Light Path Expressions and mental ray 3.11 shaders
iray 3.0 includes the ability to use Light Path Expressions (LPE) to separate different elements of your render. The iray blog has a simple explanation here: iray LPE
Additionally, the layering library of shaders coming for mental ray 3.11 (and future beta testing) will allow the specification of framebuffers using relevant LPE. This continues the bridge from the older style of rendering to newer methods with a flexible set of layerable components.
Many modern renderers use light transport to render the scene. This is a path from the light to the eye in this case. Light is transported (traced) along this path and stored in the framebuffer. LPE allows you to selectively isolate effects and paths using an expression that is consistent across renderers (the expression means the same thing regardless of render engine). This isn’t a new idea and has been around for some time, but few products have used it. Of course the trick will be exposing this in a useful way for most users and non-technical artists. But the concept of what you are getting from the renderer is much clearer in this case and more exact.
You can think of a few different (simple) interactions such as:
- Diffuse – Reflection or Transmission (translucency), and it can be direct or indirect
- Glossy – Reflection or Transmission, also can be direct or indirect
- Specular – Reflection or Transmission, this is a mirror (reflection) and is usually treated as indirect only.
In iray, these interactions can also be in different orders or number of interactions, perhaps you only want 2 bounces of reflection for example. You can also have the possibility of tagging objects and retrieving their contribution, like per-light framebuffers or separating out individual objects and/or contribution for later compositing. Keep in mind the more you add, the greater complexity and the slower your render may become.
Posted on December 6, 2012, in Example, shaders and tagged 3.11, layering shader, LPE, mental ray. Bookmark the permalink. 9 Comments.
elemental ray 
when iray 3 is going to be relased? any chance it would be part of maya?
The integration framework on the nVidia ARC website already has iray 3.3 available as a download for developer testing/usage. As for the release of iray 3 in OEM products, that’s up to the OEM like Autodesk or Bunkspeed to decide on when and how they would expose it.
The Product Manager for Maya (Cory Mogk) mentioned on his official blog that they would like to include iray at some point but that they wanted to improve the mental ray integration before taking on a new level of complexity. Sadly that doesn’t answer “when” but does show it’s on his mind.
thanks for the update
So this basically allows us to break up our renders into custom defined buffers? Different process than mr, but same effect. Did I get that right? It’s nice that it’s not shader based, so we can use materials not normally designed for passes.
The key word here is “relevant”. Anything the shaders provide will be possible through a Light Path Expression, but the rest will still rely on traditional approaches. iray’s core is where a lot of these interactions take place so in the case of iray, it’s integrated fully. You can find out more about the LPE support in the Integration Framework downloads on the ARC website.
This layering library will be a bridge to more advanced techniques so expect it to have some attributes of traditional shaders while getting you used to the structure and nomenclature of newer methods.
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