Introduction to Wood Finishes in Automotive Interiors
In the luxury automotive market, decorative materials offer more and more possibilities. Varnished wood is a commonly used material but it has certain constraints such as wear, scratches, UV aging and requires maintenance. As countermeasures, specific finishes can be applied for avoiding the mentioned degradation.
In this article, we will show how the appearance of wood evolves depending on the finish: raw or varnished. And the methods used to represent the optical behavior of these.
Figure 1 - Wood Ocean™ simulation
Challenges in Simulating Wood Textures for Automotive Interiors
Material Model Creation for Optical Applications
While dealing with material model creation for optical applications, both volume (e.g. dielectric function) and surface properties (e.g. reflectance) have to be taken into account. Considering surface ones, the visible grain of some specific materials like wood can be seen as highly challenging to reproduce. Indeed, random patterning mays sometimes cover the full surface, making it impossible to physically characterize with traditional measurement devices due to size limit (acceptable sample size is usually up to 15 cm).
An alternative method relies in measuring a portion of the grain and using a so-called tilling process. Once properly characterized, the approach consists in repeating the pattern over the large-scale surface. However, this pattern repetitions is often visible and not very realistic.
Figure 2 - Good Tilling versus bad tilling caused by the use of a pattern whose repetition is too apparent
Eclat Digital’s Innovative Approach
In view of bypassing this problem, Eclat Digital developed a dedicated approach for simulating such anarchic patterned materials. Combining specific skills of both engineers (measurements & characterization) and graphic designers (texture), large-scale randomness of materials can be taken into account while running ray tracing simulations with Ocean™.
Specific information and case study about this simulation process can be found in dedicated articles :
Creating Realistic Wood Textures
Using Procedural Textures for Infinite Repeatability
To avoid the constraints of sample dimensions,
we will create our wood texturing using Substance Designer.
We will create a so-called
“procedural” texture. With the help of pattern tools, noise and
arithmetic functions, we can create infinitely repeatable patterns without any
dimensional constraint, so without visible repetition.
How does it work ? - Setting Up Wood Colorimetry
First of all, we will find out how to represent the wood fibers using patterns and filters. The result of this work will then be used to set up the colorimetry of the wood.
The goal is to have something that is repeatable, accurate and consistent with the reality represented by the actual sample. If later on the use of this material requires a zoom it will always be possible to recreate a higher definition version thanks to the use of procedural textures.
Figure 3 - Low frequency procedurals pattern – data used to simulate how light rays behave at the surface of the material.
Figure 4 - The wood fine structures are modified and separated to receive colors information. In the right part of the graph, the separated parts are blended to recreate the full wood diffuse color.
Figure 5 – Wood Substance texture result for diffuse color
We will now detail the treatment for each wood
finish: raw and varnished.
Detailed Analysis of Raw Wood Finishes
Optical Properties of Raw Wood
We start from the texture described above to which we will add optical properties of scattering (BRDF, more information on dedicated article) supported by a measurement made with a goniospectrophotometer. We will find a treatment on the wood that gives it a slight shine and a color not very saturated.
Impact of Micro-Roughness on Light Scattering
We also note that the roughness diffuses the light, which tends to whiten the colorimetry of the wood.
Figure 6- The micro-roughness of the wood diffuses the light considerably and lightens the average colorimetry of the wood.
To test this hypothesis, we simply moisten the wood with a light film of water, the water fills in the micro asperities, smoothes the roughness and the color darkens considerably.
Figure 7- The water film fills in the roughness, the effect of the roughness is cancelled, the "real" color of the wood is observed
Exploring the Complexities of Varnished Wood Finishes
Effects of Varnish on Wood Appearance: Micro-scratches and orange peel
This case has the highest level of complexity. Because the varnish will introduce different effects to be considered.
First, the varnish will create the same effect as the water film described in the previous section: the varnish fills in the roughness of the wood and darkens the average color.
But here we have considered a slightly worn varnish. We have therefore introduced the defects previously characterized on a physical sample: polishing, micro-scratches, scratches as well as the “orange peel effect” due to the drying of the varnish. This effect is often noticeable on automotive paints and creates small waves in the reflection.
Figure 8 - Even if the varnish fills in the roughness and darkens the color, the few flaws in the varnish diffuse the light very slightly and enhance the brightness of the whole.
These micro asperities are created with the help of black and white images which are used by Ocean™ for the simulation of higher or lower areas of the material surface, thus varying the surface condition.
Figure 9- Height black&white datas for scratches and varnish micro patterns
Practical Applications: Wood Simulations in Vehicles
We used everything described above to simulate a decorative dashboard in a vehicle in the different finishes: raw & varnished.
Dashboard Simulations: Raw Wood Finish
Figure 10 - Raw wood. We notice the light color and the slight shine on the corners.
Dashboard Simulations: Varnished Wood Finish
Figure 11 - Varnished wood. The varnish fills in the roughness and the color is darker. But the surface of the varnish lightened very slightly the whole. On the left: the micro-scratches. On the right: the orange peel effect which undulates the reflection.
Conclusion - Advancing Automotive Interior Design with Virtual Simulations
We have shared in this article our know-how for highly textured materials and that with different finishes. All observations and analyses of the optical behavior have been supported by measurements that have not been illustrated here. Moreover, this process allows to accurately simulate materials with a large variation on the surface thanks to this collaboration Graphist-Scientist, supported by tools specific to each profile: Substance Designer on one hand and appearance expertise on the other hand.
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