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Spring Question

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Hello all. I have a question about spring equation. I am making a fabric simulation and I wrote a simple spring simulation code to simulate spring and dashpot (the one which stop spring from moving too much, something like shock absorber). The code is here:
vec3 spring_dash = box_array[Box_n2]->particles1[name1]->pos - box_array[Box_n1]->particles1[name2]->pos; 
float length = spring_dash.GetLength();
float displacement = length - originalLength; 
vec3 spring_dashN = spring_dash.GetNormalised();
spring_dash_f = spring_dashN*(((displacement)*K));
int extra = 1;

box_array[Box_n1]->particles1[name2]->addforce(box_array[Box_n1]->particles1[name2]->getforce() + spring_dash_f * extra);
box_array[Box_n1]->particles1[name1]->addforce(box_array[Box_n2]->particles1[name1]->getforce() - spring_dash_f * extra); 

As you can see I calculate the spring force and then apply it to the two masses, which are connected to spring. When I run the simulation, the springs get stretched too much due to masses and because I am using Euler I can't increase the K more than 90. Now the question is here: How can I make the springs to get stretched for example between 1 and 1.2 units? I mean how can I enforce the springs not to get stretched too much or gets too small? This is also very important as fabric becomes more realistic. Thank you so much in advance.

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Yes, that is correct, but I have seen some people who use Euler but he wrote a function that enforce spring to get streched, basically this function keeps the lenght between the desire numbers.
Is there any of you who know what I can do to keep the spring lenght between a specific range?


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In the linked article below, Mr. Gaffer shows the basics of springs(which I guess you already know).
But near the end of the article he mention that he will show in a future article how to handle what you are talking about, I guess.
Perhaps you could contact him and ask for a little preview?


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I did email him and invited him here, I hope he come here and give us some feedbacks.
meanwhile it would be great if anybody else know how we can do this?!

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Original post by bargasteh
I did email him and invited him here, I hope he come here and give us some feedbacks.
meanwhile it would be great if anybody else know how we can do this?!

i think the technique youre looking for works like this:

instead of using euler, a (velocity-less) verlet integrator is used, which is just as simple to code. then, instead of applying forces, constrains are enforced by altering positions of the masses to conform to the springs length. this is repeated until the solution converges.

i believe there is a paper by some guy who worked on hitman who explains this technique. google for hitman physics and youre bound to find something.

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One solution is to move the particles with some algorithms to correct the elongations; this one is harder than it seems because if you move a particle you modify also the spring length of the neighbours and it may introduce instability in the integration or even further over elongations.
In other words the risk is to introduce a cascade or a "domino" effect of over elongations.
However this method can be interesting to explore.

Another solution is to use a non linear spring coefficient: small when the spring is near its rest length, big when the spring is compressed or elongated.
This one is interesting because it simulates the physic behaviour of a real cloth in which the stretch resistance is something similar to an exponential; the drawback is that the problem becomes more "stiff" so to integrate the equation you have to reduce drastically the time step and/or modify the integrator.

When the problem is so stiff you probably cannot use Euler/Verlet but use at least the classic Runge-Kutta IV. RK IV is very simple to program but it does not work very well with stiff problems.
I could suggest you to use an implicit integrator but they are very hard to program although they are very very fast (you can use a GREATER time step). Consider that almost every real-time cloth simulation ( and not only ) use implicit schemes; be sure that with Euler and Verlet you will go not so far in these kind of problems (although they can be good enough in different situations)

Implicit schemes are usually simpler when you use a non physical correct model (like those used in demos and in games) because you can simplify the equations to program the solver.

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Thank you so much John Schultz.
When I finished reading the articles I became really confused over the whole thing, verlet and fixed time step and (e.g. "10% delta rule") :(

you were talking about:
which is look really promising. I think this structure is a lot better than the other one.
Could you please tell me what stepps I should take in order to have a fabric simulation running, using above structure?
I know you have talked about it on other threads but could you just sum up the whole thing so I will have a clearer idea about it.
I suppose:
1-I should have a verlet integrator running (currently Euler).
2-fix my time step, I am currently using this:

float t = 0.0f;
float dt = 0.24f;
float currentTime = 0.0f;
float accumulator = 0.0f;
const float newTime = time();
float deltaTime = newTime - currentTime;

if (deltaTime>0.24f)
deltaTime = 0.24f;

currentTime = newTime;

accumulator += deltaTime;

while (accumulator>=dt)
updateScene (0.024);//when i put dt here, the whole
//thing explode
accumulator -= dt;

3- change my structure to above one from:

A--B--C between A and C I have folding spring as well.

Thank you

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