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.

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.

Continuous Collision Detection

This option prevents particles from going through thin objects such as glasses, planes. etc.:

When an object is in "Shell" mode (see "Volume Mode" below), "Continuous Collision Detection" is active by default.
If the object's "Volume Mode" is "Solid Inside" or "Solid Outside you have to deactivate "Auto" and check "Continuous Collision Detection" manually.

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 values 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. You can use positive (attraction) and negative (repulsion) settings.

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 of 10 units (depends on the actual scene scale) from the object's surface will be affected by parameters like "Friction", Bounce", etc. The value is calculated automatically based on "Collision Distance". 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 – you can read more about this topic under → "Fluid-Object Interaction" and → "Filling Objects".

"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.

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". If you can see leaking particles in conjunction with thin or single-walled objects please consider activating "Continuous Collision Detection".

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