Advect Grid Node¶
The Advect Grid node moves voxel values through a velocity field over time using numerical integration. This operation is called advection and is commonly used in fluid, smoke, and motion simulation systems to evolve quantities such as density, temperature, or color according to a flow field.
The node supports multiple integration schemes that trade off between speed, accuracy, and numerical stability. It can be used for both scalar and vector grids, provided the voxel size is uniform across the domain.
Advection is conceptually equivalent to tracing each voxel backward through the velocity field by a small time step, sampling the grid value from the previous location, and assigning that value to the current voxel.
Inputs¶
- Grid
The input grid to advect. Must have a uniform voxel scale.
- Velocity
The vector grid defining the flow direction and magnitude at each voxel. The Velocity field determines how grid values are transported through space.
- Time Step
The time step used for advection, in seconds. Larger values result in faster motion but may reduce accuracy or stability.
- Integration Scheme
The numerical integration method used to trace voxel positions through the velocity field:
- Semi-Lagrangian:
First-order semi-Lagrangian integration. Fastest and most stable but introduces noticeable numerical diffusion (blurring).
- Midpoint:
Second-order midpoint integration. Balances speed and accuracy, suitable for most cases.
- Runge-Kutta 3:
Third-order Runge-Kutta integration. Provides higher accuracy with moderate computational cost.
- Runge-Kutta 4:
Fourth-order Runge-Kutta integration. Highest accuracy single-step scheme, ideal for detailed simulations but slower to compute.
- MacCormack:
MacCormack scheme with implicit diffusion control. Reduces numerical dissipation while maintaining stability.
- BFECC:
Back and Forth Error Compensation and Correction. An advanced scheme that minimizes dissipation and maintains sharper features.
- Limiter
The limiting strategy used to reduce overshoot or undershoot artifacts in high-order advection schemes:
- None:
No limiting applied. Fastest but can introduce artifacts in regions with steep gradients.
- Clamp:
Clamps values to the range of the original neighborhood, preventing overshooting and undershooting while preserving stability.
- Revert:
Reverts to first-order integration in cases where clamping would be necessary. More conservative and stable than clamping alone.
Properties¶
- Data Type
The type of data stored in the grid (e.g. Float, Integer, Vector). Determines the kind of field being advected.
Outputs¶
- Grid
The resulting grid after advection. Each voxel value has been transported according to the specified velocity field and integration scheme. The result represents the grid state after the given Time Step has elapsed.