Iray Light Experiments: Exposure Level for Low Light Scenes
RGcincy
Posts: 2,839
As is true for many DS users, I’ve had problems setting up night and interior scenes using Iray lighting. Many scenes were dimly lit and it took a lot of trial and error rendering to find tone mapping and luminance settings that worked. Setting light bulbs and flames to real-world values was nowhere close to adequate, so I would start adding 0’s to the arcane kdc/m^2 value, sometimes reaching numbers in the tens and hundreds of thousands to light a scene adequately. Then the next scene would require the same hit and miss approach again.
If you search for help, you frequently find the adage “you just have to think like a camera”. I use a DSLR’s and know how to manipulate f/stop, shutter speed, and ISO, but when I walk into a room, I don’t think about how to set my eyes, I just see with whatever light is available. I wanted something that simple for Daz Studio. I have a pretty good feel for how much light a 60 watt bulb or a candle will cast in a normal room, and default Iray render settings do not give me the expected amount of light.
So I started doing some experiments with Iray, keeping it simple. Light bulbs were set to 60-150 watts, candles to 13 lumens, flames to 150-300 lumens. Dome lighting and camera headlamps were turned off so all light came from added scene lights. Since the default Exposure Value (found in Render Settings pane under tone mapping) created renders that were too dark, I started reducing it by half and then half again until I got what I felt was an acceptable lighting level.
What did I learn? Set Exposure Level to 1 and you are mostly done. It seems to work in many situations as shown in the experiments below. At worse, you might set it to 2 if the scene is a little too bright or 0.5 if a little too dark, but it appears to me you get close to the right light level with little need for trial and error.
The results of nine experiments are described in the following posts. I’ve listed my render times for many of the images. They are based on using a GeForce RTX 2080 Ti GPU so are faster than you will get with a less powerful GPU.
I've also documented these experiments in a PDF which is attached.
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Experiment 1: Candle light
Searching Google, a candle outputs about 13 lumens of light so that’s what I set Luminance and Luminance Units to in the Surface Pane. I used default Iray render settings other than setting Environment Mode to Scene and varying Exposure Value. EV is an inverse variable; reducing the value results in more light. The camera headlamp was off, so the only light source is the candle.
Exposure Value = 13 (default). Very dark, candle barely lights even small parts of the lantern it is in. 13 seconds render time (render stopped automatically at this point).
Exposure Value = 8. Now parts of the lantern are visible but image is still too dark. 13 seconds render time.
Exposure Value = 4. The figure is visible but seems too dark for how close the candle is to her face and body. 2 min 56 sec render (5000 iterations).
Exposure Value = 1. This is more like what I would expect to see in real life. 2 min 56 sec render (5000 iterations).
Conclusion: Using an Exposure Value of 1 with real-world lumen settings gives a good result.
Experiment 2: Flaming Torch
Searching Google, a flaming torch outputs about 150-300 lumens so I used 300 lumens. Iray render settings same as experiment 1.
Exposure Value = 13 (default). Very dark, flame barely visible, no surroundings illuminated. 8 seconds render time (render stopped automatically at this point).
Exposure Value = 8. Now a tiny bit of the wall is visible but image is still too dark. 8 secs render.
Exposure Value = 4. More of the building is seen, but for the size of the flame, the area illuminated is relatively small. 8 sec render.
Exposure Value = 1. Much of the wall is lit. The steps and hints of alcove walls are also visible now. This looks to me to be a reasonable expectation for a torch-lit hall. 1 min (5000 iterations).
Conclusion: As with experiment 1, using an Exposure Value of 1 with real-world lumen settings gives a good result.
Experiment 3: Caving
A cave is a dark environment. Using the results from above, I set Exposure Value to 1 and used real-world values for the helmet light.
Helmet light set to 13 lumens.
Second 13 lumen light added behind figure. With twice as much light, I doubled Exposure Value to 2 to balance.
Helmet light set to 5 watts. Exposure Value set to 1
Exposure Value set to 0.5 to give slightly more light.
Exposure Value set to 2 to give slightly less light.
Conclusion: Exposure Values of 1-2 are also a good choice for a caving scene.
Experiment 4: Aqua Subterranea
This is a large stone underground room. It has nine hanging lanterns with a candle in each.
Exposure Value = 1. Candles set at 13 lumens each. Fairly dark. In real life I wound not expect a large room to be well lit by candles.
Exposure Value = 1. Candles set to 60 watts each. Dramatically more light, as 60 watts is equivalent to 900 lumens vs. 13 for a single candle. One could cut back on the wattage or go to a higher Exposure Value.
Exposure Value = 2. This reduces the light a bit to a pleasing level.
Exposure Value = 13 (default). This render shows how the default exposure level is not appropriate when using real-world values for candles, flames, and light bulbs as it is almost totally black.
Conclusion: Exposure Values of 1-2 are again a good choice.
Experiment 5: Motel Room
This room has a floor lamp with a 60 watt bulb as the only light. I used the same Iray render settings as for experiment 1.
Exposure Value = 13 (default). Lamp shade barely visible, no surroundings illuminated. 3.3 seconds render (render stopped automatically at this point).
Exposure Value = 8. Still very dim, not like one would expect from a 60 watt bulb. 41 seconds render.
Exposure Value = 4. The walls and floor are now partially illuminated as well as more of the furnishings. Lamp shade is getting overly bright. 59 seconds (5000 iterations).
Exposure Value = 2. This corner of the room is now fairly well lit. 59 seconds (5000 iterations).
Exposure Value = 1. This is starting to approach over-exposed, so Exposure Value of 2 may be the best choice. 59 seconds (5000 iterations).
Conclusion: Exposure Value 1 is fairly good for this case too, although EV 2 is probably a better choice.
Experiment 6: Motel Room with Spectral Rendering
Spectral rendering is supposed to be a more natural way to render light interactions with surfaces but can give darker results as it does in this case. For this experiment, I’ve set Spectral Rendering to On, chose Natural as the Spectral Conversion Intent, and used cie1931 as the Spectral Observer (these are all found in the Render Settings pane). These are all made at Exposure Level 1.
Spectral rednering = On. 60 watt floor lamp. This gives a result similar to Exposure Level 4 without Spectral Rendering. The walls and bedspread appear to me to have a richer green though. 1 minute (5000 iterations).
Added a 150 watt ceiling light (out of view). With two light sources, the foot of the bed and the edge of the chair now show light, even though the image is still dim overall. 1 minute 41 seconds (5000 iterations).
Added a 60 watt desk lamp. There is more light around the desk as expected. I’ve been in hotel rooms with dark walls like this and find this a reasonable representation. 1 minute 45 seconds (5000 iterations).
Turning Spectral Rendering Off shows how bright the room is with the three lights at Exposure Value 1. The high light level is washing out the green color of the bedspread but is showing the pattern in the wall paper.
Conclusion Spectral rendering is supposed to better reflect how light interacts with walls and other objects but it can make for darker scenes. As in real life, when that happens you can lighten a dark room by adding more light sources.
Experiment 7: Adding a Dome Environment
The bedroom experiments above had Scene Only render settings. Turning on the dome with the default map will add more light as there is a window off to the right.
As in real life, opening the drapes bathes the room in more light. Rendering with Dome and Scene at Exposure Value 1 makes for an overly bright room with the bedspread color washed out. Even though there is more light, it takes longer to render than the scene only examples. 4 min 39 sec render time.
Using an Exposure Value of 1 makes the scene much more light sensitive than the default, which is why the room above is washed out. To compensate, set Environment Intensity in Render Settings to 0.01 to strike a balance. 5 min 15 sec render time.
Conclusion: If you want to use the dome at Exposure Value 1, you need to reduce Environment Sensitivity to a very low number like 0.01 or 0.02.
Experiment 8: Lighter Color Walls
Everyone knows if you have dark walls (paint, wall paper, paneling) you will have a darker room as the walls absorb a lot of light. Replacing the dark green wallpaper in the room above with light-color walls results in a much brighter image.
Exposure Value = 1. Render time 6 min 50 sec
Conclusion: Iray will bounce light around a room. If walls and furnishings are light, you will have a better lit scene just as in real life.
Experiment 9: Another Bedroom
This bedroom has light colored walls and multiple light sources: table lamp, candles on the floor by the bed, ceiling fan lights, fireplace flames, candles on the mantel, and a floor lamp. Only the first two sources appear in the render, the others are out of view off to the right side or above. Candles are 13 lumens, table and floor lamps are 60 watts, the three ceiling fan lights are 60 watts each, and the fireplace flame was 300 lumens.
Exposure Value = 13 (default). Almost totally dark. 5 min 52 sec render time.
Exposure Value = 1. Now it is well-lit. 17 min render time (this used 15,000 iterations as the 5000 iteration version timed out while still grainy).
Conclusion: Exposure Value of 1 works well for this light-colored room with multiple light sources.
<<Jump to Experiment 10>>
There are many ways to light scenes using Iray settings. I thought I'd share the experiments above as this might offers others a starting point for low light scenes using conventional values for light bulbs and flames.
Thank you so much for taking the time to post all of this info. I found it all interesting and useful. It's pretty timely too as I've been doing a lot of low light scenes lately.
Thank you. I have downloaded the PDF and will try some of the experiments. Mosy of my renders are interiors and it has taken me a long time to find a way to work with IRay light. Even so, I am in the dark (pun, sorry) about what all the values mean so it is all trial and error. I did use Ghost Lights a lot for a while but they don't give me reflections or specularty and I find them slow when I have other forms of lighting in the scene. So now I use HDRi lighting with walls and roofs removed to let the light in. A spotlight is also useful in combination.
1st off excellent demonstration and tip. IDK why the DS default camera settings are set to dark even in direct sunlight :(
While we're talking lighting interiors, If I may ad my 2 cents as well
If you want to make your interiors instantly look nicer, and actually get fully lit with out adding extra lights, turn burn highlights/crush blacks all the way down.
a room with all the lights in it set to real world values of 40W
the room isn't too dark but the highlights are massively clipping and look just awful
change the exposure so that the highlights aren't clipping,
but we now live in a dank and miserable cave - one could add fill lights ooooorrrrrrr
the exposure values back from the firs image but I've turned burn highlights/crush blacks all the way down - no more clipping and the shadows are a bit lighter as well
..very nice, Thank you.
Interior lighting has alwasy been troublesome for me, eiher it's too dark or looks washed out.
Oh these are good tips ... keep 'em coming. :)
Thanks for those. Anyone that doesn't have RTX series video cards to do their renders appreciates the time and guesswork you've saved them.
certainly, thanks for sharing your tip!
Good stuff. Those first series are using everything default, only changing the exposure values?
I am eager to try these ideas! Thank you both for the informative and well documented tips.
Yes, the only other change was scene only, no dome.
Excellent. I coulda used these tips a few weeks back, doing a lantern promo render lol.
Rich, I have a question. As I started trying to adjust emissive lights in an existing scene, I came across "efficacy (in Watts)". What setting did you use for that in your experiments?
The default of 15. That is close to real-world light bulbs. They vary, especially LED's. We have a 100 watt bulb in our house that is 1500 lumens but a 60 watt bulb that is 800 (Efficacy = 13).
I also had difficulty finding al the emissive surfaces when modifying one set - they were hidden in many spots.
Thanks for the info.
You desperately need V3D's Iray Light Manager Pro. It will find all those emissive surfaces for you in a flash and let you change them collectively or individually.
Here is my first attempt. I think it was very successful and rendered quickly - 98% convergence in about 10 minutes on RTX 2060.
@barbult ... that is really nice, realistic-looking indoor lighting. Before I got the RTX 3090 I wouldn't have attempted something with all those emmisives but I would be able to render that now.
Somewhat OT but don't you wish that the artists would soften the razor-sharp edges of objects? For me, it is the biggest give-away that I am looking at a CG model render. I sometimes go over those edges with a blur brush just to soften them.It's not that it can't be done. Compare the wooden chair arms with the vertical edge on the reception counter.
I guess I've never been bothered too much by sharp edges. I'll be more aware of them now that you mentioned the subject.
Try using round corners radius in your shader settings - It basically fakes beveled edges.
a simle scene with 2 basiv primatives - and prfectly sharp edges
with round corner radius. a smaller value for the cube and a much larger value for the cylinder its sitting in
Also if you have "round corners across materials" turned on you can fake join seperate objects I just duplicated the cube and moved it a bit
I use it for plain abstract backgrounds
...same here but have to wait until next week when teh te,mps drop out of the 100s and 90s so I don't blow my system or GPU up.
Been working on an interior scene and having all sorts of fits trying to get the lighting to look teh wamd way I see it in my mind. With the AoA Advanced Lights in 3DL it would be a peice of cake. but Iray is a real challenge. One thing I found out is IES, profiles don't seem to work with photometric spotlights only emissives.