COMPUTER AIDED ARCHITECTURAL DESIGN
Workshop 8 Notes, Week of  October 27,  2014

BRDF  MATERIALS 

Bidirectional Reflective Distribution Function (BRDF) defined material is a physically based description that examines how incident light on a a surface is can be either reflected, absorbed or transmitted.  BRDF describes how much light is transmitted when it makes contact with specific materials (for a more complete description see An Introduction on UCLA's computer science web site )
 

This tutorial is developed after the treatment http://viscorbel.com/vray-materials-theory/  posted on viscorbal.com

1. Create a Rhino File with a ground surface, box, sphere, truncated cone and V-Ray sun light,  and create the setup below using methods covered in earlier tutorials. Create separate levels for a) the ground surface, b) the box, sphere and truncated cone, and  c) V-Ray sunlight.

2. Within Rhino, go to the menu item "Render>Current Renderer>V-Ray for Rhino" to ensure that V-Ray is active.

3. To activate the V-Ray material editor within Rhino, go the the menu item "V-Ray>Material Editor", and in the dialog box that follows, right click on the text "scene-materials" and create the a new type material, a so-called "V-Ray material". Note that  we will focus on "Basic Parameters" and "BDRF", four of the six tabs included in the material editor.

4. Right-click on the name "DefaultMaterial" and rename it "mySphere".

5. Within "Basic Parameters", the Diffuse Color establishes the base color of the material from the color selector and can also be specified in terms of RGB and HSV format. The number associated with "Roughness" controls the flatness of the material on a scale of 0 to 1. For example, preview the material by selecting the preview button, then make the color a light green, adjust the "Roughess" to 0.5, and preview it again.

Default settings:



Revised "Diffuse" setting:



Revised "Roughness"



Note that adjusting the "Roughness" appears to soften and diffuse the highlight on the sphere making it look flatter.

6. For the same material,  adjust the "reflect" color to neutral gray and preview the result. 



Note that a adjusting the reflective color towards white increases the apparent reflectivity of the sphere.



7. Apply the material "mySphere" to the sphere within Rhino and do a low resolution test rendering. Note that at full white (the value is 255 on a grey scale range of 0 - 255), there is very little remaining of the diffuse color in evidence as compared with the second rendering of step 6 above.




 Changing the reflective color to "Dark Gray" with the value 105 appears to restore more of the diffuse color to the sphere.



This is evident in the "Preview" sample of material editor as well as the revised rendering. However, note that the appearance in the full rendering is contextualized to the angle of the sunlight and the surrounding adjacent colors and objects. V-Ray adheres to a conservation of energy rule that requires that the sum of the reflective and diffuse light do not exceed 100%. For example, if the reflective light is at a relative 70% then the diffuse light is at a relative 30%.






8. The "Reflection glossiness"property blurs the apparent reflectivity of the sphere. Change the base color of the sphere to middle gray and preview the sphere with "Reflection glossiness" at 1.0 (the default value).



Lower the "Reflection glossiness" to 0.85" and preview it again.



Restore the "Reflective glossiness" to 1.0. Then note  that by changing the "Reflection" color to  red,  it is subtracted from the color of sphere as a whole, leaving the balance of the surface a slight tint of cyan. Cyan is the secondary color which results from combining the two primary colors of blue and green. 



Within the context of the full rendering, the reflective highlight appears diminished and relocated to the bottom of the sphere. A light cyan color appears over the upper half of the sphere as a whole. Once again note that the conditions of the 3D model for the full rendering thus determine the degree to which the pure "Preview" sample is represented.



Restoring the reflection glossiness to "white" in the following three adjustments demonstrate the results of changing the value of "Reflective gloss"  0.0, 0.5, and 1.0 respectively:








Within the context of the Rhino model, we get in parallel three alternative renderings of the sphere:

Note that the 3D modeling  context of the rendering appears to establish more distinctly the the preview renderings the differences between "Reflective glossiness" values of 0.0 and 0.5. 

9. The "Fresnel" parameter changes the strength for the reflection depending upon viewing angle. For example, when viewing a lake a  relatively low angle, the reflectivity is relatively high. However, when viewing it a greater angle, it decreases accordingly.  That is, the reflection is weaker as your viewing angle approaches the surface normal (the direction perpendicular to the surface), and is stronger as you approach an angle parallel to the surface.  The "Fresnel"  IOR (index of refraction) determines the level of the effect. That is, higher values indicate greater effects at lower viewing angles.  

To explore this directly, add a a "V Ray" infinite plane to the  Rhino model , and use the Gumball tool to place the plane  slightly below the existing planar surface.



Create a new so-called type "V-Ray material", set "Reflect" color to white, unlock the lock symbol "L" by clicking on it. The lock symbol "L" is adjacent to  "Fresnel reflections". Change its the value of "Fresnel IOR"  to 1.33 which has been found to be a good setting for water.



Apply the material to the original flat surface plane, and render it at a high and then at a low low angle. Note the difference in reflection between the two angles in that the reflection is greater at the lower viewing angle:

. Change the material name to "metal1", change the"Fresnel IOR" to 20, and change the "Diffuse" color to light gray.




Apply "metal1"  to the box and the truncated cone within Rhino and render it again at both a high and then at a low angle. Here the difference in angle of view impacts both the context that is reflected by the planar surface as well as the strength of the reflection.

11. Refractions also can be explored through an IOR number.  Duplicate "stillReflectingPool" to create a new material named "stillReflectingPool1", change the "Refraction"  color from black to light gray to control transparency. Change also the refraction "IOR" to 1.33 and preview it:



 Apply the material to the original flat surface plane and render (see figure on left of the three below). Note that decreasing the value of  Refraction "Glossiness" from 1.0 to 0.5 and then to 0.0 correspondingly increases the apparent frostiness of the surface. However, it also does so at increasingly  much higher rendering times. Here are renderings for "Glossiness at 1.0 (left), 0.5 (center), and 0.0 (right) respectively.

Zirconia, Cubic 2.170

13.  Dispersion of light is effected by the switching on the "Dispersion" option in the Refraction tab, and adjusting the strength of the  Abbe number,  for different materials.  Here, lower numbers produce greater "Dispersion". For example, set the Abbe number to "20" for the material "stillReflectingPool1".



Rerendering the same model shows greater coloration of the sphere and also the changes the transmission of light through the sphere onto the flat rectangular surface below it.



14. Now, go to the "Caustics" tab of the V-Ray options editor, and switch "Caustics"  to on.



Create a prisim (extruded trianglar section), orient to the sun angle as shown in the image below.




 Adjust the Refraction " IOR:  to 1.55, the Abbe number to 4, and apply the "stillReflectingPool1" material to theprism. Rerendering the image creates a prism with a few hues of color along with a concentrated area of light on the rectangular surface below. The sphere also takes on the coloration of a prism. Rendnering times increase dramatically.



15. To create water with some waveforms, extrude the flat rectangle in the ground plane so as to create a rectangular box and remove the original plane surface. In this current example, the box will be represented as it were filled with water.





Create a new"V-Ray" "Standard" material "newReflectingPool", rick-click on the material in the V-Ray material editor and add a VrayBRDF layer. Hold down the left mouse button and in the Materials List area of the V-Ray material editor,  select the "VRay BRDF" layer and move to the the top of the layer list for "newReflectingPool". That is, place it above the "Diffuse" layer.




Next, adjust the settings on the

 

15.4 Under Diffuse
Set the "Transparency" color to light gray.

15.5  Maps
Create a bump map using "texNoise and using the "Perlin" option with the frequency set to 4.0.



The "Diffuse" part of the material definition should then appear as follows.



Finally, apply the "newReflectingPool" material to the extruded box in the ground, add a "texSky" based upon the "V-Ray" sun as in the previous Workshop 6 R and re-render.