Tips and Tricks
Avoiding grid artifacts on the source surface
Light cache and dense smokes
Rendering with an external volumetric shader.
Creating the illusion for bigger resolution
Max. transp. levels of V-Ray and pure geometry mode
Complex rendering and "Circular reference" message
Simulation in 2D
Noise/slice like artifacts
Fluid simulations in hollow objects
In some cases the cells of the grid are clearly visible over the surface of the geometry used as a source. To avoid this artifact the transparency diagram should be arranged do give zero opacity at the birth value of the source channel. The example below shows two images created with different transparency diagrams. The birth temperature of the fluid is 2000K.
The light cache can cause clearly visible spots over the surface of dense smokes, so it is not recommended to be used in such cases.
Rendered with Light cache - On
Rendered with Light cache - Off
Phoenix FD is able to export its content as texture (see Phoenix texmap) and this makes it possible to use an external volumetric shader for the rendering, for example the V-Ray environment fog.
Simple setup for V-Ray environment fog:
To avoid waste of time for test rendering with different displacement settings, the shader can be switched temporary to solid mode, that is much faster than the usual atmosphere mode.
Even without fine details an image can look as if it has a lot of detail, if it has high sharpness and no pixelisation. This effect can be used to produce high resolution looking simulations with relatively coarse grids, using a properly adjusted transparency diagram.
The images below are made with the same simulation input and different transparency settings. The left one is made with simple smoke option, which gives the natural distribution of the smoke without any tricks. The second image uses a transparency diagram containing one peak near to the zero, that produces sharp smoke appearance.
Peak transparency diagram
Rendering in pure geometry mode causes high transparency level and you have to increase the Max. transp levels of V-Ray to a bigger value, given by the following formula: max transp level=grid size*100/step.
The image to the right illustrates an issue that occurs when the Max transp. levels is not high enough
In many cases when the rendering setup is complicated, the user will get an unexpected message stating "Can't make a circular reference". This message appears when the object depends on itself, mostly when some channel is exported as texture and when the same texture is used in the shading. To avoid this message usually a second Phoenix object is introduced, its input path is redirected to the first one, and the whole render setup is made with it. The first Phoenix object should be disabled for rendering, it is used only for the simulation.
An example of a circular reference
Render set-up to avoid the circular reference
Phoenix FD has the ability to perform a 2D simulation, if one of the sizes is set to 1. To keep some features like the embedded gravity and pressure decay, it is recommended to keep the z direction active.
Despite the difference in appearance both of the mentioned artifacts have a common reason - too big value of the advancing step compared to the opacity gradient. Depending on the situation there are two ways to solve the problem: to decrease the advancing step or to decrease the transparency gradient. The first solution will increase the rendering time, the second one will decrease the sharpness of the resulting image.
When simulating fluids in hollow objects one must make sure that the normals of the geometry are poinging inwards.