How to use Scale:Velocity in expressions

[Colin Braley]

Well if you want to really base blur level on the velocity of the scale property you have to apply an expression like this:

//Begin Expression
other = thisComp.layer(“square”);
s1 = other.scale.velocityAtTime( time )[0];
s2 = other.scale.velocityAtTime( time )[1];
(s1 + s2)/2
//End expression

this expression takes the velocity of the x scale of “other layer” and the velocity of the y scale of “other layer” and averages them toghether. However, this may not be what you want because if scale is decreasing velocity will be a negative number, and you can’t have a negative blur factor. After effects will give you a value of zero instead for blur factor. In order to compensate for this you would use an expression like the following:

//Begin
other = thisComp.layer(“square”);
s1 = other.scale.velocityAtTime( time )[0];
s2 = other.scale.velocityAtTime( time )[1];
(Math.abs(s1 + s2))/2
//End

in the second expression, it does not matter if scale is increasing or decreasing, all that matters is the speed scale is changing at (the speed)

~Colin

[Spritemaster]

I would recommend:

Math.sqrt(s1*s1+s2*s2)

instead. For one thing, it is more accurate physically, but that may not be so important; the main issue is that the expression you propose,

(Math.abs(s1 + s2))/2

can be zero if s1 is positive and s2 is negative, of the same magnitude. Namely, if a layer expands in x and contracts in y, you would get zero blur which may not be what you want. I suppose in most cases the scale values are linked (Xscale=Yscale) anyway and then it doesn’t matter much, but just in case…

AA

[Dan Ebberts]

Assuming x and y scale are linked, I’d do it like this:

factor = 10;
Math.abs(thisComp.layer(“scale_layer”).scale.velocity[0])/factor;

Adjust factor to get the relationship you want between the velocity and blur.

Dan