Fluid simulation software is based on real-world parameters to simulate the behaviour of the individual particles, and it is possible to store these properties. In RealFlow and the RFRK they are called “channels”. Of course you know the most important channels position and velocity. These channels can be seen in RealFlow's viewport: there are particles at specific positions and the colour gradient indicates their velocities. But, there are many more parameters which are invisible at first glance – for example pressure, density or viscosity. All this information is stored with each particle, then read out by the mesh engine, and finally used to extract vertex maps. The result is a representation of the channel's distribution over the entire fluid cloud and, of course, it changes over time.
Create Vertex Maps
[…] as Magnitude
This panel contains checkboxes for all available properties.
Precise (3DS, C4D)
This feature is used to enhance the quality of the fluid's → weight maps (the description is for C4D, but the concept is exactly the same for 3DS).
Weights (3DS, C4D)
If you have loaded two or more particle sequences the RFRK is capable of calculating the amount of interaction between the different fluids. This is of particular interest with mixing fluids, because with the “Weights” channel you will be able to create blending colours when the fluids are mixing. Under → “How to... Use Weight Maps?” you will find a complete workflow description of how to achieve this effect (Cinema 4D only by now).
In order to visualize a channel's information two materials are required. The vertex map controls how they are blended, but in many cases the result is not satisfying. With “Scale” you have an effective method of controlling the vertex map's transparency. This means that you change the ratio of both materials.
For a complete workflow description telling you how to apply vertex maps to materials, please click → here.
Create Vertex Maps (C4D)
In Cinema 4D, a vertex maps contains a channel's information. There is one vertex map tag for each channel and with this checkbox you can decide whether you want to create the vertex maps for the activated channels or not. If you want to learn more about vertex maps and how to use them please read the → “How to... Use Vertex Maps?” chapter of the Cinema 4D connectivity plugins manual.
[ … ] as Magnitude (C4D)
Some channels use vector data, e.g. velocity or vorticity. This means that the information consists of a X, Y, and Z component. If you want to merge these individual elements into a single value that represents the vector's magnitude then you can activate this option. Instead of three vertex maps, you will only get a single tag for each channel. This process is reversible and by unchecking the box, you are able to create three individual tags again.
Information for mental ray and RenderMan Users
A channel can be activated by simply checking which property you want to use, but connecting channels to a shader works differently for each render engine. With mental ray and RenderMan you will need helper nodes.
To read the channels from RealFlow cache files in mental ray you need a new node called “rfrk4_ texture_magnitude”. This is accessible from the "RenderNode" panel:
mental ray > Textures tab
The second image above shows the node tree for so-called scalar values. These values consist of just a single value, e.g. "Mass", Density" or "Age". Vector values contain more than one component, for example "Velocity". In this case you will find three values for each direction in space (XYZ). The last image is an example for a vector-based channel.
In RenderMan, channels are accessible through "primVars".