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Thread: Prism rendering

  1. #1
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    Prism rendering

    I tried rendering a realistic prism refracting some light rays without any luck (in 2018). Does anyone know if this is possible in the new LW?
    I tried using several techniques to get the light color splitting effect. I even turned on caustics but nothing worked. I used the dielectric shader and the pbr shader. Even with the dispersion turned to 10000000 the refracted licht is still white. Also giving the dielectric shader some roughness did change a thing which in my opinion is strange because I would expect the licht to get blurred by the refraction/roughness but is did not change anything. I tried using GI on/ Gi off ... still no color separation.
    Any ideas?

  2. #2
    Super Member Kryslin's Avatar
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    I think while it can be faked in Lightwave - any version, use a gradient and a weight map - it can't be done following the laws of physics in the renderer.

    I suspect that there are rendering methods that can do it (spectral rendering comes to mind), though.
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  3. #3
    RETROGRADER prometheus's Avatar
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    You should be able to get color dispersion with dielectric, you do need some roughness in dielectric for any dispersion to get the effect working, if you have that set to zero..it will not work.
    Not sure distant light would work, try spherical and area lights.

    I donīt think the native lights is actuall doing anything, using sunsky environment or hdr imaging to get a full spectral lighting condition.
    you can check use background color to hide the background environment.

  4. #4
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    As far as I can tell, LW doesn't implement correct refraction for rays of Light, which would provide spectral dispersion of Light rays in LW2018. In both LW2015 and LW2018, enabling volumetrics to see the beam, the beam of the Light passes straight through an object with an Index of Refraction (IoR) above 1.0, implementing refraction as if the Light beam were a physical object.

    For example, here's LW2015 with a Spotlight with Volumetric Lighting enabled and a luminescent red straight Rod parented to the Light. The rod passes through the Prism (IoR of 1.52), and you can see the refracted image of it. The refracted image of the Light beam follows the same path as the refracted image of the rod, which is physically incorrect.

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    In the real world, when a beam of Light enters a refractive volume (e.g. something made of glass) with an IoR that's above 1.0 (IoR of vacuum is 1.0, for air it's about 1.0003), the beam changes direction towards the normal of the plane of the surface. Because different frequencies (colors) of Light refract at different angles, the result is a spectrum / rainbow.

    Hypothetically, LW2018 implements a "physics based renderer", but the behavior of refraction for beams of Light appears to be the same. Here's the same scene converted into LW2018 with Volumetric Scattering enabled to see the beam of Light. Intensity Falloff has to be set to Inverse Distance squared (rather than OFF as imported) to see the Volumetrics!

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    So, I think to get a spectrum exiting from the prism as well as the correct change in direction for the refracted beam of light, it'd have to be faked up.

    mTp

    P.S. I should mention that "faking it up" isn't very easy for an animation where the relative angle of the prism and the light changes during the animation, and it may be impossible (or more difficult) in LW2018 because LWDG removed the Light-specific nodal control over the Light's volumetrics (a really bad decision, IMO!), and the nodal setup for the Light itself doesn't allow real-time control of Spotlight cone angle or other parameters. Here's a nice video to show the changes in the spectrum intensity and shape as the prism rotates relative to the incident light:

    https://www.youtube.com/watch?v=8eK-kFBgQU0

    Note the shadow of the prism to identify the location of the light source.
    Attached Files Attached Files
    Last edited by MonroePoteet; 10-30-2018 at 12:09 PM. Reason: Attach LW2018 scene file; P.S. on complexity of faking it

  5. #5
    Lightwave has a new physical renderer, but it lacks a number of things to make it do that trick. Dielectric still lacks true dispersion, it also needs a pathtracing engine, and to be truly accurate, spectral rendering. There's only s handful of renderers which support such advanced physical simulations, and mostly they are incredibly slow!

  6. #6
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    Yes, I can believe it'd be very compute intensive to raytrace that degree of accuracy. It can be faked up with results that are adequate for a few simple (and *inaccurate*) animations, but trying to do (for example) the physically-accurate simulation of a multi-faceted prism, a cut diamond, etc. would be very slow.

    Here's a simplistic animation in LW2018:

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    MOV file: Prism_2_FakeSpectrum_2.mov

    To re-iterate, the behavior of the refracted light (the spectrum) is WRONG. To have it behave accurately, the node network in the Nodal Motion of the SpectrumLight would need to calculate the angle between the incident SourceLight and the refractive surface's normal both for the beam entering the prism and exiting the prism. As well, the intensity and shape of the spectrum / light exiting the prism would change as the incident angle changes. Since LW2018 removed the Light specific volumetrics and the Light's nodal network doesn't provide access to anything but its various Colors, I don't think a realistic, moving prismatic refraction would be feasible even in this simple case. I think!

    mTp
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  7. #7
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    The problem in LW is that there is no way of having slightly different refracting indexes for rgb colors I would expect dispersion to fake this effect but apparently it doesn't.

    This is what I'm after. https://vimeo.com/95755327

    Maybe we need a dielectric shader 2.0.

    There seems to be something terribly wrong with refractions in LW2018. If you shine a light through a surface with some amount of refraction blur is doesn't blur the light. I would expect the light to be refracted and be blurred but it's not for some reason.

  8. #8
    The dispersion effect in dielectric does split the 3 channels, but it's inefficient and causes splits, because it the 3 refraction indeces break up, instead of spreading into a spectrum. Spectral rendering us probably a while away, but stochastically sampling the dispersion is possible.

    There's no way you're going to get it working in volumetrics, without bi-directional path tracing.

  9. #9
    Electron wrangler jwiede's Avatar
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    Quote Originally Posted by Polygons View Post
    There seems to be something terribly wrong with refractions in LW2018. If you shine a light through a surface with some amount of refraction blur is doesn't blur the light. I would expect the light to be refracted and be blurred but it's not for some reason.
    How are you setting up the surface/material where you're trying to cause refraction blurring?
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  10. #10
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    Quote Originally Posted by jwiede View Post
    How are you setting up the surface/material where you're trying to cause refraction blurring?
    It's just a prism with a spotlight on it. The prism has a dielectric shader on it. The spot that shines through doesn't seem to be influenced by it at all except from coloring it if you add some color to the dielectric.

  11. #11
    geo messy madno's Avatar
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    Just for fun a test with Iray in free Daz Studio 4.11.
    imported geometry -> set a Daz Studio build in spot light -> assigned Iray Ubershader to materials -> adjusted some sliders -> added an HDR environment -> rendered.
    Time was 18 min (one GTX 1070, one old Quadro K6000).

    Click image for larger version. 

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  12. #12
    geo messy madno's Avatar
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    Another quick test.
    I am really impressed. It just works. No hacks, just a spot light and adjusting settings and materials.

    Click image for larger version. 

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  13. #13
    RETROGRADER prometheus's Avatar
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    Quote Originally Posted by madno View Post
    Another quick test.
    I am really impressed. It just works. No hacks, just a spot light and adjusting settings and materials.

    Click image for larger version. 

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    Cool ..will have to take a look, if it works with daz studio 4 that is.
    Have to check what it is that does the trick, thanks for the heads up.

  14. #14
    Dreamer Ztreem's Avatar
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    Quote Originally Posted by prometheus View Post
    Cool ..will have to take a look, if it works with daz studio 4 that is.
    Have to check what it is that does the trick, thanks for the heads up.
    There's no trick. Iray is a bidirectional path tracer that's why it can do nice caustics.

    When a software like Daz studio has a more advanced renderer than Lightwave you start to realize which state Lw is in, it's a sinking ship(or more like a sinking small boat).
    Last edited by Ztreem; 11-09-2018 at 06:44 AM.

  15. #15
    Not so newbie member lardbros's Avatar
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    Quote Originally Posted by Ztreem View Post
    There's no trick. Iray is a bidirectional path tracer that's why it can do nice caustics.

    When a software like Daz studio has a more advanced renderer than Lightwave you start to realize which state Lw is in, it's a sinking ship(or more like a sinking small boat).
    Of course... very logical. Arnold doesn't do this either, so what does that mean?
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