The mia_material_x node should form the basis of nearly every material you build in Maya, aside from skin shaders. To find mia_material_x, click the Hypershade window > Mental Ray > Materials > mia_material_x. The standard MIA shader is a neutral gray with a sharp specular highlight. The mia material has a vast array of options. Some of them will be important to you, but a lot of them you can ignore. Arriving at a basic glass shader is relatively simple—things only begin to get tricky when you need to fill the glass with a liquid. Your success in rendering glass depends on how well you set several parameters: Diffuse, Refraction, Reflection, Specularity, and Fresnel Effect. The first thing you need to adjust is the index of refraction parameter, which corresponds to a relatively specific real-world index of refraction values that exists for all naturally transparent surfaces. If you hover over the Index of Refraction tab, a small list of approximate values for different materials pops up. Water has an index of refraction around 1.3. Crown glass has a real-world index of refraction at approximately 1.52. Set the index of refraction to 1.52. The last thing you need to tweak in the refraction tab is the transparency value. You’re creating a fully transparent glass shader, so set the transparency value to 1. Leave the glossiness value at 1.0 and change reflectivity to a value somewhere between 0.8 and 1. A little bit of subjectivity is OK here depending on the look you want in your final image, but the reflectivity value shouldn’t drop below 0.8. If you compare your current result with real-world glass, you’ll see that the surface is currently a bit too busy to be called realistic. Right now the mia_material is reflecting the environment, which is good, but it’s also computing glossy reflections based on specularity, which is bad. Specular highlights are a holdover from earlier days of CG when glossy reflections had to be faked. It’s still an important attribute in CG surfacing, but in this case, it’s giving you a less realistic result than you’d like to see. You want to retain the reflected environment but lose the specular-related highlights that are currently showing up in the renders. Find the Specular Balance attribute under the Advanced tab and set it to zero. Because the Fresnel effect is a relatively common phenomenon, the mia_material has a Fresnel attribute built into it. All you have to do is turn it on. Open the BRDF tab (short for Bidirectional Reflectance Distribution Function) in the material attributes window, and check the box labeled Use Fresnel Reflection. You should see the result change quite a bit. It’s always good to know how something is made, though. By creating the shader yourself, you learn which attributes contribute to different aspects of the shader, and you’re, therefore, more able to tweak the shader to your liking in the future or create variations on it for slightly different effects. That said, if you want to use the glass preset, simply open the material attribute window for a mia_material_x, hold down the preset button in the upper right corner of the window and go to Solid glass > Replace.
