Tag Properties
This tab contains all attributes that describe how the collision object interacts with and acts on other simulation objects (e.g., Rigid Bodies or Pyro).
If this option is enabled, the object to which the Collision tag is assigned can collide with simulation objects in the scene. Deactivation prevents any collisions, regardless of the collision element.
You already know these settings from the Rigid Body feature (Collision Shapes); they work the same way here.
For example, if you have a very high-resolution, complex Collision object, it may make sense to use a simpler collision shape for faster results.
In addition, the collision calculation with Rigid Bodies may lead to better results when using simpler collision shapes (e.g. cubes). This can have an effect on rolling balls, for example, which simply roll 'better'.
From left to right: Collision Shapes in blue: Box, Sphere, Convex Hulls (geometry accuracy = 0, but can be greatly improved with higher values), Triangle Mesh.
Collision detection calculations are generally computationally intense. However, only they lead to convincing object movements: Bodies can collide with others, come to rest due to friction, etc.
The more complex the body in its shape, the more complex its collision calculation. Therefore, there are some quick-to-calculate substitute shapes in this drop-down menu that can be placed over the actual object. In the simulation, it is often not possible to tell from a distance whether the actual object shape was used for the collision calculation or, for example, only a box that was automatically adjusted to the object size.
The order in terms of computational speed and collision precision is roughly this (from fastest/most imprecise to slowest/most precise): Box, Sphere, Convex Hulls, Triangle Mesh. In detail:
Auto
If Cinema 4D detects that the primitives Cube and Sphere have been used, they will be automatically calculated internally with the options Box and Sphere. Otherwise, Convex Hulls will be used (the default geometry accuracy 5 will be sufficient for most cases and will still check for collision very quickly).
Attention, Auto is only available for Rigid Bodies, not for Collision objects.
Triangle Mesh
The object to which the tag is assigned is defined with its complete shape as the collision shape, which makes this option the slowest, but regarding collision the best, because the exact object shape is used for collision calculation.
However, Convex Hulls with higher Geometry Accuracy can often compute similarly good collisions, but much faster.
Here, one or more enveloping hulls are placed around the object or its parts, which behave similarly to a rubber skin. The Geometry Accuracy parameter is displayed, which defines the number of "rubber skins".
By the way, this equivalent shape creation works exactly as already described for the placement tool at Accuracy.
If you want to display the collision shape, activate the Rigid Body Shapes option in the Simulation tab of the Preferences menu. To do this, you must start the simulation, or jump one frame forward from the start frame.
Sphere
A sphere fitted into the object bounding box is used as the collision shape.
Box
The smallest box that includes all object points will be used as the collision shape.
This parameter is displayed if Auto (Rigid Bodies only) or Convex envelope is selected under Collision Shapes (see also Convex Hulls).
With increasing values, the collision shape will approach the actual object shape without reaching it exactly: However, the calculation is much faster than with Triangle Mesh.
If set to 0, recessed indentations or recesses or holes will not be included in the collision calculation, but only a single "rubber skin" will be placed around the object, as shown on the left:
This attribute is also found in the collision partner. Remember that these fields are related in the two tags. For example, a piece of nylon always has the same Bounce value. When the nylon collides with two objects, one made of fabric and the other made of concrete, the Bounce value of the nylon remains the same. However, the other two objects have completely different Bounce values. The concrete provides little to no cushioning for the nylon, but the fabric allows the nylon to cushion relatively strongly.
This attribute is also found in the collision partner. Remember that these parameters are related in the two tags. The surface with which a fabric object collides, for example, can have different friction values. For example, a piece of nylon always has the same coefficient of friction. When the nylon collides with two objects, one a rock, the other a block of ice, the Friction value of the nylon remains the same. However, the other two objects have completely different Friction values. The ice block creates almost no Friction, while the rock creates a relatively large amount of Friction when it collides with the nylon.
This attribute is also found in the collision partner. Remember that these parameters are related in the two tags. For example, if one of the two settings has the value 0, the overall stickiness is also 0.
Here you can define the polygon side from which to check for collisions. Physically correct would be Both. However, more complex collisions may result in penetrations that could not be eliminated by this setting. For example, imagine a T-shirt in the armpit area of a running character: it may happen that parts of the T-shirt are pressed into the character geometry. If you set Front here now, the possibility is given that the cloth can move out of the character again, since the collision from the backside is deactivated. In addition, a small impulse is even applied from the surface to better eliminate any collision penetrations.
Back, for example, would make sense if you enclose a sphere in a larger sphere.
The polygon normals are decisive for the direction: Front is where the polygon normal points.
Liquid collision
In the following example, you can see liquid particles in a cylinder: on the left in surface mode and on the right in volume mode. The latter is waterproof in the truest sense of the word, while some liquid particles "break out" on the left hand side.
Collision objects can collide with a number of elements: e.g. rigid bodies, soft bodies, particles, etc. Depending on the element type, the collision can be calculated in different ways. In combination with the fluid simulation, you have 3 modes:
- Surface: An appropriate surface is assumed through which the liquid particles should not penetrate. Please note that the collision form and collision side parameters described above are evaluated! Please note that this mode is not as precise as the volume described below and that penetrations can occur - especially with fast particles/collision objects. The surface generally works well when the liquid particles collide ("bounce") only briefly and then move away from the surface. If the liquid is to remain in permanent contact with the collision object, the following volume mode is more suitable.
In most cases, surface is faster to calculate than volume (exception: basic objects cube or sphere).
Make sure to set the Collision Side parameter in front of it accordingly! - Volume: in this mode, a trick is used: instead of checking particles for collision with a surface, the collision geometry is replaced by a volumetric image formed by voxels and used for collision detection (more details can be found at Voxelization). Volume calculates more precisely - but more slowly - than surface and is able to simulate "watertight" collision objects in the truest sense of the word. Use this mode if you want to simulate liquids in containers: e.g. red wine in a glass, sloshing water in a canister, etc. This option works best with objects that have a volume. Here it is clear where inside and outside are. However, non-voluminous objects (e.g. a slice) can also be made voluminous in terms of collision: with the setting With open geometry, these are converted internally into a voluminous collision image.
- Off: This switches the mode off. There is no collision detection with liquids, but there is with other elements (e.g. rigid bodies) depending on the settings.
Volumes need need to be bigger to function well. Thin collision walls don't work so well, penetrations occur quickly. Note that the use of invisible (= not rendered), thick-walled collision replacement objects is often a good tool.
Here you specify the side from which liquid particles can collide with the volume. You can select from the following options:
- Outside: Collision takes place when particles hit the collision surface from the outside. This is the normal case.
- Inside: Collision takes place when particles hit the collision surface from the inside (e.g. a glass sphere filled with water).
- Both: Collisions take place both on impact from the outside and from the inside. However, this mode is less robust than the other two, which can result in penetrations!
A cross-section through a collision surface (blue) in Outside mode (in Inside Mode, the plus and minus signs would be reversed).
Here are some details about the Volume mode that will help you to better understand the next parameters described. You can see the collision surface of an object in blue in the illustration. The black, square border is a collision bounding box. This is not identical to the familiar object bounding box, but slightly larger. Several layers of voxels (red) are created around the inside and outside of the collision surface (general details on voxels can be found here). The distance to the nearest collision surface is stored in each red voxel.
Outside the red voxel layers, there are also voxels inside the collision bounding box, but these only have the "Inside" (+) and "Outside" (-) properties saved. The area of the + sign indicates the space in which liquid particles can move.
As soon as liquid particles enter the collision bounding box, collisions can be detected very precisely.
The above description applies to all mesh objects except the basic objects cube and sphere, which work without voxels. Their volume representation is mathematically defined, which is more precise, faster and requires less memory. You could therefore save resources by using these two object types.
In the Voxelization parameter, select how the voxel size is to be determined:
- Relative: a percentage of the longest object bounding box side is used here.
- Absolute: this allows the voxel size to be entered in absolute centimeters.
Depending on what you have defined for Voxelization, enter the voxel size here as a percentage of the largest bounding box length or absolutely in centimetres.
The smaller the voxel size, the better the collision mesh can be simulated, but the more complex the calculation and vice versa. Voxels that are too large lead to leaks, i.e. penetrations.
Generally speaking, the voxel size should be approximately the same size as the liquid particle size, but can also be somewhat smaller. The default liquid particle size is 6 cm, so a voxel size of approximately 3-6 cm would be appropriate. You can, of course, also go further down to better simulate the collision mesh. It may then be necessary to enlarge the voxel areas (see next parameter).
Interior Voxel Range[0..2147483647]
Exterior Voxel Range[0..2147483647]
Starting from the collision mesh, a defined number of voxel layers are created on the inside (inner voxel area) and the outside (outer voxel area). These are the red voxels at the top of the illustration.
If you select very small voxel sizes (see previous paragraph) can lead to collision problems. The reason: the liquid particles are detected too late, then overlap and bouncing behavior can occur, for example. In such cases, it can help to enlarge the voxel areas on the collision side.
Very fast liquid particles also benefit from larger voxel areas!
Other things to know about liquid collision
- It can happen that different collision methods "wrestle" with each other: imagine a rigid body floating on a liquid, which is pressed against a collision object by the liquid; who wins? Penetrations could occur here. You can influence this with the parameter Collision Priority ().
- The following applies to the Volume Mode: If you create liquid particles in a negative space, they will be drawn into the positive space within the collision bounding box with great force.
- The creation of voxels around mesh surfaces is also used with the Volume Builder (Volume Type Signed Distance Field (SDF)).
Preprocess
If you have defined triangular mesh under Collision shape, you can have the geometry preprocessed internally here by giving it volume. This benefits the volume collision mode the most, which is effectively supported by the wall thickness mode. Volumes with a clearly defined inside and outside are then created.
You can select from the following options:
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Off: No pre-processing takes place here. The geometry is used for collision as it is. This can quickly lead to problems (= penetrations) in the event of liquid collisions with objects that do not enclose a volume (e.g. a disk)!
There are a few exceptions here, such as the basic object plane, which is processed internally as a cube in terms of collision. However, if you make this editable, these special rules are switched off - but only incidentally. -
Thickness:
You can use this to turn any object that is made up of just one polygon layer into a volume. Internally, this works like the wall thickness generator. You define the wall thickness using the thickness parameter described below.
In the video above, you can see a red, fast-moving collision object colliding with liquid particles. Left is set to Off for open geometry, right to Wall thickness. On the right hand side, the collision works perfectly, while on the left hand side there are massive penetrations.
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Close holes: Use this to close holes in the mesh. Definition: Holes are where there are edges: polygons that have no connection on one side. This includes, for example, a hemisphere made of a single-walled polygon mesh, but not a hemisphere whose walls have a thickness of 1 cm, for example.
This is where you specify how thick the wall thickness created should be. The thicker this is defined, the more accurate and reliable the collision calculation will be.
However, the higher you set the thickness, the more visible the distance between the collision partners becomes. The wall thickness can therefore be shifted inwards (negative values) and outwards (positive values) with Position until it looks right again.
Load the scene above and vary the position. Switch to the "Position" camera. Figure 5 shows the effect of this parameter very clearly.
Exclude polygons
This command excludes all selected polygons from the collision calculation.
Hereby all excluded polygons are considered again for the calculation.
Once you have made a different polygon selection, you can use this command to quickly display the excluded polygons again. The current polygon selection will be overwritten.
If this option is enabled, all excluded polygons will be displayed with red edges in the editor. Internally, the collider object is always divided into triangles so that the excluded polygons are triangulated. Deactivating this option hides the polygons in the editor again.
