A visual example of using a photographed HDR with photometric numbers in Unreal
Pixel Inspector luminance readings on white sphere:
- sun light only: 4500 (cd/m[SUP]2[/SUP]) = 14,137 lux
- sky light only: 1770 (cd/m[SUP]2[/SUP]) = 5,560 lux
- combined: 6150 (cd/m[SUP]2[/SUP]) = 19,320 lux
These match (close enough to) the illuminace readings I took on location
- 18,900 lux in sunlight, facing sun
- 5,000 lux in shade, facing same direction
- sun+sky - sky = 13,900 sun intensity
My actual Sun Intensity is 18,000 lux because its color is very saturated, something else to take in to account.
Skylight intensity is at 1.0 “cd/m2”, although I still hope they remove the misleading unit label.
Pixel inspector show the deepest blue sky at about 720.0 luminance (cd/m[SUP]2[/SUP])
The skydome material emissive had a multiplier of 23,170 or 14.5 EV.
The HDR middle exposure (mentioned earlier) was at EV[SUB]100[/SUB] 13.32, so I guess you can’t just take the EV number and plug it in to your skydome emissive intensity, but it’s close-ish. Again the difference is probably due to color saturation. It’s always good to have illuminace measurements to be sure, but with most HDRs you’ll never know the camera settings or illuminace at time of shooting.
The results came out okay. I can use manual exposure or EV100 numbers and it looks pretty close to what I’d expect. Something else to be aware of is I’m not using pre-exposure, so numbers around 65,000+ will start clipping, (at least on my hardware they do).
