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Thread: user_ibl_rect with parti_volume

  1. #1
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    Default user_ibl_rect with parti_volume

    Hi,

    I'm having issues while using user_ibl_rect light shader with a parti_volume.

    Here are images to show my probleme. The scene is only one area light set to visible, using rectangular shape.

    parti_volume_ibl_rect_issue.jpg

    There is 2 big issues, but one critical :

    - the box is visible, even if it has a transmat assigned as surface shader. This is not critical, we can still deactivate primary visiblity.
    There will be no fog on the background but the production shot we are making is complety filled with gc element so this wont be a probleme.

    - The self shadowing is completely wrong ! Both render are without FG.
    You can see in the portal_light version that both casted shadows and self shadows are brightened by the fog. The haze created by the parti_volume is equal on the floor and on the objects.
    But with a ibl_rect, the self shadowing looks like it is not affected at all. There is no visible fog on the dark part of the donut and the cube.

    We don't have other possibilities to light our scene with the neon map. Using a circular area light will give a totally different look to the scene.

    Maybe we should use another method to render volumetric effects with ibl_rect ?
    I saw in the documentation that the shader "transmat" is deprecated. What should we use instead ?

    Thanks
    Hans

  2. #2
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    Default

    here is a mi file of the test scene to see if you can reproduce it

    test_user_ibl_parti_volume.mi

    and I forgot, the scene was tested on maya 2013, but the result is exactly the same in maya 2014 with mental ray 3.11.1.9

  3. #3
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    I made some other test, I was looking for a way to render the fog separatly to put it in comp.

    So I assigned a black surface shader to every thing. When using the ibl_rect, the result is really strange.
    The objects continue to receive light information. Like if they were lambert.

    Here are the render, with a comparison with portal_light

    ibl_rect:
    ibl_rect_surface_shader.jpg

    portal_light:
    portal_light_surface_shader.jpg


    Both render are using the exact same geometry and shaders. I just changed the light shader inside the are light

  4. #4
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    If you're using 2014, I might be tempted to use a texture mapped area light and the Light Importance Sampling (IS) in the core. It's a string option. And see how that goes.
    "Don't argue with an idiot, they will drag you down to their level and beat you over the head with experience."

  5. #5
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    That was a pretty good idea !

    But importance sampling in the core looks really bad. It's better thant without, but it's far from the efficiency of the user_ibl_rect.

    Here are test renders.
    some info :
    - for the user ibl, I had unified 0.5 1 100 with 8 local sample on the light
    - for the arealight (e.i. without any mr light shader), I had to boost unified to 2 1 200 with 64 local sample (it was not increasing render time for such a simple scene)
    - for the portal light, no matter what settings I put, the render was near instantaneous with this amout of grain. I was unable to get rid of it. Setting ridiculously high samples changed nothing (not even render time)
    - for the physical light, it obviously doesn't support texture in light color

    importance_sampling.jpg


    The first thing I note is that when I turn ON "light importance sampling", it kills the user_ibl ! How can such a thing append ?
    That must meen that importance sampling on override the internal sampling methods of the user_ibl. But the core global importance sampling is a lot worse than the on in the user_ibl.

    Then using a texture on mental ray light shader is baaad. Even with importance sampling it looks not supported. So how should this feature used ?
    Only the maya area light (with default no shadows and no decay !) behave correctly.
    A maya arealight without IS is better than a portal light with IS !

    But the arealight intensity looks wrong with the maya area light (if I take the user_ibl as correct)
    And no matter how much sample I'm using, I got fireflies.

    So what's wrong with this ?

    And no need to show renders with parti_volume. It's looks like it behave correctly with a mapped area light, but it's impossible to sample so it's not a option.

    Using mental ray 3.11.1.9 with maya 2014 SP1 for this test

  6. #6
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    user_ibl has its own sampling technique built into the shader, I don't think the core overrides this. (In fact, moving all of this to the core is a better idea than shaders everywhere doing its own thing)

    It shouldn't kill off the emission from the user_ibl

    The Maya area light is probably mapping this correctly. Some of the mental ray shaders are old. It's possible they aren't intended for use here. Something like a mila_light would take care of these scenarios when complete. Again, something that would be nice in the core.

    There are more settings (which escape me at the moment) for light importance sampling, try some of those. Notice the maya_state shader freaks out some while using this. Hopefully to be resolved soon. . . This version of Light IS was not integrated since it was not complete at the time (it was a refurbishment of the existing method I think.)
    "Don't argue with an idiot, they will drag you down to their level and beat you over the head with experience."

  7. #7
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    For the user_ibl, enabling IS clearly changes something in the wrong way. without changing anything exept the string options, the renders gros really bad.
    I agree that having a core function is way better than having it inside each shader, but from what I see, the IS implemented in the core is not the same as the one implemented in the user_ibl. Maybe it's has something to do with maya integration.
    Right now I will not replace user_ibl_rect with any other light if I have to use texture arealight.

    I'm waiting for the mila_light ! Hope someone can give us some news about it.

    For other IS settings, I tried "light importance sampling precomp" without much luck. It doesn't change much things. the noise pattern changes, but no improvement in quality.

  8. #8
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    can you upload your scene?

  9. #9

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    Hello MiniZun,

    thanks for your tests - which version of mental ray are you using, 3.11 or 3.12? Light importance sampling has been extended and improved in 3.12,
    for 3.11 it was working only at the level of the single lights (each light was importance-sampled separately from the others) while in 3.12 the mechanism
    works as a whole, on the whole set of lights, but you are expected to see a benefit if your scene has many lights and not just one.

    The user_ibl_rect light has its own importance sampling mechanism, and you should not use it with light IS enabled in the core - there are indeed bad
    (and unwanted) interactions between the two mechanisms.

    Best regards,

    Enzo Catalano

  10. #10
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    Here it is ! (it's a rar file containing a .ma and the map in exr, juste rename it and extract)
    scenes.mi

    Maya 2014 SP1

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