Linear light

So i have a wild theory in my head, that probably a lot of people here might have some info to chime in.

The main thing I am seeign with volumes is that there is no linear relationship between scene linear values in UE to the output of the LEDs.

so if I make a surface in UE twice as bright that does not respond to twice the LED output. at least I think so , dont know much about the controllers and drivers used but afaik they all use some gamma and then tonemapping of the UE output.

Could we get that by using linear to PQ in UE and have the led controllers etc use PQ as well, or even if the LED panels are just gamma 2.2 or someting (i have no idea what those are usually). I can just use a pure gamma transform from Linear to 2.2 (ignoring system gamma e.t.c) and then the output of the LEDs should be linear again, as it clips at 1 linear I need to multiply my scene down by a lot.

So that means , if the LEDs can do lets say 2000NIT but my scene actually has a 8000NIT highlight I need to scale it linearly so that the 8000NIT in my scene is 1 in SDR, which in turn ends up beign 2000NIT on the LED wall, then I adjust my camera exposure to match that range, probably a lot of things would be a lot darker than i real life but that shouldnt be much of a issue. :baby_chick:

Main thing is that we capture those led volumes using cameras that capture linear light, then we tonemap this to SDR if the LED panel is tonemapped allready that would give us double-tonemapping whoch makes volumes look like paintings. Running around with a luminance meter outside is really putting things into perspective, we need just way brighter LEDs to create a environment thats actually similar to the real world .

would be great to hear opinions on that,

Not sure if my answer is relevant at all, as I’m an absolute newbie in UE as well as in CGI. But I use OCIO for UE viewport and its renders. I guess it also can be used for your task.

yea Ive worked in UE quiet a bit outside of VP as well, and it was such a good thing when they introduced OCIO support (at least partially).

If your calibration, colour management and image processing chains are correct, the emitted light at the LED Wall will follow a linear relationship with UE4 exposure. Double camera exposure in the Engine and you will double light on the LED Wall. This requires not only disabling UE4 tonemapping but also encoding of the imagery according to the LED Wall characteristics to produce device independent colorimetry. What you cannot do though is working with absolute light values from the real world which should not come as a surprise given the dynamic range difference. You can however work with relative values and preserve the light ratios, this has been done for over a century now by DOPs.

1 Like

Yea thats what I am thinking, cool. I wonder why all i have seen in VP stages (I have only had a bit of VP experience in less than perfect studios). Was heavily tonemapped and it all looked like printed backdrops rather than what I would expect from a linear relationship.

Absolute real world light values work in my brain, to compare it to what a camera would see, I get what you say though its all about ratios as you said. Now I an intrigued and need to learn how film used to work and how it tonemapped the image.

I am going to try this real world in a few weeks in a real volume and will try it out before on a OLED at home and see if I can get what I want…