PlaceParticles.cs Place Unity particles individually.

gistfile1.cs

using UnityEngine;
using System.Collections;
 
public class PlaceParticles : MonoBehaviour
{
public ParticleSystem tree;  // plural. like sheep.
 
public Transform ObjectToPopulateWithParticles;
// a flatish square mesh is a good idea since our helper will take a
// random point sample in the negative Y on a pre-defined square.
// You can write better helpers...
 
// we will just start a loop until we have our particles ready to reposition
void Start()
{
StartRepeatingCheckForParticleCount();
}
 
void CreateTreesFromParticles()
{
// should be 1000 in my case...
int partNum = tree.particleCount;
 
if (partNum == 0)
return;
 
// make a list, of the same length
ParticleSystem.Particle[] ParticleList = new ParticleSystem.Particle[partNum];
tree.GetParticles(ParticleList);
 
for (int i = 0; i < ParticleList.Length; ++i)
{
// "5" is the tree sprite height offset we want, ignore it. change it for your tree height
// GenerateRandomLocationInArea and ShootRayReturnParticlePosition
// just find a place in a block and shoot a ray down,
// nothing fancy, just puts it on the floor.
 
Vector3 newPlace = PlaceParticles.ShootRayReturnParticlePosition(PlaceParticles.GenerateRandomLocationInArea(ObjectToPopulateWithParticles)) + (Vector3.up * 5); // magic number erk!
 
// actually assign the location to the particle in the list
ParticleList[i].position = newPlace;
}
 
//actually assign the particles, with new positions, back to the system
tree.SetParticles(ParticleList, partNum);
 
CancelInvoke("CreateTreesFromParticles");
}
void StartRepeatingCheckForParticleCount()
{
InvokeRepeating("CreateTreesFromParticles", 0, 0.02F);
}

// helper bits
// 256 is my "magic" block size
public static Vector3 GenerateRandomLocationInArea(Transform chosenparent)
{  
return new Vector3(Random.Range(10, 246) + chosenparent.position.x, 35f,
Random.Range(10, 246) + chosenparent.position.z);  
}
 
public static Vector3 ShootRayReturnParticlePosition(Vector3 location)
{
Vector3 targetPosition = Vector3.zero;
RaycastHit hit;
 
if (Physics.Raycast(location, -Vector3.up, out hit))  // ugh.
{ targetPosition = hit.point; }
 
return targetPosition;
}


}