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View Full Version : Modeling & topology vs. Realistic skinning


redwhitejacket
02-22-2009, 12:53 AM
If we were to take a real human face and zoom in on a micro level to observe the flow of skin cells, what would we see? Would we see a similar topology flow as we do according to many 3D modeling tutorials? That is, do skin cells line up in a circular arrangement around the lips, eyes and ears?

My argument (which might be completely wrong, idk) is that real skin has no particular topology. It is merely pulled, stretched, and scrunched around the face and the body based on the underlying muscle tissue and bone beneath.

So, from what I've seen and done, 3d artists focus primarily on the skin.

- worrying about topology of a model and eliminating 5 sided polys or triangles when possible
- pulling vertices and edges of the skin to form expressions for blend shapes
- using normal maps and texturing to show wrinkles, etc.

But, why not have a method in which artists are focused on creating the underlying facial muscles and skeleton, and let the skin act as a drapery over that (where you could define areas were the skin is thinner and tighter or bunched up and loose)?

I know in newer versions of Maya, I think there's a muscle plugin, but I'm not sure if that is also for the face. Either way, I wonder if or when the day could come where facial modeling tutorials involve artists beginning with skull/cartilage and muscle primitives which they manipulate and tweak to perfection, and let the 3D app create a skin over that which the artist can also tweak of course but instead of pull and pushing CVs, edges, and faces, they'd be worrying about skin properties such as the thinness, thickness, and density of the skin.

Thoughts?

GrogMcGee
02-22-2009, 03:48 AM
Topology isn't about recreating reality; it's about creating something that mimics the reality that is needed. As such the purpose of the topology is organize the mesh in such away that deforms cleanly and is easy to manipulate.

Now, as Laa-Yosh points out here (http://forums.cgsociety.org/showpost.php?p=5665660&postcount=800), the face here has little to no classic topology. It's just really dense. However, that kind of method is not always possible, eg, in video games.

Muscle systems are used help preserve skin volume during deformation. And because deformation looks best when the principle direction of deformation is perpendicular to edge loops in question you can either create a really dense mesh and not worry, or you can follow the general topology method or mix the two.

Topology has no real relation to the real structure of the face in a some sort scientific sense, just a useful functional system for describing the face or other body parts.

The reason for the whole all quads thing has been explained about a bazillion times: triangles and n-gons smooth in unpredictable ways. Further, mudbox and z-brush don't really like triangles and n-gons.


Edit: Regarding the construct and tweak bone and muscle and then drap skin over it. That seems like a whole lot of extra work for very little benefit. Further most of the time we don't tend to look at organisms and think about the bone and musculature that describes the skins motion. Imagine a whale? Do you think of something like this (http://www.whale-images.com/images/killer-whale_breach.jpg) or this (http://z.about.com/d/goaustralia/1/0/g/f/oldtom.jpg)? I betting on the first one. The second ... honestly, I know it's a whale skeleton ... but it doesn't make me think whale.

ansemsnobody
02-22-2009, 04:50 AM
Many 3D artists base the flow of polygons on the facial muscles under the skin.

http://catalog.nucleusinc.com/imagescooked/9658W.jpg

http://www.yorku.ca/earmstro/journey/images/facial.gif

(Just random facial Muscle images from a Google search)


It helps in animation if the polys are already shaped like the muscles you will attempt emulate. (well, sort of shaped like those muscles...)

redwhitejacket
02-22-2009, 05:12 AM
Edit: Regarding the construct and tweak bone and muscle and then drap skin over it. That seems like a whole lot of extra work for very little benefit. Further most of the time we don't tend to look at organisms and think about the bone and musculature that describes the skins motion. Imagine a whale? Do you think of something like this (http://www.whale-images.com/images/killer-whale_breach.jpg) or this (http://z.about.com/d/goaustralia/1/0/g/f/oldtom.jpg)? I betting on the first one. The second ... honestly, I know it's a whale skeleton ... but it doesn't make me think whale.

Ok, so you say "drape skin" method I am suggesting is extra work for little benefit. I disagree with you, and here's why. Consider how much work it takes to create blend shapes (morph targets) for every character in an animation. I mean, for main characters, you can literally have hundreds of subtle blend shapes that must first be created and named, and the animator has to surf through all of those to find the right ones to form the expression they desire for every keyframe (major pose) of the animation.

Having a muscle-bone-skinDraping system would provide a simpler tool set by allowing animators to easily access the muscle and bone level, moving manipulator handles to place the face in any pose they desire. I think, if done right, it could save tons of time. You could have a split screen (or dual monitors) where the animator is moving the muscles and bones on one screen, and watching the characters face change on the other.

I also think it would add for more realism. Again, right now with blendshapes, the artist is responsible for every possible motion of the skin, so all subtle motions must be added by the modeler as well. Sometimes, you might have stiff or plastic looking characters as a result of this.

I believe with the method I am describing, a new level of realism would be added to characters, because simply moving the underlying muscles would manipulate many different areas of the draped skin in both major and subtle ways.

As for the whale example you mentioned, remember you still have access to the skin layer, so you can view it and manipulate it if you want. But you'll largely be creating the creature based on bones and muscles, which would really help give a clear and obvious understanding as to how the character can move while providing added realism.

GrogMcGee
02-22-2009, 05:37 AM
That's just a good rig and if you need the cg to be indistinguishable from reality then you're likely going to use something more than just blendshapes - like a muscle system.

Muscle systems are extremely computational expensive though. You're effectively running a cloth sim - sure computers are getting more and more powerful, and perhaps in the very near future we'll easily be able to run 200k triangle skin simulations in real time - but we can't right now. Furthermore, in something like the face there are hundreds of muscles subtly interacting with the skin above them and muscle around them. Imagine setting that up? All the muscles needed to make a smile or you could sculpt that in 20 odd minutes.

edit: Also, nice Barrak model :thumbsup:

redwhitejacket
03-04-2009, 08:25 PM
That's just a good rig and if you need the cg to be indistinguishable from reality then you're likely going to use something more than just blendshapes - like a muscle system.

Muscle systems are extremely computational expensive though. You're effectively running a cloth sim - sure computers are getting more and more powerful, and perhaps in the very near future we'll easily be able to run 200k triangle skin simulations in real time - but we can't right now. Furthermore, in something like the face there are hundreds of muscles subtly interacting with the skin above them and muscle around them. Imagine setting that up? All the muscles needed to make a smile or you could sculpt that in 20 odd minutes.

edit: Also, nice Barrak model :thumbsup:

Thanks :)

Well, ray tracing and global illumination are already standards for the most part. We have cloth sims, fur/hair sims, water sims, muscle systems, etc. I don't see why adding a skin sim would be all that difficult, it seems like it would be less computational than a cloth sim actually.

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