LuxRender vs LuxCoreRender (Prism Scene)

[Jack Eden]

Hi, First off I want to say thank you to the development team for their amazing work on this project. I am a huge fan of LuxRender and worked on a number or tutorials and wiki pages a number of years ago. I stepped away from 3D work for a while but now I am trying to stage a comeback and would love for the new LuxCoreRender to be a part of my toolkit. This is my first post to the LuxCoreRender forum, but I am using the same name and avatar as the previous LuxRender forum, I don't know if anyone will recognize me.

I thought I would start off by converting a few scenes over to LuxCore, rendering them and uploading the results to help do some promotion work. This was my first attempt at doing that and I have run into a few snags. I think I was able to fairly accurately convert the scene over but the multi-spectral colors are just not present the way they were in LuxRender. There are some noticeable band gaps in the colors and it doesn't seem to be fully spectral either, just three bands of color, red, green, and blue. I am using a blackbody emitter (10,000K) as the light color, so I should be getting all the colors across the visible spectrum, but yet the band gaps persist. Any help with how to solve this issue would be very much appreciated.

Again, thanks for everyone's hard work and I am so glad to see that this community still exists and that this renderer is still under active development, and I hope to be able to offer a meaningful contribution to the project.

[B.Y.O.B.]

Welcome back, Jack
Can you upload the scene so we can play around with it?

[Sharlybg]

First welcome back and wish you all the best on the new engine.

About the scene there is a huge diference in tonemapping is it possible to show both under same treatment i'm curious about it. in fact i like the smooth natural tone transition in Lux 1.6 but also like the clarity of luxcore.

[Theo_Gottwald]

I can confirm that from own experiments.
To get the full spectrum - like in my Forum-picture,
I had to overlay a second - shifted version - of the refraction.

Testing-Scene. Compare left side (rgb) and right side on the Prism (full rainbow).
There is no big diffrence between BIDIR/CPU and "PATH TRACING" about this.
To me it looks like the "Spectral rendering" just uses 3 colors (R,G,B), maybe at least 10 Colours would be more accurate.

A2.blend

(195.79 KiB) Downloaded 281 times

[Dade]

Some of the difference in the original scene looks like may be related to different max. path depth

[Theo_Gottwald]

Looking at the details also here the "Mix-Colours" between RGB are missing, while having been there in the old version.

Generally the light dispersion is stronger, even though the overal lighting is less.
Let me add that the glass itself looks better in the new version.

In the Luxrender-Example-Scenes is the best example.
Look, it just shows Red, Green and Blue instead of the full rainbow.
Luxrender Samples

[Jack Eden]

Sorry for the delayed reply on this. I've spent quite a bit of time messing around with settings to see if I might have missed something but I just can't seem to find anything that might be causing the problem.

I've attached the two blend files and tried very hard to make them as identical as possible. The IOR of the glass in the two scenes do not match because if I set them to be the same then the place where the light was refracted to did not match. Also, if the power value for the two lights is set to the same across the two scenes then they don't match either, this is one of the reasons for the mismatch in tone mapping.

Thanks for any and all help.

[Dade]

It is a bit hard to say by looking at this scene. Old Lux didn't had lasers light sources, as far as I remember, so they were built using a "tunnel" object and that was both very inefficient and heavily affected by the max. number of bounces.

Can you make a comparison using a very basic setup like this => https://github.com/LuxCoreRender/LuxCor ... enes/Prism ?



Or something even simple, projecting the light on a plane instead of using volumes like in the above images.

[Jack Eden]

I simplified the scene greatly, I took the light source out of the "tunnel" and moved it so it is only refracting through one prism, and I applied an IES file to it to constrain it to be very similar to a "laser" light. The mesh acting as the light source is barely visible in front of the prism, a thin sliver of grey. I think I am still getting the same results, basically just red green and blue with not much in between. If the light coming off the prism is over exposed then some of the secondary colors start to appear, so maybe it is a power issue, that RGB spectral bands are over-represented and that other spectral values are under-represented.

[Dade]

Can you post this test scene ?

Have you tried to replace the IES light source with a true laser light in LuxCore ?