Particle expression problem?

[breeder]

Hi i posted this question to one other forum but don;t get response.

I have particle expression after dynamics:

if(particleShape1.particleId==0)
{
float $wp[]=worldPosition;
setAttr locator1.tx $wp[0];
setAttr locator1.ty $wp[1];
setAttr locator1.tz $wp[2];
}

this expression tells locator1 to follow first particle.

But locator1 stays behind one frame.

Why this happens?

There should logical explanation for this , i don't beleave it is bug,etc.

[joojaa]

Well in general you dont want to use setAttr, or getAttr, or any melscript in expressions. Especially when you dont need to. Yeah the values in the world position arrays are the values for last frame, true. Thats because the procession of calculation is not yet computed It will happen just before drawing screen.

There should logical explanation for this , i don't beleave it is bug,etc.

Its not a bug per see, its quite consistent with how maya operates in general. Its how maya categorizes the computation. See maya works like follows. whanever something changes maya marks it as needing to update (the term used is attribute being dirty). Maya does this to avoid computing many things over and over again per frame.

Normally maya updates each attribute when its first asked for. However asking for the single value of a array atttribute, is not the same as asking for a normal attribute.  The expression for particles work on the internal representation of the node this sckips the graph optimization (it does not have to but the ndoe would need many more conditionals if not making it a wee bit slow as it would nee dto exit the optimized code loop). Since the node knows in advance its going to need to update the world positions last for the visualisation anyway its foregoes doing the relatively expensive computation 2 times by marking the attribute clean ahead of time. However more importantly is ensures maya wouldnt go ahaead and use 1 frame old particle positions on screen sicne the alternative option is to not update the world postions after the after dynamics script. (well ok this COULD be fixed with yet another attribute but why slow computation down for a borderline case)

In general you almost never want to operate on worldspace positions of components tough for many other reasons. Its very easy for you to circumvent this and use position instead of worldPosition, position is already up to date and you can jsut simply calculate the world position yourself, use a node (i recommend this as secondary alternative it gives you better performance IF you ever hide the locator), using one more particle node OR restructure your situation so that the particle node really is allways in worldspace position. I recomend the restructuring because theres little sane need to ever move the particle shapes around in the scene because thats like trying to do the einstein experiment that led to the theory of relativity.

PS: its actually better to unpack the data yourself in a nonparticle expression node this actually forces the data to be up to date. But better yet it wouldnt get suspended when scrubbing backwards, or when using cached partice shapes.

[breeder]

Thanks for answer.

Another question.

I want to get vector array data in multi float or multi double3 attribute.

Only way i can think of is for loop.

float $wpp[]=getAttr particleShape1.worldPosition; 
for ($i=0;$i<size($wpp);$i++)

{

setAttr worldPos.wp[$i] $wpp[$i];

}

*worldPos is node , with wp float multi attribute.

But this is slow.

Maybe faster solution would be setAttr worldPos.wp[0:$i]  $i0 $i1 $i2......?But how to do this?Only i can to is to put induvidual elements ?

[joojaa]

Not if you intend to use mel.

Only i can to is to put induvidual elements ?

The slowness is not about calls your NOT supposed to use setAttr in expression nodes (period), because this would update the undo queue. Maintaining the undo IS SLOW.* But tahst insignificant to the speed hit you get because: you cant let maya optimize graph evaluation so your expression node need sto fire each frame (theres little reason to ever use the allways update option) and the fact that you cicumvent the entire logic on HOW mayas supposed to work. There is a secondary reason and thats the trouble you get when your working with other files than jsut this project (it is quite litarary sucidial form a long term perspective).

Make a custom node. Its pretty simple.

But this is slow.

well if you work against the system you can bet your ass its going to be slow. Its going to be slow as long as you enbrace the mechnaism and not try to implement your won mechanism. Its counter productive and it will frustrate you to no end.

*See expressions have this language call, expressions are NOT mel so reading the mel reference for doing expressions is jut ballanty worng.

object.attribute = expression.

This is wildly different form asking to setAttr, yeah you can not loop their assignemnt because its a explicit connection and maya NEEDS to know the connectiosn up front. You cant cahange connections runtime.