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Please take a look at the image below. It shows the traditional, completely grid-based approach we had in RealFlow 2012. The view represents a 2D view of a domain with an interacting object. As you can see, there are gaps. These gaps occur, because the object is rasterised as well and it is not possible to fill cells only by half or one third. The size of the cells also control the distance between the object's surface and the fluid's surface. If the interacting object is positioned incorrectly you will observe a rather large gap:
RealFlow 2013 2014 uses a completely different approach. There, the outlines of an object are represented as a distance field. The quality, or better resolution, of this field depends on the number of grid cells you have determined for the object in its “Cell size” parameter. As illustrated below, the domain's particles follow this distance field and create a rounded surface matching the object's shape:
This means that Hybrido 2 works with two different cell sizes: one for the domain and another for the object. It is important to know that the domain's “Cell size” is the higher-ranking parameter. So, even if you use very small “Cell size” values for the object, you will not achieve any improvements with regards accuracy, because the solver cannot “see” these small cells. Values of 0.1 (object) and 0.2 (domain), for example, do not have any influence and you only waste memory. The result in terms of fluid-object interaction remains the same:
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When you decrease “Cell size” to, let's say, 0.05: the map is shrinking, but it is also much smoother. Additionally, the red ring is much thinner. With even smaller values, the map almost fits the object's surface. But, the object's “Cell size” parameter is not the only value that can be used to adjust the distance and the already-mentioned “Surface offset” is of, at least, same importance. When you enter a value of 1, for example, the map will be enlarged by 1 meter at each side. Another thing you will observe is a smoothing effect. This can be seen clearly with edged objects:
The offset depends on the object's "Cell size" value and the map can only follow the grid in steps of "Cell size", e.g. 0.3, although it is possible to enter any value. An example:
Imagine an object with a "Cell size" of 0.1 and an activated "Slice plane" option. To shrink the field and fit it to the object's borders exactly you enter a negative value, e.g. -0.085. Now you start the simulation, but the particles still have a slight offset, and it seems as if your settings did not have the desired effect. The reason is that object is subdivided into cells of 0.1 metres and this is your step size for the "@ Offset [XY|XZ|YZ]" parameters. In other words, the only working values are integer multiples of the current "Cell size": 0.2, 0.1, -0.1, or -0.2 etc. So, if you want to adjust the SDF, take a look at the current "Cell size" and work with this value to adjust the SDF. Values, different from the step/cell size, are rounded by RealFlow.
Evaluating Objects with Distance Fields
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