Category Archives: shaders
One of the features artists have asked for is the ability to do dispersion inside mental ray without needing a custom shader.
Unexposed in the main UI is an option for “weight tint” that is a luminance you can control inside a mila_mix or mila_layer node to get the effect. Slightly offsetting the ior of the transmission nodes is what controls the spread of the dispersion effect.
I will update this post again when I have a useful phenomenon to provide to make this simpler, but in the meantime you might come up with your own experiments. Updated below.
You can find the three band (color) phenomenon here. (Copy and paste if your browser doesn’t download the file. Place in your “include” folder for mental ray shaders)
The phenomenon exposes the following controls for the user as a transmission-only component. You can layer this with other components manually by connecting it through the hypershade. The incarnation of MILA in Maya right now isn’t setup to use custom phenomenon flexibly just yet. By altering the phenomenon you can include other controls. For example you can add a tint control instead of having only colorless transmission. Or for nicer looking dispersion you can create six bands instead of the three in this version (Think ROYGBIV).
You can see the power here of Phenomenon and MILA. You can create and store constructs and expose the controls you want for yourself or others. Using the conventions outlined in the MILA documents you can continue to build complex materials with all the benefits of importance sampling and light sharing.
*hint: if you find the result is a little “green” (assuming you have connected this as RGB) then you can reduce the ray cutoff to solve it. It’s not always necessary and is based on the model. The link to the string options for MILA are HERE. Eventually such controls will be exposed in the UI natively.
In Maya 2015, the Layering Library (MILA as we will call it) was integrated for use. This is an initial integration and will evolve through advancements and feedback.
Part One will discuss some basics before getting to the UI inside Maya.
You can find other details on the Official mental ray blog with Bart’s first post on MILA.
Why MILA? Isn’t the MIA (Architectural) material good enough?
Artists have noticed when using the MIA (Architectural) material that complexity adds render time. In fact, the more you layer and tweak the MIA material, the worse it can get on an exponential level. The MIA material would spawn rays to sample the scene at a high rate even if it was a third or fourth level of trace depth. When layering the MIA material, each layer would complete the entire job of shooting rays and sampling lights before combining output, even if that work wasn’t visible in the final result.
The MILA material avoids this. It uses better techniques to reduce work done by the renderer. This means layering the material or using high trace depth has less (or no) impact on the render time or quality. This is important for better and easier realism in an image.
The MIA material also had a few features that users have decided they didn’t like or were rarely used. The glossy reflection curve had a drastic change near the 0. point, making it hard to paint textures for. The interpolated reflections weren’t used often and required tuning. And other features like tuning reflection rays weren’t always intuitive.
MILA moves towards a “roughness” parameter for glossiness that is more linear and easier to texture. It also has a more accurate glossy reflection model. Rarely used settings or “tricks” were removed in favor of straightforward operation. And in keeping with the mental ray shift in usability, simple Quality controls are used to tune the result of the material instead of tweaking numbers. The Quality controls also understand the settings in the material itself and attempt to self-adjust for better renders from the beginning.
Why component based?
Many times an artist would want to add a simple reflection layer or maybe some dirt to a MIA material. This meant adding a completely new monolithic material on top for a single and simple job.
Component based selections mean effects or interactions can be added as necessary.
MILA also follows the design of the Material Definition Language (MDL). Coming in mental ray 3.13, the MILA materials will lead towards this rendering workflow.
Understanding concepts and terms.
In using MILA there are a couple things to talk about. Before getting into the actual components, there are types of interactions to think about. This will make creating a material easier to understand later.
Reflection versus Transmission
-Reflection is a ray that strikes the material and bounces off. This can be the traditional “reflection” we’re used to as well as diffuse reflection where light strikes and comes off an object back into the scene. This lets us see the reflected light from the object(s). A specular or mirror-like reflection is easy to understand. Diffuse reflection is the same thing but the light reflected back is diffused (not sharp). So the concept is the same in graphics.
-Transmission is when a ray passes through an object. This is traditionally noted as something like “refraction” but covers all types of transmission. Diffuse transmission is commonly known as translucency.
The names of the components and the direction of the energy (ray) can be described visually using the image below:
Why these names?
As mentioned above, the move to MDL makes for renderer agnostic materials. Using these names also helps associate with Light Path Expressions (LPE) which are also renderer agnostic. For example, “reflection” can mean a lot of things to different renderers. But “diffuse reflection” is specific.
Direct versus Indirect
-Direct effects are a result of light directly affecting the material. The light path is from the light to the material with nothing in between.
-Indirect effects are a result of light that has interacted with something else before reaching the object.
Components have these names in their description to help you choose the effect you need:
More details can be found in the previous post about the Beta testing of the Layering Library. Integration has changed the UI and the workflow. That process will follow in the next parts of the blog.
On to part 2 with explanations on the Base components and more.
The Layering Library (MILA) shaders are currently in Beta and are not designed for production use. However, here at The Mill in LA, we spent some extra care (and time) in the lookdev process to use the MILA shaders in a commercial spot for Norfolk Southern.
You’ll see many trains rendered using the MILA library for mental ray.
Feedback was very positive and we were able to better layer effects on objects with different material types and labels easily all while keeping render times under control (some complex 1080HD frames with motion blur were about 2+ hours a frame with brute force settings and the Environment Light) Artists also found the glossy reflection much easier to handle and faster to use than the mia_materials.
One artist said of their experience with the MILA material, “I will never use the mia_material again.”
Take a look at the spot below.
As the MILA shaders evolve, features will be updated here when possible. These shaders are inspired by the Material Description Language and you can find more on that from the GTC conference here: nVidia Material Definition Language for Coordinating Materials (Thanks to Saycon for the link in the comments)
This latest release includes some more user-inspired changes like:
- A non-physical global clamp for reflection to eliminate hotspots (fireflies) from lights and interreflections
- Independent direct and indirect contribution sliders on components to create non-physical but art driven look
- Further moving of controls to global Quality String Options both for all MILA Quality as well as specific controls like “mila glossy quality” etc.
- Continued work on the Quality controls adaptability meaning fewer tweaks, just set the shader look and hit render
- Creating a Diffuse Reflection Detail Quality to replace Ambient Occlusion controls
- Moving controls off the shader components to make them clean and easy, a move toward greater simplicity for mental ray (to be continued)
- The elimination of unused or infrequently used controls
- *experimental* propagation of additional buffers in secondary rays
The clamp option can be used as on or off (default is off) for controlling overbright highlights and reflections. It is a non-physical effect. This means it purposefully changes the energy in the render to avoid artifacts from insufficient sampling. This is usually caused by a very hot HDRI used to light a scene or a bright area light and the resulting indirect reflections.
This clamp option may also reduce render times because fewer samples are taken to resolve hotspots that cause variance in the image. The below image had an HDRI with a high value of over 7000 for the sun. Rendered at Quality 0.20 for Unified Sampling to amplify the effect of poor sampling. This also affects the resulting framebuffer contribution. Notice the areas that are not overly hot are maintained.
Direct and Indirect Contribution
A direct and indirect slider can be used to change the look of the material non-physically. Recall that Direct Reflection is the result of the light. Indirect Reflection is the result of the light from other objects in your scene. The easiest way to think of this is the direct diffuse is historically your diffuse pass and indirect diffuse is the indirect pass or “color bleed” from nearby objects reflecting light and measured by something like Final Gathering.
Below is an example with the effect of direct at 0.00 contribution and then indirect at 0.00 contribution on glossy reflection. You can adjust these independently to achieve a non-physical but pleasing artistic look in a material. This will also affect the resulting framebuffer contribution. These can be texture mapped as well for special effects.
Diffuse Indirect Detail
Diffuse Indirect Detail replaces the Ambient Occlusion controls with an On and Off switch globally, a distance parameter, and quality. Below is an example of on and off at default values.
Framebuffer Indirect Contribution Writing
*experimental* Additional buffers can now be rendered as seen in a reflection or refraction (indirect). This means you can get the matte color of an object written to its own buffer even if it is only seen in a reflected or transmitted ray. Below is an example where the turn blinker (indicator) writes its resulting color matte to the framebuffer despite being behind the lens cover of the light. It also contains information where it was reflected as well. This is useful to isolate and alter elements after rendering.
You can also pass the matte through another node like a rayswitch to further define the mattes generated through transmission (blue) or reflection (red) as seen below.
Now apply Glint!
Keep in mind that this object would be “baked” into the transmission framebuffer derived from the beauty. So changes after rendering would still be limited. This may also increase the render time as an extra color framebuffer is considered for anti-aliasing. If you wish to avoid that you can turn off “contrast all buffers” in the Quality Tab -> Framebuffer Rollout of your Render Settings.
Overall the goals for MILA are continued flexibility and simplicity based on MDL. Redundant and unused controls are moved and clarity in settings is being improved.
Why all of the “Quality” Controls?
Also notice the increase in controls for “Quality”. This is important since the underlying method or algorithm can be hidden under a simple control. This is already true of Unified Sampling and the Native IBL.
Why is this useful?
By hiding the method, developers can later change or improve it without introducing new controls or altering the old ones. This was true of the refinements made in Unified Sampling in 3.10. Artists can continue to work as the renderer improves without learning new techniques.
This is part of how mental ray will continue to simplify the workflow for users without sacrificing flexibility or speed. This will also make the integration of new features much easier in OEM products like Autodesk Maya since documentation and UI changes will be unnecessary with added improvements to existing features.
Unified sampling in the Autodesk 2014 products represents a first step toward mental ray use simplification. The layering shaders, etc,next
— mental ray (@mentalray) April 23, 2013