Position Attribute in Expression Question (from Learning Maya Dynamics 5)
 

float $nz = 3* noise (nurbsPlane1ParticleShape.position*.1 + time);
nurbsPlane1ParticleShape.position = nurbsPlane1ParticleShape.position0 + <<0,$nz, 0>>;

Hi, that expression is used to control the y value of each particle in a grid of particles that has been parented to a nurbs plane. The result looks kind of like cloth blowing in the wind. What I don't understand is how the part that is in red can possibly be used to assign each particle its own y coordinate. Isn't position a vector attribute? How then can it yield a scalar value for $nz? And furthermore, how is it that it yields a different y value for each particle in the field?

:confused: :confused: :confused: :confused:

The answer lies in the second line. The scalar value is used in a vector <<0, $nz, 0>>. Why does it yield to a different y value? Well, because the vector is added to the initial position of a given particle.

Thanks! What I still don't understand though is how an equation that uses a vector quantity with three distinct numerical components (nurbsPlane1ParticleShape.position) can yield a single decimal numerical value (float $nz).

If, for example, nurbsPlane1ParticleShape.position is equal to <<2,3,4>> and that value is then inserted into the equation, then shouldn't the equation also yield a vector quantity?



ps. Nice Quake III model btw.

Ah... Now I see what you mean. It is because of automatic type conversion. In this case the .position vector is automatically converted to a float by calculating a magnitude (length) of it.

These two will yield to the same result:

float $nz = <<1, 2, 3>>;
float $nz = mag(<<1, 2, 3>>);

Now I get it. Thanks!

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