This is the place where the collision between objects and fluids is defined and adjusted. Only objects with a "Collider" tag attached can be seen by fluids.
The panel is subdivided into three sections:
Collision
Enable Collision
Enable or disable the fluid-object collision on demand, e.g. by animating this option.
DY Collision distance
This parameter describes the distance between the particles and an object's surface.
Collision Geometry Detail
Here you find five levels of quality. The higher the level, the longer the simulation will take, but the fluid-object collision will also be more accurate. Changing the level influences and updates the "Cell Size" value found in the "Volume" section. In most cases, the calculated value provides the best balance between simulation speed and accuracy. If want to control "Cell Size" manually deactivate the associated "Auto" checkbox.
Interaction
Friction
A value of 0.0 creates absolutely no friction and a perfectly even surface. Higher values can even stop particles from moving. This parameter accepts all positive between 0.0 and 1.0.
Bounce
A value of 0.0 creates perfect elasticity, while higher settings make the particles lose appropriate amounts of their energy. The maximum value is 1.0.
Sticky
It can be seen as a “glue factor” to make particles stick on the object’s surface. There are few things you should know:
- You can use positive (attraction) and negative (repulsion) settings.
- Use a value similar to gravity as a reference and then adjust it further.
Roughness
This value ranges between 0.0 and 1.0 and adds randomness to the object’s polygon normals to produce a slightly different collision direction.
Collider Velocity Factor
With this parameter it is possible to amplify the object's influence on the fluid and create more turbulence.
- accepts any positive value including 0
- is a very sensitive parameter and should be altered carefully.
Interaction Distance
With 10, for example, particles within a distance 10 units (depends on the actual scene's scale) from the object's surface are affected by parameters like "Friction", Bounce", etc. The value is calculated automatically based on the "Collision Geometry Detail" level. For manual control uncheck "Auto".
Volume
Cell Size
RealFlow | Cinema 4D's fluid engines are cell-based and therefore, colliding objects have to be rasterized internally. The smaller the cells, the better and more accurate the collision, but at the cost of longer simulation times. By default, "Cell Size" is calculated automatically based on the "Collision Geometry Detail" level. For manual control uncheck "Auto". With "Auto" turned off, "Cell size" can be animated. This allows you to refine collision detection over time and speed up the simulation, e.g. from coarse to accurate.
Surface offset
This parameter creates a solid extension around the object and prevents the fluid from penetrating its surface:
- The offset can be visualized with Scene > Display > Show Collision Geometry
- Both positive and negative values are accepted.
Domain Offset
In order to improve fluid-object interaction you can increase this parameter. The effect is that the fluid will “see” the object earlier. This can be interesting for fast-moving fluids. By default, "Domain Offset" is calculated automatically based on the "Collision Geometry Detail" level. For manual control uncheck "Auto".
Volume Mode
Here are the three possible modes:
- "Solid inside" makes the fluid collide with object's outer surface.
- "Solid outside" creates a hollow object and fluid interacts with the inner surface.
- "Shell" is normally used with open objects like glasses, vases, but also planes. When active, an invisible layer is added around the object's surface. The thickness of this layer is
Cell size * 3. As a consequence, there will always be a gap between the fluid and the object, but it is watertight.
By default, "Volume Mode" is determined automatically by analyzing the object's geometry, but this test is not always reliable. For manual control uncheck "Auto".