Light Intensities and Units

[chafouin]

Hello,

So I've been looking at light intensities and I appreciate that LuxCore tries to be more scientifically accurate. Power and Luminous Efficacy (and thus, lumens) is really clear.
If we consider that Cycles use a Luminous Efficacy of 683 (Luminous efficiency 100%), then I discovered that my addon is wrong by 3 EV...

The sun and sky work fine as well, at least if following the sunny 16 rule (bright daylight Exposure value = EV 16 (f/16, 1/100s, ISO 100). I do think it's slightly on the bright side however. When taking pictures with a digital camera in bright daylight, I end up with exposure values closer to 14 than to 16.

However I have been trying to use HDRI I captured and that's where things become a bit less clear.

My HDRI is exposed (in Photoshop) so that, when used in Cycles with a default Emissive intensity of 1, it has the correct intensity with the proper camera exposure values.
However, when used in LuxRender, I have to push the Gain to 10 to match the same exposure. Is there an scientific explanation behind that value?

Secondly, I need to set the Sun Light Gain to 0.1 to get proper intensity. I know that there were a couple of thin clouds in front on the sun during the HDRI capture, but does it make sense to have to do that? It feels that the sun is a bit too bright in general, as mentioned above.

I also wanted to try the Distant light, and I had to use a Gain of 10 000 000, for a Size of 1. Does this have a unit? Changing the size also affects the intensity.

Thanks!

[B.Y.O.B.]

I also wanted to try the Distant light, and I had to use a Gain of 10 000 000, for a Size of 1. Does this have a unit?

It's the half angle in degrees, similar to the Cycles sun in 2.8: https://docs.blender.org/manual/en/dev/ ... #sun-light (just half of that).
We should probably use degrees as unit in the UI.

[chafouin]

It's the half angle in degrees, similar to the Cycles sun in 2.8

Sorry I wasn't clear, I was asking about the 10 000 000 gain.
But yes, having a unit for the size is always welcome.

[lighting_freak]

Hi,

thanks for hearing someone else asking for units and values.
Thanks also for sharing your findings according to luminance values in your render result.
May I ask how you're reading those numbers from the image. I guess youre using the *.exr result somehow.
Do you have a tool that allows you to this?

Have you ever been thinking about adding a human eye tone mapper to your add-in?
This is only one of several papers about this topic:

krawczyk05sccg.zip

(1.63 MiB) Downloaded 179 times

Having right values with orrect interpretation may exceed luxcores abilities in virtual prototyping a lot.
Maybe also some application in ergonomics seems to become doable.

Thanks for your feedback.

BR

[chafouin]

My tests aren't scientific, I'm eyeballing a lot, but here is my process:

- Capture an HDRI with the full range (including the sun, as much as possible)
- Place that HDRI in LuxCore/Cycles.
- Set LuxCore/Cycles Camera Exposure to match one of my bracketed photograph that I use as a reference
- Adjust the HDRI exposure in Photoshop and reimport it in LuxCore/Cycles until the exposure visually matches my reference

Now I have my HDRI exposed to what I would call "absolute radiance". I render an image that I will use as "ground truth" reference.

- Make another HDRI version where I paint out the sun.
- Use the sunless HDRI in LuxCore/Cycles
- Add a sunlight, adjust its rotation, color and intensity to match my "ground truth" reference

To check local lights intensities, I do the same, just not with 360 HDRI images.

You can probably watch my video here as well which talks a bit about that process: https://youtu.be/PcDTLF8hk_g

I quickly read the paper you linked, and, correct me if I'm wrong, but they only mention localized tonemapping, the rest is about reproducing glares and the lack of details in some parts of the dynamic range, which is different from tonemapping.

About localized tonemapping, this is something that makes sense in real time applications, because the camera moves, and it's not elegant to use global fill lighting for all your dynamic objects and characters to make sure they don't look too dark in shadows. Here is an interesting read from God of War (currently the only game I know of using localized tonemap): https://bartwronski.com/2016/08/29/loca ... nemapping/
For still images, localized tonemapping isn't needed because the tools are there in Lightroom and others: you can just lift your shadows area, and tone down your highlights areas.

[chafouin]

I wanted to let you know that I updated Photographer to work with Cycles and LuxCore:
https://gumroad.com/products/FWQf

Here is some information from the changelog that could be of interest for LuxCore users:
- Now that Blender 2.8 is clearly explicit about the lamp units being Radiant power in Watts, I adjusted the default exposure value to make sure exposure is physically correct.
WARNING: This means that old scenes that were using previous versions of Photographer will look darker by 1.61 EV.
- Compatible with LuxCore. You can now easily switch between Cycles and LuxCore using Photographer and physically based light intensities.
NOTE: – LuxCore uses Watts in combination with Luminous Efficacy. Use an Efficacy of 683 Lumens/W to match Cycles light intensities.
– Point lights will match perfectly between the two render engines, but Area lights and Spotlights will unfortunately render slightly differently.
– HDRIs in LuxCore require a gain of 250 to match Cycles HDRIs with a strengh of 1.
– Photographer doesn’t use LuxCore ImagePipeline Tonemapper and relies on Blender’s exposure the same way it works with Cycles, for ease of use. It will force LuxCore Tonemapper to do nothing by using its default exposure settings. Do not be alarmed if the LuxCore Tonemapper doesn’t reflect Photographer Exposure settings.

[zeealpal]

Awesome, Photographer looks pretty handy. I like using real values for things where possible, thanks for supporting LuxCore.

Specifically I have a recently been rendering images for HMI's, (Industrial controls/icons are often so dated) and rendering realistic systems based on the CAD files looks so much better. I can't share the images, but I rendered a orthogonal image of a micro-brewery system, and we used that as a background for a HMI/Scada, and it was very well received at a demonstration stand we had. I'll donate some $$ as it'll be very useful

[Dade]

I wanted to let you know that I updated Photographer to work with Cycles and LuxCore:
https://gumroad.com/products/FWQf

Chafouin, if you want, can write a "News" in forum news section (just update the addon description "This add-on adds new Camera settings for EEVEE and Cycles" + LuxCoreRender ).

[chafouin]

Haha, thank you for pointing it out, I updated the description.
I also fixed some issues with LuxCore Depth of Field F-Stop in the new 2.2.1 version.

And please post it in the news sections if you want, if it can help users

Awesome, Photographer looks pretty handy. I like using real values for things where possible, thanks for supporting LuxCore.

Thank you zeealpal, I would be really interested in seeing what people create with the help of my addon as well, if that's something you can share!

[marcatore]

@chafouin
you wrote that you're not using luxcore tonemapper, right? if yes, I have this question: if I've set my camera with Luxcore ISO,shutter and f-stop and I enable your addon with your manual controls, should I disable the Luxcore controls?