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Ab anaticulae existo or The Slightly Philosophical Approach to Sub-Surface Scattering

By Kosta Atanasov

Part 0:
Introducktion

If you want to make a transparent object you are painting appear realistic, it’s certainly not enough to know where the layer’s opacity slider is. Light has a distinct behavior within translucent objects, and just like in anatomy, where the artist’s understanding of underlying forms makes his paintings more convenient, the understanding of this behavior will help us identify the effects we see. More than that, it will help us prospect how an imagined, non-existing object would look if it was to exist. But what exactly is existence?

Click on the image below for enlargement (of the image):

FIG.-1

Click on the pic below for a better look (on the image, sorry):

We paint what we see, and what we see is nothing more than reflected light. The image we perceive through our eyes is transferred to the brain through the optical nerves, where it gets processed and the object is recognized. Things we imagine, however, are also produced in the brain and trigger exactly the same “strings” as the pictures we receive through our eyes. So, since we have no way to tell the difference between things that are real, and things that we imagine, how do we know what is real? Well - we don’t. Throughout the ages, a great number of scientists and philosophers have wondered does the world we see actually exist, or is it all in our minds (fig. -1). For the purpose of this tutorial, we will just have to assume that the world we live in actually exists and is guided by some certain physical laws and principles, a small part of which I will try to explain in the following study.

Part I:
What is SSS?

So, what does triple S stand for? Nope, it’s not one huge S. Those tree letters mean SubSurface Scattering. And don’t get fooled by the name either – if you envision the mechanics of subsurface scattering somewhat like the one on the right (fig.0), I’m sorry to tell you that you are wrong once again. To put it simply, SSS is a property of light. It occurs in basically every object, which allows light to pass through its surface. And that doesn’t count for objects you have to drill a hole in first. While with solid, opaque objects (fig.1) light reflects without penetrating their surfaces, with translucent ones it enters inside the object. Once there, light doesn’t just continue traveling in a straight line, like we imagine it would. The light rays, just like very small and somewhat fast pin balls, bounce off the tiny particles, which build the objects structure and scatter, changing their direction and color (fig.2).

FIG.0


FIG.1	FIG.2	FIG.3

When you cast a strong light on the back of a finger, you can see a visible glow on the edges of the side, opposite to the lighted one, where light shouldn’t be visible (fig.3). When light enters the object, it hits a particle and splits. The split rays also splits, and so on. Reflected from particle to particle, light can actually travel inside the material mass, no matter what curves the upper has (fig.4). Because of that, light often comes out of the object in a place, where we normally wouldn’t expect it to, if it was traveling straight. That causes another effect – since light can exit in unlit, darker areas, objects with SSS seem to have an inner glow.

Dependent on the color of the material, the exit light color changes as well. For example, human skin is full of blood vessels, and light colors red. So if you’re really, really curious if kings have blue blood, you don’t have to kill one to find out – you just have to make one sit still and light him properly. Another myth busted!


FIG.4	FIG.5	FIG.6

All materials have maximum depth, at which the light can penetrate to. In the ideal case of having a single consistent material (let's call it "Translucency depth") (fig.5). For example, lets say a fly makes an emergency landing in your cup of milk. You would be able to see it clearly when on the surface of the milk. The further into the milk-mass it sinks, the blurrier it gets and the more it fades into the milk's color, until it finally disappears (remember not to drink that milk) (fig.6). (Note that no flies were harmed in the creation of this tutorial. They were drawn rather then drown.) A good analogy of SSS effect is like having a fog (with the material's main color) and depth of field effect(focal point at the surface) combined. However, though for our purposes we can treat the blur and the colored fog as the same distance, their translucency depth may differ in real life.

The translucency depth depends on the density of the material (which, in the upper example, depends on the species of the milked animal and the date it took place) (fig.7). Light intensity reduces exponentially from A to B. In dense materials Translucency depth is shorter, and longer in the more translucent materials. This SSS value is also interconnected with light intensity. The brighter the light – the more translucency depth will stretch and light will be able to pass deeper into the object.


FIG.7	FIG.8	FIG.9

With SSS materials we also have the effect of shadow depth. If we have a cast shadow over the translucent object, we will note that its edges aren’t crisp, but it blurs into the underlying object (fig.8). The dark of the shadow seems to penetrate the surface, scattering underneath it. Since shadow is actually the absence of light, light enters the surface around the shadow area, and spreading wider, causes the shadow to narrow in depth. This effect will also be visible when another object is inside the translucent one – the non translucent object will cast narrowing volume shadows (fig.9).This effect will be close to what we see when bright light enters a dusty room from a window. Any objects, blocking the light’s path, will cast visible volume shadows. The sole difference would be that the environment, in which these shadows are being cast, is much denser, and due to SSS of light we get the narrowing and diffuse look of the shadow.

Part II:
How to paint SSS?

Knowing the these principles, you can paint SSS using any technique you want. The purpose of this tutorial is to explain what to do, rather then how to do it, so I will not enter into much detail in this part, and presume you are familiar with the software you use. Here is how I paint when I'm aiming to get realistic results: It's pretty much software independent, though I personally use Adobe Photoshop- all the upper illustrations for this study were created using this technique.

1. First, paint the object like you would a normal non-translucent object.


FIG.10	FIG.11

Normally, I start with a quick sketch (fig.10), then clear the outlines and proceed to filling it with color. Usually, I pick neutral grey. I then shade it, using the Burn and Dodge tools in Adobe Photoshop (which basically darken and lighten the image). For the first shading I paint a soft shadowing ambient – It’s the kind of lighting you nearly always will have present in the unlit, darker areas, no matter what lighting you chose. I use soft brushes with different brush sizes. When the ambient lit image is done, I add colors, using colored layers with different overlay modes (fig.11).


FIG.12	FIG.13

2. Now it’s time to kick some triple S! As explained previously, where translucency depth is longer than the volume thickness, light will pass through your object. The thinner the volume, the more light will pass – and the more of the colors of the background will show through. Imagine that your translucency depth is a stick, colored with gradient, white at the base (where light enters the surface) and transparent at the end (where the material will no longer allow light to pass).

I try to keep in mind the volumes of the object, while mentally sticking the stick from back to front of the object, so it’s perpendicular to the camera, facing the eye of the viewer (fig.12),. I paint in the color of the translucency depth stick in the point where it hits the surface facing the eye. Don’t feel bad about backstabbing the object that way – you are painting the SSS equivalent of ambient lighting – something that you will have in nearly every lighting condition. There is no need to be precise here – just paint approximately what you think the passing light intensity is at the exit point.

In most cases you can enrich the image, by adding another layer – background bleed – just use the fig.13 layer, by applying “levels” make the gradient much thinner and closer to the surface, and use it to select and copy over the object the background layer colors.

Again, you can go through this process any way you want to. I use white to transparent gradient because I often use the layer’s transparency afterwards for selections, and because I can see the layer underneath (the object itself) as opposed to white-to-black gradient. The illustrations here are presented in white-to-black for viewing conveniece only.


FIG.14	FIG.15

3. After we did the overall base for the image, its time to get into the real lighting. Next step is basically like the previous one, only you must stick your translucency depth stick somewhere else. Nope, guess again. Place it along your key light vector. In this picture we place our key light on the left, so we point the sticks from left to right along the Path of the light Beam (fig.14). Once we get this layer done, we’re ready to finalize the painting.

4. Normally, I would use “overlay” mode for the two layers, playing around with the opacity value (quick example - fig.15). If needed, you can duplicate one of them to strengthen the effect. If you have a really translucent material, you can select the layer transparency of the first SSS layer, copy the background colors on top of the object (depending on the level of refraction, you can offset or even flip the image vertically) and blur them according to the material density. Don’t be afraid to experiment with the layers, you might find your own, even more convincing way to paint SSS. Be brave – Undo is a mighty weapon to have at your disposal, a luxury classical artist do not posses. Your image should be now more or less ready.


FIG.16	FIG.17

5. At this stage I would normally flatten my layers (fig.16) and play around with Dodge and Burn tools again. I’d also add a specular layer, and an ambient color (environment lighting) layer (both shown combined at fig.17). Try changing your Dodge settings to “highlights”, and refine the SSS areas a bit more. Do it with a larger brush and a single stroke, so you can use the “Fade” option from the “Edit” menu. It allows you to adjust with what opacity to apply your last stroke – very useful when fine tuning your image.

6. You should be ready to print it and show it to your girlfriend. Watch out with the translucent rotting zombies, but if you’ve painted a cute gummy heart, you should be in for a good time.

Remember, your painting doesn’t need to be 100% accurate. Stay close to what you think is true, and you will probably get results which will look good enough – and as long as they are convincing, they don’t need to be physically correct. In fact, I don’t pretend that what I’m suggesting is scientifically correct; it just helps me produce believable results. I’m not a scientist; most probably you are not one too. After all, we’re doing artwork, not a scientific study. Besides, if this world doesn’t actually exist, you’d be wasting your time, which you don’t actually have, because there’s no such thing as time, right? Then why waste something you don’t have? It would make no sense, would it? Anyway, in case it does exist – thank you for reading, and I hope this study has been of help to you.

Click on the image above to see better (the image):

FIG.+~

Kosta Atanasov, 2006

The author would also like to thank all the Vray render (Chaos Group) magicians, both for taking the time on several occasions to discuss with him the SSS effects , and for buying him that tripe soup that last time. He also would like to thank Neil Blevins (Pixar) both for his feedback and for his wonderful SSS research, which you can read HERE if you are interested in a more in-depth understanding of the nature of subsurface scattering and it’s appliance to 3D graphics (plus viewing very useful photo examples of SSS). The author would also like to thank Rod Green (Offset Software) both for his feedback and for his great edit, and last, but not least, Bon Jovi (New Jersey) not ‘cause they have any relevance to subsurface scattering in particular, but just 'cause he feels like it. Thanx guys! Rock on, all of you!

Author’s note: I actually plan to publish this tutorial in an offline media. The upper part concludes it - the stuff below is just for fun’s sake, and hasn’t got that much relevance to the matter, as you will no doubt note if you choose to read on.

ADD YOUR COMMENT

The Slightly Off Topic, But Fascinating Facts Welcome to Kosta's Not-a-lot-of-people-know-this:

About the Author:
Can too much heavy metal indeed make you paint cute stuff? Read on to find out:

To learn more about Kosta
click anywhere on the pic, except on his face

Kosta Atanasov has been fascinated with subsurface scattering since early childhood, when he lost his first underwater goggles in the waters of the Black Sea, by the shores of which he currently resides. Since then, he has scattered many other objects below it’s surface, including an unfortunately high amount of silver bracelets and earrings (though he’s a guy). While emerging from the subsurface once, he hit his head on a buoy, discovering the hard way the Archimedes buoyancy (a painfull law of nature). Since then, he has began to write in this particular manner. Other than that, he has worked in game development more than 7 years doing concept art, illustration, modeling, texturing, rendering, rigging and animation, plus another long list of development tasks he should not be doing at all, like, to name an example, renaming long, boring series of gifs. He has recently graduated “Animation Direction” it the “New Bulgarian University”, and currently works as a Character Artist on Project Offset - a highly innovative nex gen epic fantasy game.

About Archimedes:
A swell guy or an evil genius? The man behind the legend:

To learn more about Archimedes
rotate the handle of the mechanism below:

One of the greatest scientists ever lived, he was born in Syracuse, once ancient Greece – now in Italy. He was a genius philosopher, physicist, mathematician and engineer, and many of his inventions are being used even nowadays, like the pumping screw mechanism (which is not what you just thought it is, but the mechnanism on the illustration). Anyway, he is not as widely known for the discovery of the principles of density and buoyancy he found while taking a bath (to which the author is referring in the beginning), as for the fact, that in his excitement he went out and roamed the streets naked, yelling “Eureka!” (The ancient Greek chicks must’ve digged that). Obviously, he was a scientist far ahead of his time. People are only beginning to do that now in the stadiums, and they even don’t scream anything worth mentioning. Another legend about him tells us about his ingenious usage of polished soldiers’ shields. Making an array, focusing and reflecting the sun at the roman fleet, attacking Syracuse, he sets them on fire, saving the city (Another reference made by the author). Never mind his seeming addiction to weapons and the improvement of their destructibility, he was indeed a great scientist (some claim, that he is a past incarnation of the more famous Leonardo). Unfortunately, he is dead. Legend has it, that Archimedes was slain by an ignorant roman soldier, while drawing circles in the sand, and his famous last words were "Noli turbare circulos meos“ (Do not disturb my circles). Interesting fact: same were reputed to be the last words of a little grey alien, trespassing on a crop field, to a farmer in Texas.

About the Rubber Ducky:
Groundbreaking insight! Its occult nature unveiled:

To learn more about rubber ducks
click on the eye of the smaller one, facing east:

The history of the rubber ducky is shrouded in mist, and goes a long way back in time. Though the first rubber ducky encounters have been documented in the early 1800s, rumor has it, that in the lost book of “The Necronomicon”, attributed to the mad Arab Abdul Al Hazred, rubber ducks are mentioned as “corruptors of the equilibrium”, spawning in this world “from beyond the gates of Urizen”. Another source, renowned author and researcher Elena Blavodkaskaya, speaks of it as a “forsaken remnant of an early, catastrophically failed genesis” and disputes the “frighteningly absent expressions of these horrid objects” and “their terrifying squeaking, when sat on”. Another reference we find in the bestseller book of Eric Fon Heineken “ChEErios of the Gods”. He writes: “These huge, ancient drawings of them on the Nazca plateau (southern Peru), these vast geoglyphs, can be nothing else than a energetic buoy for the ships of the arcane star travelers.” Other than that, a fascinating fact by itself is the mere existence of rubber duck races, in which the last are released into a stream of water, and the fastest ducky emerges victorious.

The Way, Way Off Topic Stuff Well, why the hell not?

As always, when trying to paint a photo-real rubber ducky, I try to put myself in its place. To see what it sees. To feel how it feels. To become a rubber ducky myself. I ask myself questions like “What if I didn’t like the chick I was forced to bathe with? What’s worse, what if it was a guy?” It was one of those moments, when inspiration hit me on the head (just above the buoy scar) with the spiritual equivalent of a sledgehammer and I wrote this short, epic poem, answering all these questions. Here goes:

One small yellow dot

The borderless sea is in horrible rage
In maddening anger it's seeking revenge
To hold on to it strives a small yellow dot
The currents below want it tied in a knot
And guided by hate are the pillaging waves
To smash it to pieces each one of them craves

They’re lifting it up and then throwing it low
They try to subdue it, but they simply don’t know
The taller they rise, all so scary and black
It will not be frightened, and will never turn back
Determined and proud, it will always go on
Now walls of its prison are banished and gone

Forsaken all boundaries it’s finally free
One small rubber ducky set course open sea
And strange as it sounds, sure as hell wouldn’t trade
The pounding strike of just one single wave
Its rampaging rage and its bone chilling cold
For all the hot water bath-tubs in the world

Thanx for reading, and remember:
Catapultam habeo. Nisi pecuniam omnem mihi dabris, ad caput tuum saxum immane mittam.
(If you happen not to speak latin, click here for the translation)