How complicated could a candle be?
biggerhammer_3af0e1eec5
Posts: 14
So I've been playing around with Daz. A scene that I had in mind was an actor (Caprice) in the Alchemy Chasm fromscene builder in the basic Genesis package. I was hoping to have dramatic lighting with candles.
So far, I've had really dim lighting with candles. Using twenty candles doesn't seem to improve matters much. I suspect that I would have to have Caprice roasting directly over the candles, inches away, to get much light. And that isn't the kind of scene I had in mind. 
I tried to figure out IES, but everything I could find was either childishly simple or fiendishly complex. I have as much physics understanding as your average Joe who does not work in industry, so that has so far been a dead end. If anyone knows where I could find an IES file for a candle, I'd love that- Google hasn't turned one up.
What I want is a candle flame that emits light, very brght up close, fading as distance increases but still provide at least enough light to get shapes at around six feet distance. I have figured out emitting surfaces, but the light fades out at such a short distance that it is not useful.
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I cranked the single candle I'm testing with to 12000 lumens, which is around as much as an American street lamp. It bares lights her face when held at arm's length. So lumens is not the key.
The key is moving the emitter out of my model's ankle. False alarm.
I still have a lot to learn.
Iray is a PBR renderer, so light falloff should accurately emulate the real world.
While you said you solved the issue, to get some nice candle light you should turn the light's temperature way down low (like, 2000 degrees) and set the lumens to real-world values (about 15 lumens). Then you use the tonemapper to adjust the exposure. If you're just starting out, all you need to do is adjust the Exposure Value, but if you know what you're doing you can use real world values for shutter speed/aperture/ISO (as long as you adjust the "CM/D ^ 2 Factor" to 10.0 to compensate).
Thank you. I'll try the simpler solution now and come back to the more accurate one later.
I don't think the default values for lights have any relation to real world values. A 100W light bulb is enough to give bright light to a large area, but if you set 100W light in DS, you can see that there is a bulb, but it doesn't light up anything.
I use 120000 Lumens/90 degrees for my spot lights and that is usually enough to create a "normal" lighting indoors.
What were your tone mapping settings? I've used a photo of a room as a guide to what they should be and the render seemed reasonably lit then - if anything a bit bright, as I recall, but that may have been down to the lamp shade and different reflectivities.
I'll check when I get back home. Haven't changed them since I found ones that have given me the results I was comfortable with.
My tone mapping settings.
I remembered using ISO 200 as that was what I usually used on my camera, but when I struggled with the lighting, I don't remember seeing much difference (if any) by trying to use values that I would have used on a real camera, although I haven't tried changing the cm^2 Factor in here.
My usual lighting (preset scene) is 3 spotlights at 120000 Lumen/Sphere/90 degrees, one in front and two back on both sides and that produces "normal" lighting for indoor scenes. Environment mode Scene Only.
Those are not indoor settingsd, certainly not indoors with artificial light. They seem quite tight even for a bright day outdoors.
For this shot, I mimicked a real-world candle. Point light. Geometry set to Sphere, width to 1cm. Lumens at 15, temperature at 2,000.
I mimicked the exposure settings Stanley Kubrick used for Barry Lyndon. Shutter speed of 48 (twice the framerate) and aperture of 0.7 (so low he had to order special lenses from NASA). The ISO is the last resort; if you get your other settings right, you crank that up until the image is bright enough. Here I set it to 800.
With the CM^2 Factor set to its default, 1.0.
With the CM^2 Factor set to 10.0.
Here's a still from the movie, which was filmed using only candlelight, for comparison.
As you can see, if you increase the lights tenfold, it brings the values up into the expected range. I don't know why Iray operates at a tenth of real-world values, but I've tested this with HDRIs and scene lights, and it does.
I think they are pretty much the default ones that DS uses, which makes one wonder why they have chosen such as default.
Setting the cm^2 Factor to 10 made all the difference, I was able to take the lumen values down to 1200 and the scene was still brighter than with my original values and now even the candles looked like they contributed.
The last time I fiddled with the settings was over a year ago, when I was still rendering on CPU with every render taking hour(s), so I lost patience quite soon and once I found ones that gave me enough light I saved them and have been using them ever since.
One caveat I forgot to mention: HDRIs naturally load with a value of 2.0. I don't know why that is the default, but it makes them twice as bright as they need to be. If you don't have any scene lights, you can set the CM/2 Factor to 5.0 (i.e. half) to get the same light level. If you do have scene lights, leave the CM/2 Factor at 10.0 and just bring the HDRI Environment down to 1.0.
Because the default environment setting corresponds to a bright day outdoors, for which the default exposure is reasonable..
I did some candle renders a few years ago. I went back and updated them as I now have a better understanding of the lights and render settings in Studio Iray. I only changed the skin and hair settings.
2021-08-14 00:40:58.840 Total Rendering Time: 2 hours 36 minutes 20.63 seconds
Emission Temperature 1850
Luminance 80 Watts
Luminous Efficacy (lm/W) 12.76
One Candle
2021-08-13 21:59:51.050 Total Rendering Time: 1 hours 56 minutes 37.96 seconds
Emission Temperature 1850
Luminance 80 Watts
Luminous Efficacy (lm/W) 12.76
One Candle with four instances
2021-08-13 17:16:32.021 Total Rendering Time: 2 hours 7 minutes 1.36 seconds
Emission Temperature 1850
Luminance 80 Watts
Luminous Efficacy (lm/W) 12.76
One Candle with four instances.
Click on image for full size.
Settings
Environment Scene Only
Emission Settings for candle flame.
A search for exposure settings candlelight turned up several pages, the ones I looked at seemed OK with 400 as the ISO and went for a much slower shutter (four to a maximum of fifteen, 1/4 to 1/15 second) and a somewhat more open lens (lower f-stop).
It depends on the light settings for the flame. If I change the wattage or the Efficacy I would need to change the Tone Mapping settings to compensate which then changes the reflections and shadows.
As a test i loaded the Baroque Grandeur Iray version, and added some (floating) tableware. All the emissive flames were set to 1 Lumen, ISO 400, f-stop 2.4, Shutter speed 4 (1/4 second). One render uses cms^2 factor of 1 and gamma of 3.4, the other uses cm^2 Factor of 10 and gamma of 2.2. The Gamma 3.4 is a little grey, but it's also lighter than the cm^2 10 version so I'm not sure if that is an actual drawback of using gamma or just that the one is lighter than the other. Both had about the same render time and convergence.
Gamma 3.4, cm^2 Factor 1
Gamma 2.2, cm^2 Factor 10
The gamma 2.2, cm^2 factor 1 version, not shown, was overly dark I think - and very noisy when it finished rendering, perhaps because the drakness was allowing large areas to be effectively converged very rapidly while the bright spots were still very far from converged - so I agree that applying the suggested values for a candlelit scene was not spot on here - however, many of the surfaces are quite dark and others are very shiny in a large room, so much of what they relfected was dark due to distance from the lights. On balance I would still try rendering without adjusting the cm^2 factor in the first instance, but with an awareness that some adjusting (though that or through gamma) might well be needed.
I'm biased (obviously) but I think the second one looks more realistic. The contrast between pools of light versus deep shadow are exactly what I expect to see with candlelight (especially those light streaks on the tableware, which look great), whereas the faint gray ambient hue you mentioned in the first picture just looks like the gamma was cranked up.
@Richard Haseltine
You have to be careful of what settings are used for the flame. One lumen is not the output of a candle, it is actually 12.6 lumen which is 1 Candela.
https://www.quora.com/Roughly-how-many-lumens-is-a-candle-flame
Thanks, I had found soemthing that said they were the same - I didn't entirely believe it but went with it for lack of anything better.
Yes, that's what I meant by saying it was grey - but they aren't really matched, so i wasn't sure it meant adjusting the one value is better than adjusting the other or if it was just that they were not quite doing the same thing. Still, if my lights were too dim anyway it may well be that neither adjustment was necessary.
Playing about with the Gamma settings in images doesn't really help either.
https://www.cambridgeincolour.com/tutorials/gamma-correction.htm
The idea being that a higher gamma pulls up the darker tones, at the expense of compressing the lighter, to give a more useful result. it shouldn't be overdone because it tends to make things grey (as anyone who has overdone the grey slider in a levels dialogue will know) but it can be useful to tweak. That page is about camera capture, so not really the same thing, though it certainly doesn't mean that gamma adjustments (beyond the default 2.2) will be useful either.
Use the Burn Highlights and Crush Blacks to do that. Which is basically using the Contrast settings in image software.
Isn't that the opposite - collapsing values close to the limits to the extreme (0 or 255)?
Lowering the black on its own lightens the shadows. Lower the Burn Highlights on its own darkens the blacks. Balancing the two can vary the results.
I usually just keep mine on .96 for both which seems a good setting to me.
OK, I'd always taken them as like the black and white point sliders in Levels/Equalisation.
Isn't that all the Contrast slider does? It lowers and raises the tonal contrast between the Black and White mid ranges.
https://pixelsandwanderlust.com/the-difference-between-clarity-sharpness-and-contrast-sliders/
I don't think so, I believe the Contrast effect actually chnages the shape of the histogram while the black and white points just clip it and stretch it out to fill the full width again - but that page isn't very specific about the algorithm used.
Well, the answer appears to be maybe - depending on how the image editor in question actually performs contrast enhancement https://iq.opengenus.org/contrast-enhancement-algorithms/