4.19 Physical Lights

[anonymous_user_675dcd7a]

@pf_breton

solution: internally normalize the input image to force a range from 0 to 1.0, evaluate the Illuminance at an imaginary horizontal plane and scale by a user defined factor, resulting in lux .

I didn’t fully catch that earlier, sorry.

@pf_breton

It would be a a luminance matching the defined horizontal illuminance, so direct correlation with physical units is actually brought to the surface.

I think I see what you’re saying. I could go outside on a cloudy day with an incident light meter and measure maybe 1,000 lux facing straight up, then capture an HDR panorama of the sky. Back in Unreal the HDR panorama will be normalized and I could plug in my measured 1,000 lux. Right? It’s not a bad idea as long as there’s still a way to get a sunny and non-sunny texture to use the same scale if I wanted (they would output/measure different results in lux obviously).

@pf_breton

AFAIK the whole point of HDR imagery is to capture the full range of luminance, thus eliminating the notion of exposure entirely ( a pixel value becomes fully linear making abstraction of non linear aspects introduced by cameras). then what is left is a notion of scale, like zdepth or height maps.

Correct, any HDR texture used as a light source should have the full range (unclipped, linear), but we still need to know how those pixels relate to real world cd/m2. Most stitching/processing software (such as PTGUI) don’t output in absolute luminance as cd/m[SUP]2[/SUP], but scale output based on the middle EV or some other scale, so the HDR is already “semi-normalized”.

@pf_breton

Besides, I don’t understand what a HDR image captured at EV10 means for you. Can you describe what this means in terms of hdr pixel intensity? What luminance a pixel set to 21.56,21.56,21.56 means in this case?

If I shoot an exposure bracket and merge to an HDR, and the middle exposure was at EV[SUB]100[/SUB] 13 (let’s say ISO 100, f/8, 1/160s), I know that there is *some *relationship between those EV[SUB]100[/SUB] 13 pixels and luminance. PTGUI says that 1.0 in the middle EV should correspond to white in the output HDR. So presumably I could take 21.56,21.56,21.56 measured in my HDR and scale by 13 EV (*8192, or some number/formula) and get 176,619 cd/m[SUP]2[/SUP] for that pixel. (looks like a pixel near the sun)! Another pixel from that HDR might be from the blue sunset sky and measure 0.04 (0.0301,0.0503,0.0860), but when we scale that knowing it was captured at EV 13 we get 327.68 in luminance (246.57,412.05,704.51).

I’m sure I could be making some wrong assumptions about the math, but when I go through the process on HDRs I’ve captured with corresponding incident light readings I’m within a stop or two of where I should be. I can also compare to Unreal’s procedural atmosphere and check luminance with the pixel inspector, it’s close. I’ll leave the exact math to the researchers :). What I do know is that labeling sky intensity as cd/m[SUP]2 [/SUP]as it currently is will confuse people. I am glad you folks at Epic are at least trying to figure out ways to make physical lighting units less confusing