Get HDRP light intensity at specific distance from a light source

Hi,

Is it possible to get light intensity at specific distance (specific point ideally) from a light source? On the CPU I mean.

Cheers!

 

One year later, still wondering the same question. I'm most unsure about the rectangle and tube area light.

 

It's gone through a few name and iteration stages since ~2019.3 LTS

But it may be worth checking out the Render pipeline Debugger under windows > Analysis/render pipeline > render pipeline debug ( LTS /tech stream in use depends on location and naming) firstly.
DOCS

This may give you a good indication of how intensity is calculated at different stages.
See automatic histogram too

This all goes through the HDRP Volume framework on custom passes for the most part so great for manual debugging purposes.
If you require your own scripted feedback, the Volume framework API may also help to do something similar.

The two may help you get some insight on how to get to what you want to achieve.

 

Thanks for the tips! I will check it out.

I need to do this in built player though, at runtime, so I hope it will be possible to port these debugging solutions somehow.

I can't simply use raycasts since I need to know not only light occlusion, but specially the actual intensity. That can be influenced by so many things... light cookie, fadeDistance etc.

 

Couldn't you just calculate it for the specific light?
I mean it's basically the usual distance squared falloff with an additional distance falloff based on the light range.
So in general it's;

Code (CSharp):

1. attenuation = pow(saturate(1 - pow(distanceSquared * range, 2)), 2) / distanceSquared

All needed functions can be found in;
https://github.com/Unity-Technologi...elines.core/ShaderLibrary/CommonLighting.hlsl

Also worth reading;
https://catlikecoding.com/unity/tutorials/custom-srp/point-and-spot-lights/

 

Yes, you are correct, this works well with point light and cone shaped spot light.
Pyramid and box shaped lights can also be handled relatively simply I guess.
But then you have rectangle shaped area light where falloff changes based on angle etc. No idea how to handle that one, at least not with a solid accuracy.

 

I tried searching HDRP shader code for Rectangle Area light calculation.

However I failed to find the exact piece of code that determines how much light intensity diminishes at specific lit (within light range) point. I'm not really extra proficient at shaders. I only managed to find BoxDistanceAttenuation function in CommonLighting.hlsl

So I reproduced it on CPU but the results don't really match. Not sure if that's because I got it wrong, or because that's not even the function I should be looking into

Maybe it's the PolygonIrradiance function that handles this? From my experiments it seems that the BoxDistanceAttenuation makes sure we quickly attenuate at greater distances close to the light range. It almost doesn't change at lower distances, even though it's there that the light intensity diminishes the most in my case.

The worst part are variables called 'ltcValue', 'diffuseDimmer' etc. I have no idea what they mean and how they might or might not affect the result I'm looking after.

Anyone? I call for thee Mr. @bgolus ?