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You can find a parameter with exactly the same name under the Hybrido domain node's “Fluid Dynamics” panel. This is no coincidence, because both values describe the resolution of the used grid and are given in metres [m]. Small values create a higher number of cells and increase the simulation's accuracy: the parameter determines how accurate the fluid solver “sees” the object. As always with RealFlow, higher accuracy also means longer simulation times.
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This parameter is comparable to an object's “Collision distance” for particle fluids. The given value acts like an invisible cache or solid extension around the object and prevents the fluid from penetrating its surface. It is often a good idea to play with “Surface offset” a little before you start to change “Cell size”. Complex objects can particularly profit from this parameter, because the buffer acts like an even wall and ignores small structures – it smoothes the surface. Another field of application is fast moving objects or fluids. In these cases, you normally have to increase the number of substeps to grant proper collision detection, but it is better to increase “Surface offset” instead, because with an increased number of substeps you also change the fluid's behaviour. "Surface offset" also accepts negative values and is measured in metres [m].
Domain offset
For the creation of the object’s “Signed Distance Field” (SDF), its bounding box is used. The SDF determines whether an interacting particle is inside or outside the object, or on its surface. With this parameter you can easily extend that bounding box to improve particle-object collision, for example in combination with the “Waterline” emitter. An increased SDF might change the simulation, because the solver will “see” the object earlier. This value is given in metres [m].
Volume mode
You can choose from three different options. The default setting is “Solid inside” and the body acts like a completely filled object. Particles react with the node outside and bounce from its surface. With “Solid outside”, the object is hollow, while everything around its borders is considered solid. A Hybrido fluid that is created inside such an object will be trapped and cannot leave the body. Finally, there is “Shell”. In this case, the object acts like a non-solid, hollow body. Fluids inside the object will not be able to leave it, while fluids from the outside are reflected at its borders. A balloon is a good example of an object in “Shell” mode. When "Shell" is used, RealFlow adds an offset above and below the object's surface to make sure that the particles will not interpenetrate the object. The size of this offset is
Surface offset = Cell size * 3