Ask : Fluids and Objects

Owned by thomas (Unlicensed)
Last updated: 03 May, 2018
3 min read
Why is there a gap between the particles and the collision object?

RealFlow | Cinema 4D uses an internal proxy representation of a collision object – the so-called collision geometry. In order to refine this proxy object you have to add a "Volume" tag and decrease → "Cell Size" or choose a better → "Collision Geometry Detail" option. You can also control the shape and look of the poxy through the → "Collider" tag's "Show Collision Geometry" option.

If your object is an open, single-sided shell then you will always see a more or less small gap between the object's surface and the fluid. The reason is that RealFlow | Cinema 4D creates a layer around such an object to make it watertight. The thickness of this layer is Cell Size * 3 on each side of the polygon shell.

Why are simulations with rigged characters, point level animated objects (PLA), or objects with scale changes slow?

RealFlow | Cinema 4D creates an internal collision geometry from the object for the interaction with particles. When the base object has deformation/PLA, or a change in scale the collision geometry has to be recalculated during the simulation. And, depending on the complexity of the base object, this takes some time and slows down the simulation. The most effective workaround is to reduce the number of polygons as far as possible or use low-res proxy objects.

The quality of the collision geometry also plays a role and you can adjust in the → "Volume" tag's parameters. → "Cell Size" and → "Surface Offset" are the crucial parameters. And when you click on → Collider > Display > Show Collision Geometry you can see the internal object.

I can see particles going through my collision object. What's happening here?

With fast moving particles and/or a very small number of simulation steps per frame it might happen that a particle in a starting position goes to a final position in the next simulation step that is at the other side of the collision geometry:

  • Go to Scene > Solver and disable the → "Auto Params" option to get access to the "Min Substeps" and "Max Substeps" parameters.
  • Increase the values to 4 and 8 and simulate again.
  • If the particles still leave the object you have to increase both values again in moderate steps of 2 until the object is watertight.
  • An additional option is to increase the → "Surface offset" value of the collision object's "Volume" tag. A good starting value is → Cell Size * 0.5.
  • Finally, activate the "Collider" tag's → "Continous Collision Detection" option.
I have enabled "Continuous Collision Detection", but there are still particles going through my collision object. Why?

If this is the case the problem can be fixed by

I have activated the "Animate Preview..." option, but still I cannot see the wetmap. Why?

Wet-dry maps require an UVW tag attached to the object. You will get this tag automatically when you make your object editable.

Why can I see the wetmap in the render, but not in the viewport?

Open the → material panel and go to the "Editor" section. Then enable "Animate Preview...".

The wet map is distorted, mirrored or repeated. How can I fix this?

Wet-dry maps require proper UVW coordinates and this is within your responsibility. RealFlow | Cinema 4D does not perform any changes on UVWs or plausibility checks.

I have applied an object to the "Object" emitter, but there is no emission. What's wrong?

When the body's polygons are very small it might happen that you will not get any particle emission. To fix this, increase the "Fluid" container's "Resolution" parameter to create more particles. You may need high settings (100, 200 or even more).

Are RealFlow | Cinema 4D particles able to influence Cinema 4D dynamic bodies (rigid and soft bodies, cloth)?

No, RealFlow | Cinema 4D does not change the motion or shape of Cinema 4D dynamic bodies, but they are able to collide.

I see exploding particles when they interact with objects, for example when I try to fill a glass. What's going on?

RealFlow | Maya works with adaptive substeps by default and this means that RealFlow determines the actual number of substeps. When particles settle, the number of substeps decreases, and this means less precision. As a consequence physical effects inside the fluid start to accumulate until a "pop" occurs. After the explosion the particles settle again, and RealFlow increases the number of substeps - the beginning of a new cycle. To avoid this effect increase the number of → "Min Substeps" and/or → "Min Iterations".