Full spectral.

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Skywise
Posts: 2
Joined: Wed Oct 02, 2019 7:13 am

Full spectral.

Post by Skywise »

Any plans to restore full spectral capabilities to Lux? Way back with LuxRender
that was a major selling point for me to choose it as my render engine of choice.

But I just discovered that the new LuxCoreRender is not fully spectral and only fakes
it.

I only dabble in rendering and was away from it for a few years. Finally getting back
into it. But I only just now noticed the loss of this feature when I decided to
re-create my Pink Floyd prism logo from 12 years ago so I can render it bigger, only
to be stymied why the rainbow was all wonky.

After some quick research I found the discussions on this so just wanted to chime in
and add my voice to those desiring this feature.

I never noticed before on anything else I did, but now that I know, I will always feel
that my images aren't 'perfect', as my interest is in photorealism, and accurate
dispersion is an important element to that.

Brian


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bnzs1
Posts: 8
Joined: Sun Jan 20, 2019 9:31 pm

Re: Full spectral.

Post by bnzs1 »

hi
not fully spectral and only fakes it
I wonder what makes you think that?
is there somewhere a table-comparison of some functions that are not supported in the LuxCoreRender?
as opposite i can quote this viewtopic.php?p=15472#p15472 "LuxCore does spectral rendering on demand, only when required, both with BiDir and Path: the end result is the same of old LuxRender but it has far less noise and it is a lot faster"
+
something similar to your statement has already been discussed here
viewtopic.php?t=2255

as well as already is written above:
Please upload a testscene that allows developers to reproduce the problem, and attach some ...
Skywise
Posts: 2
Joined: Wed Oct 02, 2019 7:13 am

Re: Full spectral.

Post by Skywise »

not fully spectral and only fakes it
I wonder what makes you think that?
One of the developers themselves, B.Y.O.B. viewtopic.php?p=4745#p4745: "LuxCore does not support spectral rendering currently. The glass materials uses some tricks to simulate it, which are activated if the "dispersion" value of the glass is greater than 0." and viewtopic.php?p=4756#p4756 "Old "Classic" LuxRender is indeed spectral, but it is no longer developed. The new LuxCore(Render) is currently an RGB renderer."

Also, reference the following Github ticket: Incorrect color spectrum https://github.com/LuxCoreRender/LuxCore/issues/262, which references some specific code as a source of the problem with violet not dispersing/rendering correctly.

And as requested, a sample scene and file.

LaserPrism2.png
LaserPrism2.blend
(1.58 MiB) Downloaded 515 times

Sample of 'laser' lights going a through a prism and the dispersion hitting a screen. Volume scattering is used to visualize light beams.

The first 7 are lights colored using RGB values in the usual ROYGBIV rainbow. I can forgive the violet/purple one splitting into red and blue as, since it was defined using RGB, how is the render engine to know violet from purple?

The next 14 are emissive light panels. Each is using irregular data to define the spectral output. Each is 10 nanometers wide centered on wavelengths 650nm through 390nm at 20nm intervals.

Note the lack of smooth change in dispersal pattern from left to right. Only that each dispersion is composed of varying intensities of red, green, and blue. Yellow and cyan are absent in the dispersed light, being composed instead of combinations of red/green and green/blue. On the far right where violet is, the dispersion is only blue and dispersion never changes regardless of wavelength. Also note that there is always some red present just past green whenever there is any red involved (notably in orange).

It is quite apparent that the light source spectral data is converted to approximate RGB values, then those RGB values are used to calculate the dispersion disregarding the actual spectral content. The resulting rays are themselves RGB values with no spectral content. Else why would a pure yellow spectral light source disperse into two bands of red and green with no yellow content?

BTW, Blender 4.0.2, LuxCoreBlend 2.8.

Brian
kintuX
Posts: 812
Joined: Wed Jan 10, 2018 2:37 am

Re: Full spectral.

Post by kintuX »

Sad, but true...
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