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Thread: Light Path Expressions

  1. #1
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    Default Light Path Expressions

    Light Path Expressions are implemented as a subset currently to control specific MILA light path passes in mental ray. This document should be valuable in understanding LPEs. How to specify and better use them.

    nvidia_iray_light_path_expressions.150818.LTR.pdf
    nvidia_iray_light_path_expressions.150818.A4.pdf

    Note that in the Maya 2016 integration for MILA Light Path Passes, each typical pass name has a tool tip specifying the LPE. For example, over direct_diffuse, we show L<RD>E. That represents light transport path starting from the light (L), then interacting in a reflecting diffuse manner with an object (<RD>), and then hitting your eye (E). Read the above document to more fully understand how to read the light path expression.

    Although we used the term direct_diffuse for the light pass historically named diffuse, note that it could be even more specifically named direct_diffuse_reflection. We're taking you there a step at a time, as historically the term reflection in integration UI has several meanings.
    Last edited by bart; August 20th, 2015 at 18:33.
    Barton Gawboy

  2. #2
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    Default

    Hi Bart,

    I have some questions on LPE.

    I've created a scene with Maya 2016 SP2, and I've only used Mila_Material: I'm trying to set Passes with the Light Path Expression. Now, if I use the standard LPE I have no problems, but if I try to create a custom LPE, the result is always a black render.

    I want to clarify that I've created one UserFrameBuffer connected to miDefaultOptions, and a mentalrayOutputPass connected to the camera's Output Shaders: I've used the OpenEXR to save my buffer.

    To create the custom LPE I have relied on Iray for 3DSMax 2016, but in Maya don't work properly. Basically I'm trying to extract from my scene: caustics, translucency with shadow (mila_diffuse_transmission), transparency, raw shadow, and the contribution of individual lights.

    I carefully read the pdf document you posted, and I saw that the LPE supports the "handles". I tried to use them to extract the contribution of a single light in the scene (<L'light_right'>.*E), but in Maya don't work.

    What am I doing wrong?

    Thx in advance

    Last edited by mayaboy; August 25th, 2015 at 17:30.

  3. #3
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    Default

    LPE is a generic technology that can be implemented to a given level in a given renderer. Although the goal may be more complete, any given render may have a different amount of how much is implemented. And then the integration of the renderer may also present both restrictions, as well as options.

    As mentioned above, in mental ray for Maya 2016, we only have some preset LPE passes implemented through the new UI with mila materials.

    There is no custom LPE, yet. Please use the UI and don't be connecting nodes and random attributes. If we wanted custom LPEs to be used right away in mental ray for Maya, we would have provided a more complete easy-to-use UI for it.

    That said, we do implement the same concept with light handles in a bit more of an semi-automatic preset way. The light handles are implemented in the UI with passes that know about specifically named light sets. Again, this is handled primarily in the UI, including the specific names. All you have to do is move the lights into the right set. Do you see the option to use light sets, Split Direct Light passes?
    Barton Gawboy

  4. #4
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    Default

    Hi Bart,
    thank you for your prompt reply: light sets was exactly what I was looking for!! I did a quick test, and I was amazed how nicely it worked! It would be great in addition to Light Set also have a Object Set, to have a Render Pass system to be able to partition the scene, similarly to render layers of maya.

    I think I noticed an error on LPE emission pass, because don't extract the irradiance produced from the emission of mila material (mila emission), and in any case don't affect the indirect pass: I've tried with mila object light, but in this case the emission pass is empty

  5. #5
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    Please start a new post in the Maya Application Specific section to follow up on emission pass. It should work with the mila_emission, but object light, and other lights directly seen by the eye, will not contribute correctly to the emission pass yet. This is a known issue for which we are contemplating an easy-to-use solution. More info in the new thread we start.
    Barton Gawboy

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