 

Using Texture To Drive Glossiness

As soon as the object leaves its packaging and a person touches it, the reflections are no longer equally glossy. The areas touched by hands or scuffed against something rough are slightly more blurry from oils, scratches, or any other interaction with the world.  Try to use a texture or a map instead of a simple color– it can have very little variation in brightness, but it is important to pay attention to these little details.  Otherwise, the result will look artificial.

Generally, it is a good idea to derive your glossiness map from the reflection map (you can overlay a different texture to make it more interesting). The areas that are less reflective will probably be slightly more blurry as well. This is not a hard rule.  You can break it, as long as the result looks believable.

This example still needs some Bump to look realistic, but we’ll get to that a bit later.

The next parameter is Subdivs:

This setting determines how many samples V-Ray can use to clean up the noise in blurred reflections. Basically: more samples = cleaner reflections.

Note:  Most V-Ray users prefer to use the Adaptive DMC sampler for rendering their images. This means that the actual number of samples needed for a clean image changes, depending on the DMC settings: 1/4 will need lower values than 1/100.   We recommend leaving this value at the default, 8 subdivs, if the models or materials will be used by other people. Everyone has their own workflow and will adjust it accordingly. Otherwise, it can get frustrating hunting down a material with a too-high subdiv count that doesn’t work with a particular render setup.

 

Reflection Depth & Exit Color

 

 

This option sets how many times the reflection is traced before it is converted into the exit color. This helps to speed up the renders by reducing the amount of calculations V-Ray has to do for reflections. Here’s an example with exit color set to blue:

The default settings work well most of the time. If you have a lot of mirrors or other reflective objects, you might need to increase the max depth, though going higher than ~20 isn’t usually necessary.

If your material has blurry reflections, you can make it render a bit faster without losing quality, by reducing the Max Depth as follows…

  • Glossiness 0.9-0.99 = max depth 5

  • Glossiness 0.8-0.89 = max depth 4

  • Glossiness 0.7-0.79 = max depth 3

  • Glossiness 0.6-0.69 = max depth 2

  • Glossiness 0.35-0.59 = max depth 1

Since the reflections are blurred, there will be no negative effects on the image. The values we’ve provided are more like a rough guide, so you can adjust them if needed.

Lesser-Known Parameters

That about covers it for the basic Reflection tab– Interpolation is no longer needed, since it’s much faster and easier to use light cache for glossy rays in GI settings. Dim distance and Affect channels are only used in some very specific cases (more related to scene optimization, not material creation).

BRDF

There are a few other options hidden a bit lower, in the BRDF tab and the Options tab.

BRDF is basically a mathematical model that is used to calculate the reflections and specularity for your material. There are three types available for you to choose from – Blinn, Phong and Ward. Each one has their own specific uses.

As you can see, the main difference is in the way they treat highlights. Phong is the sharpest, Blinn is a bit more blurred, and Ward is much softer.

There really is no hard and fast rule for which of these to use, but our general recommendation would be to use Ward for metals and anisotropic materials, and Blinn or Phong (whichever you prefer) for the rest. The only exception is that it is not recommended to switch to Ward for metals, if the metal is highly polished or has very sharp reflections (like chrome, gold jewelry, etc.).

Anisotropy

 

 

Anisotropy is used to simulate stretched out highlights. In the real world, these are caused by elongated micro-scratches that go in the same direction. Below are a couple of example photos; this effect is seen mostly on brushed metal:

Anisotropy should be set in an interval between -0.99 and 0.99.  With values of -1; 0 and 1, it will not do anything.

The effect becomes stronger as the value approaches 1 (or -1). The difference between negative and positive values is the direction of the stretching. Positive values stretch reflections horizontally (simulates vertical scratch pattern). Negative values stretch the reflection vertically (simulates horizontal scratch pattern).

You can also rotate the the stretching effect to any angle you want by using the Rotation parameter.

For even more control, you can choose the axis that is used for calculations.

For it to work correctly, Anisotropy needs blurred reflections. If your Reflection glossiness is set very high, the effect will not work.

Just as with other aspects of V-ray, we can use Maps or Textures to drive the Anisotropy parameters as well.

You can use an Anisotropy texture with reduced strength to fine tune the exact amount of imperfection it introduces. Keep in mind that texture maps only work as positive values, so it’s best to combine them with positive Anisotropy strength. For example (below), we’re using Anisotropy 0.6 + 20% of a texture. The result looks a bit more natural than just pure Anisotropy.

Rotation maps can be used to change the direction of the simulated scratches. This is good for creating things like circular patterns or metallic flakes that reflect light in random directions. Smooth gradients make the rotation gradual, while patches of different colors make the transitions sharp, with each shade of gray having a different rotation value.

 

The Options Tab (Reflections)

Before we finish with Reflections, here are a couple more options to consider. Scroll down to the Options tab and take a look at the settings there:

The outlined options (above) are all affecting the Reflections.

First of all, you should never turn off the Trace Reflections option, since it is essential for a realistic result. If you turn it off and use only the fake specular highlights, reflections are just that: round, fake highlights, regardless of the shape of the lights or the environment.

Next is Reflect on back side. By default, it is turned off and that’s fine for most materials, since it helps to cut down on the render time. However, if you are creating glass or other transparent materials, you have to turn this option ON, otherwise, the result will not look realistic.

And finally, let’s look at the Energy Preservation mode. The default setting of RGB is physically correct, however, there might be some cases where the result is hard to predict. For example, a white material with blue reflections (below).

The Reflection amount is subtracted from the Diffuse color. For example, lets take white Diffuse [230;230;230] and blue Reflections [0;0;230]. So, what do we get when we subtract? We get Yellow [230;230;0], and that is exactly what we see when rendering this particular example:

Switch the EPM to Mono and you get a much more predictable result – white Diffuse and blue Reflections.

These types of materials are not common. Change this option only on the rare occasion that you have to create colored Reflections on top of a bright Diffuse color.

Next week: V-Ray Material, Part 3: Refraction

This series of tutorials was made with our friends at Viscorbel.

If there are any topics you’d like to see in a future edition of  TurboTips, let us know in the comments below, or Tweet your question to @TurboSquid with hashtag #TurboTips.

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