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PBR Lighting Question Extracting Sun and sky intensities from real world data

What guidelines does Epic have to create a well-calibrated lighting scene to test materials? How would you determine the a correct scalar sun intensity value that matches real world data from a calibrated HDRI and lux values of the sun and sky? For example, if you have a lux value of 120000 on a sunny day with a sky value of 8700, You will get a sun to shadow ratio of 13 ( at least that is what I measuered). With lux information, camera metadata, and an stitched hdri of the real world location, can you extract approximate sun intensities? If the sun intensity in unreal has no basis on real world values, is it easier as an alternative to photograph an 18% grey ball and match in game? Still Any employees at Epic, I welcome your responses.

While there are several ways of guessing we’re using values close to lux, the results always end up not convincing at all. Physical directional light and skylight was on the to-do list on the old Roadmap, but having physical lights isn’t a priority for Epic so they’re gone. So basically the best thing to do is as you mentioned, you take a picture from a grey ball, also create a grey ball in UE4 and try to match the lighting to that of the picture. You have to modify the settings so the picture you take are very close representation of how your eye sees it.

Check the sildes beginning at 95 : UE4 Lightmass for Large Console Games (UE4 Lightmass Deep Dive)

Square Enix tried to use “Illuminance-Based Lighting” to unify the different light intensity metrics.

Thanks, [MENTION=698017]Mr R[/MENTION]abbit. This confirmed some of the information that I have found so far with regards to outdoor lighting in Unreal and the problems of matching real world lighting. I am finding it challenging to match the lighting visually to a real world reference in a test map using only a directional light, a skylight, and a calibrated hdri map to a skydome. Skylight intensity and sun directional intensity that look right do not match any sun to shadow ratio that I have measured. I get decent results with a hdri skydome that has a sun included, but with it painted out the indirect light doesn’t look correct at all.

There’s a post where Epic suggested a 4:1 ratio for Sun to Skylight. I also remember then mentioning 12 for the Directional light was close to physically correct.

@Zacd. I have read that post before. That is not enough information. RyanB say “skylight intensity is relative to the HDR source so can’t really give a number there.” There problem is that there is no correct way to derive the skylight intensity from a well-calibrated HDRI and apply it to unreal. Back to the link postted by MrRabbit, SquareEnix sought to use illuminance, the amount of recieved light on a surface, as unit for all the different kind of lights used in Unreal. If you follow their logic, they came up with a sun intensity of 2012.8 and then adjust the exposure as necessary. In practice, that logic doesn’t work. Unreal 4’s lighting isn’t physically accurate and I can’t find a process to even approximate correlating values to real world data. I measured the sky yesterday, on a hot sunny day, and I got a sun to sky ratio of 8 to 1.

In Realtime engines even if the terminology is ‘PBR’ it is almost always going to be 90% eyeballing everything (so many reasons for this). in fact its 50% eyeballing in offline renderers like Vray and Arnold and that says a lot.

There is also the artistic side of things, you can have 100% physically based scene but that doesn’t mean it would look right.

I understand that in the end there will artistic liberties taken. If your statement is correct, I still want how thew 10% physically accurate calculations work in Unreal 4 when you have real world data. Maybe the only accurate way for Unreal 4 is photographing a grey ball in the real world and eyeball match it in game.

Well my practice is to have all three balls in there most of the time: White, Grey, Reflective (I sometimes add dark grey as well + one subsurface skin). so yes if you wish to match a photograph lighting then having as many refs to compare to is the best way to go. This process can be easier if you are the one taking the pictures and setting up the live references. I also agree with some of the previous points regarding sun to sky ratio usually looking right at around 4 to 1 sometimes 3 or 6 to 1 ratio for skylight, depending on the skylight texture.

I just hope Epic realize the importance of physical lights one day… Physically Based Rendering + Physically Based Materials + Physically Based Lighting is the proper PBR setup. The point of PBR rendering and materials is to achieve more true to life picture, and the lack of true to life lighting breaks the whole point of it.

I’m with you Max!
I keep seeing your posts all over the internet in discussions concerning this and other issues with UE lighting.
Sometimes it feels like we’re still in the dark ages despite so many successful creations.
Here’s to a bright future of Unreal and to a proper workflow! :smiley:

Hi. Check out my video tutorial. I have made some tools that can help setup a physically based lighting workflow.

https://youtube.com/watch?v=Tj5IerFef0k