what is spectral rendering exactly?
CMKook-24601
Posts: 200
I am trying to understand it's function... I mean when I did a sample render on a project i was working on I could see that it rendered things differently in the sence that the pixels came in a differenet way but the quality didn't seem all that much different... but admittedly it was a sample render and kind of small
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Seems to about rendering a scene using red, green and blue wavelengths, which can then he combined inside a photo editor. This page explains it reasonably well:
https://en.wikipedia.org/wiki/Spectral_rendering
oh! okay, so its just a "More Real" light bouncing and interaction with in a scene... nice
The reverse - a standard render uses a mix of red, green, and blue while a spectral render will specify the colours exactly using their frequency.
okay, but the long and the short of it is that Spectral render makes the light behave more realistically as it bounces off surfaces such as skin and clothing... is that what I am to understand?
No, nothing to do with bounces...it uses the full spectrum/more wavelengths. Basically, with 'sunlight' it will use the whole range from infrared to ultraviolet.
right a more realisitic light... i guess light source... eh I get what you mean I know what I mean LOL :P
So why wouldn't we always use spectral then?
I found the Spectral Rendering, but what's faithful vs natural, and the difference in the two Observers? (that sounds creepy.) And please explain in simple terms, not jargon. I'm not technical at all.
Faithful, as far as I can tell is 'scientifically accurate', while natural, is more 'photographic'.
The two observers are different algorithms for the calculations...not sure of the differences, yet.
And why not use it all the time...slowness.
This took about 6 hrs (architectural and caustics samplers enabled, too) while the non-spectral version took about an hour
What if you just use the spectral feature and dont turn on the Architectural rendering, is it faster or no?
The conversion of RGB colour values to spectral values, but this is not an unambiguous process. The Natural and Faithful conversion intents are two different ways of going from colour value to spectral value - the former is smoother, but can produce quirks such as over dark reflections of saturated colours. Faithful is less smooth but avoids those artefacts.
The observers do the opposite -they convert from spectral values back to colours for output. They use two different models to do that job:
cie1931 = uses the CIE 1931 2 degree standard observer as color matching functions (https://en.wikipedia.org/wiki/CIE_1931_color_space)
cie1964 = uses the CIE 1964 10 degree standard observer as color matching functions (https://en.wikipedia.org/wiki/CIE_1964_color_space)
So you're going to love me asking this, but I think others may want to know too- architectural and caustic samplers do what- and when combined with the spectral, do they change it in the same manner as when they are stand alones?
I did an 'outdoor' render with an HDRI and while slower, it wasn't as slow as that interior. The interior without Architectural wasn't any faster. It would probably be a bit faster with more light (levels). It's not going to be a simple just do this and....it will take experimentation to find the best settings.
The Architectural sampler can boost the speed in some renders...other's it's not helpful. But it does help clear graininess on interior renders.
Running some tests here in my Art Studio thread. There's not much I can contribute so far as technical, but I can do test renders
The typical render without Spectural Rendering vs with it- Spectural took THIRTEEN MINUTES OFF THE RENDER TIME.
Now we need spectral profiles for light sources. ;)
Spectral rendering changes the way light is broken down and handled. In traditional rendering, we use Red, Green, and Blue light values. Everything color related is based on those three primary light colors.
But light isn't like that in reality. RGB uses 3 frequencies of light, to represent ALL light. Real light uses a near infinite variation of frequencies between 430 and 770 THz (390nm to 700nm wavelength).
Spectral rendering instead uses MORE frequencies to better represent light that exists in the scene. Light color (and surface colors) are converted to CIE representations, and more accurately broken down and IOR/abbe values utilize the actual frequencies rather than just plain old RGB. Lights that use actual Temperature (K) values will actually emit the right color frequency, rather than a RGB approximation, which will then be used to drive the caustics/IOR/abbe equations.
Spectral rendering doesn't do much without transparency, correctly defined lights, and caustics. With it, transmitted light, bent light, and caustics will be much more accurate and realistic.
As far as the Architectural and Caustics samplers, refer to this: http://blog.irayrender.com/post/51722647664/the-architectural-and-caustic-samplers
From my own observations: Without the Architectural sampler, areas of more light are completed faster than areas of darkness or low light; With the Architectural sampler, all areas are completed at the same rate. Before getting the new computer with GTX-1080, I preferred using the Architectural sampler, whether or not it increased the render time, because I could stop the render when the light areas stopped looking grainy, and the dark areas were equally done.
I have an image I created last year that I've been reworking using Kindred Art's Iray Ghost Light Kit. The last version is posted in the "Iray Ghost Light render and support" thread in The Commons, and was rendered using Spectral Rendering On, Faithful, cie1964. Previous renders used Architectural Sampling On.
The original image is 2560x1440, so I set the render size to 1280 by 720 to test the Spectral Rendering settings. Other settings I used include Max Samples to 5000, Quality "off", and Max Time "0". I then rendered the image 6 7 times, with the following settings:
I didn't bother trying all the Spectral Rendering settings with Architectural Sampling On as the Spectral Rendering alone took little more than 1/3 the time to render. At only 5000 samples, all images had some grain in the dark areas—probably only noticable at high magnification—but the Spectral Rendering images were less grainy. The cie1931 setting, whether Faithful or Natural, left the corridor behind the open door and acolyte, (upper left,) completely black. After I determined Spectral Rendering On, Faithful, cie1964 was the optimal setting for this image, for both speed and final appearance, I set the size back to 2560 by 1440 and rendered to 15K samples. I don't have the exact time for the original, but the Spectral Rendering at that size took about half the time. I do not, as yet, have a render without either Architectural Sampling or Spectral Rendering.
(The final image is in my gallery, here.)
ETA: Added actual render times, and also a "control" image without either Spectral Rendering or Architectural Sampling on. Also to mention I'm using one GTX-1080 on an 17-6900K based PC. Changes are in blue.
I have a question. I've noticed when I try and render a scene with "Faithful" turned on it renders light from every light in the scene, but if I render using the "Natural" setting it only renders light from the sun/HDRi scene lights and surfaces with 'emitted' light. No spotlights, point lights or distant lights will render. Is that the way it is supposed to be?
Although I don't know the answer to that question specifically, I did notice in my test render that a spotlight in the background was ignored when the Spectral Observer was set to cie1931, but used when Spectral Observer was set to cie1964. Have you tried rendering the scene set to Natural with cie1964 also selected?
Excellent info here - time to play with this new feature.
It is noise however, not grain.
Iray is physically based, but that is not the same as unbiased. Unbiased engines are always in Spectral mode, such as Octane. Iray isn't in Spectral mode by default. In most cases images from Iray usung only RGB will be visually similar enough to unbiased rendering that it will be acceptable. But this spectral lighitng is just one of the subtle differences between truly unbiased and merely physically based rendering. So what I am saying is that enabling Spectral Rendering is actually removing bias, increasing accuracy, and often taking longer to complete the render. It would also mean more noise, because there is less cheating involved with the light calculations. Overall, the new lighting is a good thing. Getting Iray speed at full Spectral to compete with Octane speed at PT or PMC is a fair test comparison as the quality of the renders is now going to be much more closely matched.
Which is awesome, because it gives you more control to make decisions about realism vs. speed of render and whatnot.
One of the things I like about 3DL is that I have the ability to go from 'not at all realistic' to 'reasonably realistic,' and decide where I want to dial my render to. If it wasn't the fact that highly realistic 3DL was approximately like Iray in CPU mode, I would be inclined to stick with 3DL. But, of course, GPU rendering changes the decision.
I really need to make a spectral render with a ghost... ;)
Hmm. You know, I'm doing a test with a prism, and for the life of me I can't see any clear difference between spectral rendering and not, despite this being an obvious situation where you think there would be.
The setup is a very tight spotlight (.05 degree arc) aimed straight down into a prism with abbe (dispersion) and a little SSS.
On the left is just caustic filter. On the right is caustic + spectral rendering.
I suppose there's a very slight difference, but... huh.
Refraction and caustics is still controlled by the material IOR (and Abbe number). It's not clear if spectral rendering has been implimented any further than just surfaces, reflections and indirect.
The Observer Intent seems to work as expected:
CIE 1931, reduces blue significantly and adds a bit of green and a bit more red.
CIE 1964, all the colours get a tiny boost, but blue is boosted less than green which is less than red.
Can you confirm what I see, (I've been looking at these images too long) 1931 has made the floor more purple, 1964 has made the whole image a bit brighter .
I just notice an improvement if i use Spectral Rendering. I had these strange fireflies on my scene that disappeared when i use Spectral Rendering. Same rendering time -- 2 min 100 iterations. Fireflies start to appear around character nails after 50th iteration.
I use spectral rendering by default it also makes the translucency/sss of your models react much more realistically!
EDIT: Sorry, for necroposting.
In my experience the folk that wrote the theory & algorithms for spectral rendering might claim improvements in the realistic qualities of your renders but what I noticed with my own eyes it that it flattens out the perceived depth of your render and makes the lighting much, much dimmer that not using spectral rendering. At least it has for my with DS PB 4.12+
I believe there's something wrong with special rendering in the Beta.
It's never been the lights dimmer previously....