FAQ
Frequently Asked Questions
Why isn’t the movement in the viewport smooth?
The viewport playback speed is entirely dependant upon your CPU speed and the complexity of the simulation.
If you are simulating orbiting planets, say, you should be able to sustain a reasonable 25 or 30 fps on almost any computer. However, if you have designed a detailed scene with many physical objects and full collision detection, expect the frame rate to slow down. Also, don’t expect speedy results if you have loaded an existing scene and simply added some dynamics; it is far more efficient to design a dynamics simulation from scratch.
Here are some tips:
How can I make a sail that will move a yacht when the wind blows it?
To create a yacht’s sail you will need to use soft bodies.
First of all create your sail and mast, then add the soft body and rigid body tags to these. The easiest way to attach the sail to the mast is by using rigid springs; if you make these very short and very stiff they will act like a short piece of rope connecting the sail and the mast together. We recommend that you use the RBS Draw Tool to connect the springs between the edges of the sail and the mast.
Depending on the type of sail you want to create, you may only want to pin the sail at three or four points or you may want to have regular ties all along the mast to keep the sail attached firmly. Using many springs to connect the sail will give pleasing taught ripples through it as the wind blows.
Why do the objects penetrate each other when colliding?
No object in the world is perfectly rigid. By allowing objects to penetrate each other slightly you will give the impression that they are actually deforming under the collision, which can give a natural look. However, this is not what you want if you are using very thin, or transparent, objects.
To achieve more rigid collisions you may wish to increase the Oversampling value in the Solver object. This will cause more frames to be calculated between each real frame, increasing the chances of a ‘correct’ collision. Alternatively, increase the Eps value; this will enlarge the object’s collision boundaries so Dynamics will think that the objects have collided slightly earlier.
Why do the objects levitate?
If gravity is present in your scene and you have the camera positioned exactly side-on to an object on a perfectly flat surface, you may see that it is actually hovering very slightly above the surface.
This is due to the Collision Eps value of the Solver (in the Details tab), which determines how close objects must be before they are considered to be touching. Reduce this value to allow the objects to get closer together.
How can I make a brick wall and then knock it down?
One of the difficulties in setting up a brick wall is getting the bricks to keep still before they are knocked down.
You could arrange the bricks so that they have a small gap between them, this gap being the same size as the Collision Eps value in the Solver. Alternatively, you could drop the bricks onto each other and initialize the scene once they are at rest. This will then start the simulation with the bricks in this state.
How you knock down the wall is entirely up to you!
Why does the scene explode and/or objects disappear?
If you find objects suddenly disappearing into the distance or soft bodies violently rippling then it is likely that your Solver Object’s integration method is set too low.
The level you need for the Integration Method largely depends on the relative speeds of the objects controlled by the Solver. For example, a dart hitting a dartboard hits it at a relatively high speed; this means that you need a very accurate solver to make sure it stops when it reaches the board.
I have no idea what values to use for the friction coefficients.
Here is a useful list of friction presets that is not intended to be physically accurate but should achieve the effect you want:
| Dynamic Coefficient | Static Coefficient | |
| Shoe on carpet | 70% | 95% |
| Shoe on ice | 20% | 20% |
| Shoe in grass | 50% | 65% |
| Tire on dry tarmac | 95% | 98% |
| Tire on wet tarmac | 75% | 97% |
| Tire on ice tarmac | 30% | 80% |
| Air hockey puck | 5% | 5% |
| Brick on concrete | 80% | 90% |
| Soap on enamel | 10% | 50% |
| Football on grass | 55% | 65% |
You can use static coefficient values over 100% to simulate a very coarse, or ribbed surface, such as polystyrene or a stony surface.
Why do I get different results for the same scene on the Mac and the PC?
This is an inherent problem of using cross platform software. Macs and PCs both use very different processors and the FPU (Floating Point Unit) varies between them. This can often give slightly different answers to the same calculation because of the differing rounding algorithms and precision used.
So, if a pinball hits a flipper and bounces off, the Mac may calculate the ball has left the flipper at one angle and the PC may come up with a different value. This may be a fraction of a degree. However, by the time the ball reaches the top of the table, it could mean the ball hitting a completely different part of the table. As the simulation continues, the ball ends up taking a vastly different route. For this reason you may wish to bake your simulation or use a network that has only one type of processor in it.
Why does the cloth look like rubber?
The most likely cause of this is that the springs are too stiff and have a low density mesh. The default cloth springs use all three types of spring: structural, flexion and shear. The flexion springs can stop the material from creasing like cloth usually does.
If you have a cloth with few subdivisions the flexion springs will spread themselves quite widely and, because they only link to every other point, they cannot then fold. To get around this you can either remove the flexion springs altogether or use a finer mesh for the cloth.
Why does the cloth fall apart when I use only shear or flexion springs?
If you take a closer look at these two types of springs you’ll notice that they are all criss-crossed. Both types of spring actually consist of two layers of springs that are in no way connected. This means that if you grab the cloth by setting a single point to have a mass of 0, then only one layer of springs will be held whilst the other layer is allowed to fall. So the answer here is to add some structural springs.
How can I connect a rigid body to a soft body?
Currently the only way to fix a rigid to soft body is by using a Rigid Spring tag.
Why do I have to initialize the scene?
Dynamics simulates movement starting from an initial set of values; subsequently everything happens automatically. The scene needs to be initialized because Dynamics modifies the position, rotation and shape of objects independently of CINEMA 4D‘s object system.
Suppose you’ve played a simulation through a few frames. What will happen now if you pause the playback and move an object? Should the object start its animation from that position the next time you play the scene? You can see that it is important that Dynamics’ position and rotation data is held separately from CINEMA 4D’s data.
Scene and object initialization is done automatically where possible (such as when you drag an object into the Solver). You should only need to initialize the scene when you manually change the starting positions.
Why can’t I blow part of a rigid object to spin it?
All forces such as wind, gravity or drag will only affect an object whose centre of mass is inside the field and these forces will affect the entire object. This means you can’t blow, say, half a page of paper to make it spin; the paper will move linearly in this case. In certain cases it may be possible to use a drag field.
My simulation is nice, but takes too long to calculate. How can I speed it up?
This is usually a scale problem.
Suppose you’ve created a scene of a ball falling on a plane. It would appear to behave completely differently if you used a 100 m wide ball than if you used a 1 cm wide bouncy ball (parallax). The easiest way to adapt this behavior is to increase the gravity strength. Increased gravity will cause objects to fall more quickly and this is often enough to make the simulation run as you expected.
The objects don’t react when they collide with each other. What’s going on?
This is likely to happen if you have scaled an object using the Object tool; Dynamics does not work correctly with objects that have had their system scaled. You will need to select the object and choose Functions > Reset System to return the object to the correct scale. You may then use the Model tool to scale the object as you need it.
Why do highly damped rigid springs explode?
Spring damping uses a similar algorithm to the drag object and you will notice that too much drag will cause objects to gain energy in an uncontrollable way. This sounds counter-intuitive but is due to the fact that if you try to remove too much energy from a system in a short space of time some of the objects will try to continue at high velocity, destabilizing the system. This also affects soft body springs.
I have an object at an angle. Why don’t the start values make it move in the direction it is pointing?
All Dynamics objects work using the world coordinate system, not the object’s local coordinates. The only exception to this is constraints, which use the coordinate system of the parent object.
Why does the console say ‘Maximum number of contact points reached!’?
This message is telling you that Dynamics has exceeded the maximum number of points that it can simultaneously calculate for collision detection.
This will often not influence the scene in any noticeable way. If you see this message and notice an object has not collided when it should, try reducing the collision type from Full, or try using a proxy object.
When a soft body collides with a rigid body, the rigid body pokes through the cloth. How can I stop this?
This is due to the different ways in which collisions are calculated between soft and rigid bodies. Soft bodies do not have collision detection along their edges, only their points may collide.
Increasing the Collision Eps setting on the Solver’s Details tab will give the colliding objects a ‘thicker collision skin’ and will therefore help solve this problem.
Why has the object stopped moving?
How can I create hair?
You can make soft and flexible hair by creating a long ribbon shape and turning it into a soft body. Then use an alpha map on the object to give the impression that it has many hairs. Self collision detection can be used to prevent the hair from passing through the body and itself.
Why do I have to use polygon or spline objects in Dynamics?
All objects must be in a polygon form for Dynamics to work; this is because parametric objects contain no real geometry for the Dynamics engine to work with.
A spline object may be used as a soft body because it is only the points that matter when soft body calculations are made by Dynamics.
Limitations: