Birth Fluid operator

The Birth Fluid operator can be used to birth particles inside of a (Phoenix FD) fluid grid based on grid voxel and particle properties.

Fluid Birth

  • Fluid object: the input fluid grid object.

  • Start/End: controls the time range in which to birth new particles.

Input grid channels

  • Temperature: when enabled, grid cells with a temperature value above 300 will be able to spawn particles.

  • Smoke: when enabled, grid cells with a smoke value above 0 will be able to spawn particles.

  • Velocity: when enabled, grid cells with a velocity value above 0 will be able to spawn particles.

  • Fire: when enabled, grid cells with a fire value above 0 will be able to spawn particles (FumeFX only).

The higher the temperature/smoke value per cell, the higher the probability that a particle will be spawned in the cell.

  • Rate per grid: The theoretical rate of particles births that can happen per simulation step over the entire grid, given a per-cell probability of 1. The actual rate of particle births will be lower, and the birth probability per-cell is dependent on the total smoke and temperature values of each cell.

  • Rate per cell: Same as rate per grid, but the probability is calculated per cell instead of per grid.

“Rate per grid” and “rate per cell” are easy to understand once you grasp what they are doing. “Rate per grid” means that if every single cell of a grid is completely full of smoke/velocity/etc (depending on the input grid channels you’ve selected), then the probability of each cell getting a particle is the rate divided by the cell count: [rate per grid / total cells]. So, if a grid has 1000 cells and “rate per grid” is set to 1000, each cell will generate exactly 1 particle if each cell is completely full of smoke/velocity/etc. The “rate per cell” setting is similar, except the probability of a full cell getting a particle is identical to the “rate per cell” value (ie, it’s not divided by the total number of cells). In practice, a grid will have lots of empty cells, or lots of semi-filled cells, so the total number of particles will be lower than the explicit “rate per grid/cell” settings (hence why the rate values are probabilitistic and don’t define an exact number of particles that will be generated).


Enabling these channels will override the default cell spawn probabilities.

  • Temperature/Smoke/Velocity/Fire: the threshold override channels.

  • Min/Max/Variation: the range of values for each selected channel that will control how many particles are spawned in each cell.

Input Particle Channels

PhoenixFD grids can contain both voxels and particles. To birth particles that will be copies of PhoenixFD grid particles, enable the relevant particle channels.

  • Channels: enabling these channels will birth copies of particles contained within them, if such particles are available in the cache of the input object.

  • Channel percentages: controls the percentage of copied input particles to birth.

  • New only: controls whether only new particles will be birthed each frame. A particle is considered new if its input ID has not been processed on a previous frame.


  • Seed: the seed value for all varied parameters.