Birth Grid Operator

Restrict By Mesh Volume - When on, and you've specified the reference geometry (see following), Birth Grid generates particles only at grid-knot locations inside the reference object. This means, of course, that the reference object must be at least partially inside the operator icon.

Example:  BirthGrid_RestrictByMeshVolume.zip

Tip: Use this feature to create a particle array with the shape of a mesh object. With the Delete Internal Particles option (see following), the particle array can take the form of a shell that defines the object shape, such as in the following illustration:

You can also use nested mesh objects, combined into a single object with Attach, for layered shells. The way this works is that, when using Restrict By Mesh Volume, the operator places particles only inside each pair of concentric surfaces, so you can get hollowed-out sections without using Delete Internal Particles.

For the following illustration, we nested (from the inside out) two cylinders, a box, and a sphere. To get the open top and bottom, we reduced the Birth Grid icon height and positioned it so its top and bottom were inside the nested shapes' bounds.

Note: The process of determining which grid knots are inside of the reference geometry can be lengthy, depending on the number of grid knots and complexity of the mesh topology. During the calculations a running progress bar appears in the status area of 3ds Max interface (near the bottom). To interrupt the calculations, click the Cancel button.

When Restrict By Mesh Volume is on, the following controls become available:

• Delete Internal Particles - When on, generates only the outer layer of particles within the mesh volume.

• External Layers # - Controls the thickness, in particle layers, of the shell defined by the mesh volume.

• Interactive Update - When on, changing a Birth Grid setting such as Grid Size updates the grid immediately. When off, to see the results of changing a parameter, click Calculate Mesh Volume Limits.

  When dealing with a large number of particles, you can work faster by turning off Interactive Update and recalculating the grid only when necessary.

  Note: Turning on Interactive Update forces an immediate update.

• Reference Geometry - Click the button to specify an object whose mesh defines the volume for restriction of particle generation.

• => - To select the reference object assigned in Birth Grid, click this button. This selects the object in the scene and makes it easy to transform the object or access its parameters.

• Calculate Mesh Volume Limits - Click the button to specify an object whose mesh defines the volume for restriction.

  The first time you specify a reference object, Birth Grid automatically adjusts the operator to create particles only at knots inside the object. Thereafter, if you transform or modify the object or specify a different reference object, you must use Calculate Mesh Volume Limits to register the change and update the grid.