Ask : Fluids and Objects

Why is there a gap between the particles and the collision object?

RealFlow | 3ds Max 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 → "RFVolume" world-space modifier and decrease "Cell Size " or choose a better " Geometry Detail" option. You can also control the shape and look of the proxy through the "RFVolume" world-space modifier's → "Display Volume " 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 | 3ds Max 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.

What is the "RFCollider" object and where do I find it?

The → "RFCollider" is necessary when objects should be considered by RealFlow | 3ds Max and used in simulations. Without a "RFCollider" an object is invisible to the particles in terms of interaction. If you want to use an object in conjunction with the → "Fill" emitter to create an initial particle volume without further interaction, then the "RFCollider" is not required.

The "RFCollider" can be found in the world-space modifier list.

What is an object's "RFVolume" world-space modifier?

This set of parameters is located in object's "Attribute Editor" under "RealFlow 3ds Max". When objects interact with RealFlow particles or when they are used to create an initial particle volume, the plugin creates an internal representation of the object - the so-called collision geometry. With these parameters it is possible to adjust the collision geometry and refine its shape. Furthermore you can specify whether the object is solid, hollow or consists of an open, shell-like surface. This is important for filling objects or to define whether particles should interact with an object's inside or outside.

Why are simulations with rigged characters, vertex animation/deformation, or objects with scale changes slow?

RealFlow | 3ds Max creates an internal collision geometry from the object for the interaction with particles. When the base object has deformation, 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 object's → "RFVolume" parameters. "Cell Size" and "Surface Offset" are the crucial parameters. And when you click on → the "RFVolume" world-space modifier's → "Display Volume" 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 try 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" parameters. A good starting value is Cell Size * 0.5.
• Finally, activate the "RFCollider" element's → " Continuous 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

• increasing the number of → simulation steps (aka "substeps")
  increasing the "Volume" parameter's → "Surface offset".
• decreasing the particles' velocities through → "k Speed" and → "Drag Force" daemons.
• check the → "Volume mode" to see if it is set correctly.

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 | 3ds Max 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).

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

RealFlow | 3ds Max 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".