Select the particle_emitter and Simulate > ICE > Create menu choose Emit Particles from Selection… With the PointCloud still selected, press [Alt] +  to open an ICE Tree. Before you can assign any goal or paths for the particles you have to choose a criterion to use to separate the particles. In this example you’ll be using three different objects so it makes sense to divide the particles into three groups, one for each goal/curve. As each particle is born it’s also assigned a unique identification or ID. By feeding this ID into a Modulo operation and set the devisor to the same value as the number of groups you want to create, you’ve efficiently created the 3 collections.
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While the weights in the weight map will be controlled from within the ICE Tree, you have to create the actual weight map manually. Select the Grid object and from the Get > Property menu choose Weight Map. Select the Grid object and press [Alt]+ to open an ICE Tree and from the Create menu choose Simulated ICE Tree. Press  to open an Explorer and drag and drop the Sphere object to your ICE Tree. Continue by getting a Pass Trough node, a Get Point Position node and a Get Closest Location node. The Get Point Position node returns the position of each of the points of the grid which is then used by the Get Closest Location node to determine whether the Sphere touches any of the vertices of the grid object (or is within a specified distance). If the Sphere is within this value the corresponding points’ weight should be set 1. If it’s not, the weight should stay unaffected (the default value is zero). Open its PPG. Check the Enable Cutoff Distance checkbox and set the Cutoff Distance to 0.1 and close the PPG.
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