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Explanation of Cloth/Collision Parameters

For use with Poser 7 as well as Posers 5 or 6

The way cloth works requires some thinking about real world objects. This section will be a tutorial in the sense that I'll be instructing you to do things, but not things with Poser. This is a 'life' tutorial in a sense. We're going to take each property and figure out what it means in real life, so that we can apply it to Poser!

by Paul Jenkins

The way cloth works requires some thinking about real world objects. This section will be a tutorial in the sense that I'll be instructing you to do things, but not things with Poser. This is a 'life' tutorial in a sense. We're going to take each property and figure out what it means in real life, so that we can apply it to Poser!

Collision Offset:

In the real world, collisions occur almost instantaneously, but if one goes down to the atomic level, you find that objects actually start moving before they are pushed. Odd, I agree, and difficult for most physics professors to either explain or understand, but true. This is sort of the nature of collision offset.

The collision offset parameter tells Poser to start figuring out what will happen before it happens, so that it won't have to do the calculations on the fly. For instance, if a hand will touch a sheet, Poser can either start calculating the effect and working toward that effect well before the contact occurs (high collision offset), or wait until it happens risking things not looking right (low collision offset value).

The benefit to setting this high is that the effect will be more fluid and natural. The drawback is that calculation time will increase as Poser needs to think more about what will be occurring, and think ahead.

Collision Depth:

The Collision Depth is a sort of "buffer zone" between the two colliding objects. I think of it like a pair of magnets. Ever try pushing two magnets together when they have the same polarity? You usually end up with them floating slightly apart instead of actually touching.

In the same way, Collision Depth tells Poser to create a zone around the object you'll be colliding against, so that the cloth won't touch it. This helps with avoiding accidental poke-through, and making clothes appear to stand off the skin a bit as they would in real life. If you look at the shirt you are wearing, it doesn't touch your skin at all points. This is because the universe is using a slight collision depth value.

A higher setting may cause the cloth to look static, or less movable, but will make the object much less likely to "poke through."

A lower setting risks poke-through, but may cause the cloth to look more "fluid" and natural.

Static Friction and Dynamic Friction:

This is where the real life tutorial begins. Take your Poser manual and set it on the carpet. Push it slowly along the floor. Feel free to make "Toot! Toot!" noises.

What you're looking for here is the Friction. There are two types, static and dynamic. Static friction is the amount of applied force it takes to push the book, and dynamic friction is the amount required to keep it moving. Try this experiment with other things, as well. You'll notice that while it may be difficult to get your couch moving, once it is in motion it takes far less exertion to keep it going. This is because the static friction is very high, and the dynamic friction is not so high.

In Poser, friction will be used in the same way it is in real life. By increasing the friction you can make it "sticky" or "slick".

It is important to note that friction will also require density. A heavier object will be more affected by friction than a lighter one. A good example of this is the "sliding" problem with cloth on a figure. At first I tried to fix this by increasing the friction, but that's not enough. The object needs weight to have friction affect it. A good example is to take a silk shirt and drape it on a chair. It might be hard to keep it draped, even though it has a medium-high friction. Do the same with a leather or vinyl piece of cloth and you'll find it will be easier to keep in place. It has more weight to keep it stuck, even though it has less friction than the silk.

Fold/Shear/Stretch Resistance:

These three concepts are the hardest for me to understand personally, and will take some time to keep clear in mind. To start, we need some cloth, time to raid the closet.

Coming back from my closet I have a leather jacket, a wool jacket, a silk shirt, corduroy pants, and a pair of mittens. Now let's start seeing what I can do to figure out what "folding", "shearing", and "stretching" means.

Folding isn't too tough. The ability to fold. Flexing my clothes I find that leather has a high fold resistance while silk has a low fold resistance. It takes more to fold the leather and wool than the silk and corduroy, and more still to fold the mittens. So, thinking about this I might make a table to show me the difference in "fold resistance" that looks like the one below:

Lowest: silk

Low: corduroy

Medium: wool

High: leather

Highest: mitten

So if I want to simulate silk, I need to set this value really low.

Shearing is getting more complex. A shear isn't like bending or folding, it's more like "support." It's the ability of the cloth to hold its natural shape. For instance, when I drop my leather jacket on the floor (a fairly common occurrence) it still somewhat maintains the shape it was in. When I do this to my silk shirt, it collapses into a heap.

We can also consider this with the mitten, which actually requires me to stomp on it to lose shape. The wool jacket has a medium to low sheer resistance, caving in on itself pretty easily.

So our chart for Shear Resistance would look like this:

Lowest: silk

Low: Wool

Medium: corduroy

High: Leather

Highest: mitten

Stretch resistance is pretty easy, but much harder to analyze with what I have, so we'll raid the sewing cabinet. Our search reveals a rubber band, a piece of thread, and a piece of leather.

To measure stretch resistance I'm going to try to stretch these objects. I pick up the rubber band, pull a little. Pretty simple. Low stretch resistance. Doing the same to the piece of thread I can pull it somewhat after some tugging. It stretches, then breaks. High stretch resistance. Of course, in Poser the thread will never break, but then, that would probably make us mad if it did. ;)

Now I try the leather. I grunt. I puff and pull. I strain. I quit. This won't stretch. Leather has a very high stretch resistance, apparently.

Stretch damping is one of those things I'm still a bit off with. I think that this term involves how much the object pulls back when you stretch it. I'm considering the rubber band again. When I pull it it gets harder and harder to stretch it more and more. I think that this value increases the stretch resistance as the object is stretched. This would explain why it looks "syrupy" at high values, and "unpredictable" at low ones. The unpredictability would be very predictable if you consider that the object will just keep stretching until its resistance overcomes the force acting on it. Thus, a low value here might make the cloth stretch very, very far with very little force.

Other Dynamic Values:

Cloth Density is how we determine what the cloth "weighs." This dial literally means X grams/cm^2 where x is the dial setting. That didn't help me much, so I sat down and started calling experts. I called Joanne Fabrics and Hobby Lobby and said "How much does a square centimeter of silk weigh in grams?" Surprisingly, they didn't have the answers, but promised to find out if I bought some fabric and sewing supplies. I'll make a full list of material weights as soon as possible.

It is important to note, however, that adding weight affects many things. It will increase friction and the effect of gravity. It will make it less likely to be pushed by the wind. While Poser figures don't have to worry about wearing 400 kilos of clothes, the effect of extreme weights will be very noticeable.

Cloth Self Friction is just like the other forms of friction but applies only to the cloth affecting itself. Cling Wrap is a perfect example of high self friction. It alleges to stick to everything, but all it ever seems to stick to is itself. ;)

Air Damping is the same as saying "air resistance." A good example is the difference between a feather and a piece of leather. While a piece of leather is barely affected by wind (high air damping) a feather is very affected, to the extent it falls slowly and is blown wildly about by the slightest wind (very low air damping). Another example of low air damping is silk. Anyone who has worn a silk shirt on a windy day knows exactly what I mean.

Collision Friction is a poorly labeled box. This should say "Check this box if you would prefer to use the collision friction settings. Leave it unchecked to use the cloth's friction settings." I think you get the point.

I hope this helps. I'll follow this tutorial in the next few days with dial settings for real cloth types.

Breaking Wind!

Heeheehee! I love that title. Anyway, it's time to get into the physics of wind! I'm pretty excited about this, because it means that maybe all those dull fluid dynamics classes had some bearing on my art. The thing is, you won't need to be an aerospace designer to understand this. If you are, you probably don't need this tutorial.

To begin, we need to forget calling it wind. It's not. It's water. Huh? Well, tell me, what's the difference between moving water and moving wind in how it affects the world? I'll give you a hint. There is none, but because we can't see wind, it's harder to think about. So, let's stop thinking about wind, and start imagining water. For all practical purposes, they're the same thing.

To start with, we need to figure out what things mean. The four properties we are most concerned with are Amplitude, Spread Angle, Turbulence, and Range. Some of these may be apparent, but what might not be apparent is how they interact, so, even if you know what these mean, please follow along carefully so to avoid overlooking something.

First of all, we have amplitude. A better way of considering this is how much force do we have? Take your hose and start spraying. Turning the knob to the right increases the pressure and the speed that the water is coming out at. This is the same. Higher amplitude means the water (or wind) is going to be moving faster and with more pressure on things that it hits. I tried to sketch this relationship in the below image. With the illustration I'm considering the "cloth plane" attached at some point just above the wind force.

Spread Angle seems apparent, but might require a deeper look. The angle at the beginning will affect things at the end. In Sketch 2 I've presented a description of how the water flows over the object in its path at different ranges. For a fan, spread angle would be large and the range short. For a straight in your face wind the angle will actually be rather low and at long range.

Turbulence is far more touchy than anything else. Turbulence is (or should be) based on a random seed. That means that the higher this value, the more random all other factors become. A highly turbulent wind will move in many different directions and with greatly varied force. This means that the angle an object is being struck at will change from place to place with a high turbulence, and the amount of amplitude will also vary from place to place.

Sketch 3 attempts to illustrate what's going on with variations in turbulence. This effect is what makes a plane rock, shift, drop, and raise suddenly in flight as more pressure is added or taken away from different parts of it's wings.

Range is the length of the force. In real life a wind just slows down until it ceases to be moving. In Poser, however, it's more like it travels at the same speed from the start until it simply hits a wall and stops. I've tried to show this in Sketch 4.

So how does all this work together? Well, if we imagine a few types of water or wind, we can get a good idea of what are settings should be. For instance:

Fan: high amplitude, low turbulence, short range, high spread angle.

Summer breeze: low amplitude, low-mid turbulence, long range, low spread angle.

Hurricane: Very high amplitude, very high turbulence, long range, high spread angle

Now, for a bit deeper look at how wind will interact with gravity, we'll need to consider vectors.

A vector is simply a direction and amount of force. A good way of considering vectors is to look at sketch 5. I have drawn lines (vectors) to illustrate the direction in which gravity is moving and the direction in which our "wind" is moving. The dashed line is the "resultant" vector, or the direction in which force will be moving. This assumes that our forces are equal in strength, and is pretty easy to understand- but-

What if they're not equal? What if our wind is stronger? Well, in this case the force will be moving more to the side than straight down, so the dashed line moves toward the wind line. Conversely, if the wind is less than gravity, it moves toward the gravity line. Basically, the resultant vector will always be closer to the stronger force as shown in sketch 6.

While it's easy to consider this from forces moving at a 90 degree angle, I took some time to sketch out other angles in Sketch 7. The important thing to understand is that colliding forces negate each other, while forces going in the same direction add to each other. While I doubt you will spend hours drawing out every force in a scene, it is important to making forces work to at least have a basic understanding of how those forces interact.

Tips:

1. To speed calculations close your library palette and switch to cell-shaded tracking.

2. Start by working with one variable at a time to simulate real effects.

3. Remember that dynamics work over time. Try running your simulations for 30 to 60 frames and take your pick of the best for the final still.

4. If you get an unexpected result, write down what you did to create it. Track all of your dial settings and post them so that others can try. Sometimes unexpected results require many people to troubleshoot, and sometimes a result you don't want might allow someone else to do something new and creative.

I hope this tutorial assists you in all your Poser work. I'm sure that in time you'll be a master. Patience, hard work turn into deep understanding and skill.

Paul