particle AI

[cire]

Hi there! This is for maya gurus out there. I'm doing a little test in which I want to give the movement of the particles a some sort of behavioral pattern. What I'm aiming here for the moment is to make each particle "aware" of the other particles so that they don't bump into each other. I'm thinking of doing it using expressions. Say, I have four particles which I randomly assign an initial velocity through an expression. Now, for each particle, I'm going to compare its position to other three particles and if they fall below a certain threshold, a parameter e.g. velocity is altered to avoid collision. Now my question is, for each particle, how can I access the position of the three particle. Is it possible to, like, increment particleId so that it refers to the next particle to be computed. How can I do this in maya expression (or MEL if needed):

if  (particle#1.position - particle#2.position < 1)
	....
if  (particle#1.position - particle#3.position < 1)
	....
and so on and so forth...

Any ideas and suggestions would be most welcome

cire

[msimon]

To find the correct MEL syntax for selecting individual, or multiple particles and changing attributes, go into component select mode, and select a particle. Then look at the output command in the script editor.

[dons]

i would think you would fill a global vector array with the particles positions in a runtime expression on the particles using particleId to increment the array.
then use another expression on a locator to parse the full array.
the first expression executes once for each particle, the second one parses once per frame.

[msimon]

The approach you use would depend on how many particles your talking about. For four particles, you could simply have the expression compare all four particle positions each frame. For a variable number of particles, you would need to run a nested for-loop to cycle as in the following code. Note that you need to do a little vector math here, and that the "getParticleAttr" call returns a float array, not a vector. I did this somehow at work without the float array I think, but I don't have that script here. I'll check that tomorrow morning. Also, I did not simplify this code on purpose and I didn't test it, but something close should work - I've done it before.

int $i, $j;
float $minDistance = ;//your minimum cutoff value here
float $maxDistance = ;//your maximum cutoff value here
int $sizeParticles = getAttr particleShape1.count;

for ($i=0;$i{
float $partPos1[] = getParticleAttr -at position
particleShape1.pt[$i];
for ($j=0;$j {
float $partPos2[] = getParticleAttr -at position
particleShape1.pt[$j];

float $temp1 = $partPos1[0];
float $temp2 = $partPos1[1];
float $temp3 = $partPos1[2];
float $temp4 = $partPos2[0];
float $temp5 = $partPos2[1];
float $temp6 = $partPos2[2];

//vector math
float $pos1 = pow($temp4-$temp1, 2);
float $pos2 = pow($temp5-$temp2, 2);
float $pos3 = pow($temp6-$temp3, 2);

float $pos4 = $pos1 + $pos2 + $pos3;
float $pos5 = sqrt($pos4);

if (($pos5 > $minDistance) && ($pos5 < $maxDistance))
  //your routine here
else
  continue;

}
}