4.19 Physical Lights

[anonymous_user_b81e37271 (anonymous_user_b81e3727)]

@post:16

Hi,

Some more details on the new exposure settings in 4.19 :

• The exposure value from the viewport EV100 slider is defined as Exposure = 1 / (1.2 * 2^EV100)
• The relationship with the PPV camera setting is EV100 = log2( N^2/t * 100/S ) where N is the aperture, 1/t is the shutter speed and S is the ISO.
• The scene color is defined in cd/m², before the exposure is applied. It can be inspected by using the PixelInspector tool.
• To isolate the effect of exposure on the final color, disable the “tonemapper” post process in the show flags.
• Directional lights and sky lights units are in lux.
• When using the “manual” metering mode, the scene can go dark because the default values define an EV100 of 9.91 (where as most scene are made for an EV100 of -1.2).
• When using auto exposure with realistic lighting values, increase “Max Brightness” to 10K and “Histogram Log Max” to something like 16. Otherwise, it will go white.
• Prior to 4.19, the point lights units suggested to be in lumens but it was not in practice.
• IES profiles work in the unit type of the lights.

@
The sky light is definitely not using units of Lux, it actually appears to use units of cd/m^2, could you please clarify?
I tested the sky light by capturing a perfectly white sky sphere (measured with the pixel inspector to have a scene color / hdr luminance of 1.0) and using this to light a perfectly diffuse white sphere. Testing the luminance of the sphere with the pixel inspector reports a luminance value that is almost exactly 1.0 cd/m^2. Which suggests that the sky light is, in fact, using units of cd/m^2 and not lux (lm/m^2). A directional light of intensity 3.14 is required to get similar luminance values on the brightest part of the sphere. This makes sense since a purely diffuse BRDF would just multiply the incoming Illuminance (Lux) by 1/pi to convert to outgoing Luminance (cd/m^2). So either the sky light is actually a luminance value (cd/m^2), or someone forgot to multiply by 1/pi somewhere, but I’m guessing it’s the former.

The second issue I have is that the HDR Luminance values I read from the pixel inspector don’t seem to match up to a given EV100 value. For instance, I created a room lit entirely by a single point light of 1400 Lumens with a temperature of 3000K. The room was roughly modeled off the dimensions of my bedroom, the lumens and temperature values of the light bulb were taken from the actual bulb that lights the room. Using my phone camera (it has a manual mode where I can check ISO, shutter speed, and f-stop) I know that I need an EV100 of about 5 for this room to look correct. Using these values the room does actually appear correct, however, when I look at the pixel inspector I see an average HDR Luminance value of around 20. From the Wikipedia page of exposure values (Exposure value - Wikipedia, table 3) I can see that for an EV100 of 5 I would expect an average scene luminance of about 4 cd/m^2 which does not match up at all with the values I am getting.

I’d also like to get some more info about this exposure formula (Exposure = 1 / (1.2 * 2^EV100)).
Where exactly does this come from? in particle the 1.2 value?
I assume by exposure here you mean the average scene luminance in cd/m^2 that the game will adjust to?

@Daedalus51

EDIT: okay, interesting find! The 1000 lumens light was suuuuper dark, so I had to use 100 000 lumens instead. I thought, what happens if I use candela instead? So with a converter, I found out that my 1000 lumens light would translate to 75 000 candela. After I put 75 000 candela into the lights value in Unreal, it actually looked correct! Like what the heck is happening here? XD

I think your math might be a bit off here. A 75000 candela spot light with a cone angle of 45 degrees would be equivalent to the same spot light with about 138000 lumens. So your 100000 lumen light actually matches up with the candela value you used.