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4.19 Physical Lights

Also LPVs don’t work when using manual metering mode. Reverting back to non-physically based lighting. Thanks for the hype epic. :3

@Uriel.Doyon
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
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.

@Uriel.Doyon
First off, 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 intensity 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 on 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 does this come from exactly? particularly 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
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 35000 lumens. When you set the spotlight to lumens now and decrease the cone angle, you are concentrating the same luminous power into a tighter beam, and hence will get a brighter spot and higher candela value. However, you’re probably not wrong to think that the spotlight doesn’t look correct for a given lumens value. @Uriel.Doyon There appears to be a bug in the code for the SpotlightComponent, in the GetCosHalfConeAngle function. This function is currently returning the cosine of the outer cone angle of the spotlight, when it should actually return the cosine of half this angle. Since this code is used for all subsequent lighting calculations, all the values will probably be wrong. If the devs don’t get to it before me I will try to fix it and submit a pull request to epic.

THANKS for sharing your settings!! Beautiful lighting.
So the sky brightness in this scene is 5000 cd per square meter. What value would this be in lux? Lux is lumen per square meter and the internet seems to suggest that I can simply convert 1:1, therefore 5000 lux? This would be suspiciously simple. :slight_smile: So the skylight has 4% the intensity of the sunlight?

@Xharthok
If you have the average luminance of the sky, then you should be able to multiply by pi to get the corresponding average Lux value (Note that in my above post I actually deduce that Unreal’s skylight units are already in cd per square meter and not Lux as stated by Uriel). You can test this in unreal by placing a perfectly white, 100% rough, 0% metallic, 0% specular, ball in the scene and checking its luminance values with the pixel inspector. A directional light with a lux value of 3.14 (pi) will produce a luminance of 1 at the highlight. Creating a perfectly white sky sphere (make sure it’s luminance is 1 when hovering over sky with the pixel inspector if using bp_sky_sphere, as it has weird output) and using it as the input for a skylight, will produce a ball with luminance values of 1 spread over the entire surface of the ball.

Did physical lighting become obsolete already? Epic?
Thread dates back to January and there’s still no support.

Maybe we have to move this to the “Feedback for Epic” section and bump like crazy? :smiley: Maybe even push this thread via Twitter? I think if we make enough people see and react to it, Epic HAS to respond at some point before it turns into negativity. I don’t like doing it like this, but man, if they continue to ignore this, let’s fight the good fight! :smiley:

@kenpimentel @Amanda.Bott [USER=“14973”]Chance Ivey[/USER] @Kalvothe

Twitter post is out! Share and retweet as if your lights depend on it! :smiley:
#AllLightsMatter :stuck_out_tongue:

Retweeted :slight_smile:

Hey all,

Let me catch up on this thread and I’ll get back to you with one direction or another! Just so you know, I have not been avoiding it, just a super busy time with things going on, I apologize for that!

<3 <3 <3

Thx @Kalvothe !!

Hey guys,
I have pinged some of our teams on this. We’re in the thick of getting stuff ready for 4.20 so give us some time to put together a response. I know you’ve been waiting patiently and I promise I’ll stay on top of it, but I ask for a bit more time.

:slight_smile:

Thanks!

Thank you! Just a suggestion, it was mentioned above, but I’ll say it again. Please, make working presets. It’s a best way to show how everything works together. Scene with super simple exterior and interior. Light scenarios-bright day, cloudy, night. Artificial light in the interior. Dynamic and baked versions. I doubt if anyone will be unhappy with it.

so I may have found why the exposure isn’t working properly. I was looking through the trello ( https://trello.com/c/d1MUZwNk/218-physical-light-units ) today after the 4.20 pre-release came out and found this under the 4.19 physical light post. It looks like the EV100 option under metering mode is missing.

@Daedalus51

That’s just one issue with all of this. You can ignore the exposure in the Post-Process Volume and use the viewport’s EV100 slider as a workaround.

I’ve had decent results with interior lighting and EV100, but exterior lighting is still a little awkward. No clean way of measuring cd/m² for the Skylight/HDRI and intensities for the sun don’t feel correct even with the proper EV. While the Pixel Inspector is super useful, a viewmode for highlighting ranges, similar to false color, would be more beneficial I think.

Have a look here for some context :slight_smile:

https://www.scantips.com/lights/evchart.html

Cheers!

Have a look at this for some context :slight_smile:

https://www.scantips.com/lights/evchart.html

Cheers!

So…completely ignore manual and auto-exposure in PPV and just use EV -1.2?