Expression Question Dan Ebberts?

[Sam Moulton]

value +

as in [value + [0], 300]

this would let you animate the X position using keyframes while holding the Y position to 300 pixels….

[Dan Ebberts]

Yes, value is the key. As an example, this expression will cause a layer to orbit its keyframed position:

r = 50; //radius (in pixels)
f = 1; //orbits per second

angle = time*Math.PI*2*f;
value + [Math.sin(angle)*r, -Math.cos(angle)*r]

The important point is that whatever you add to value needs to be a 2-dimensional quantity. Normally it will be in the form

value + [a,b]

where ‘a’ will get added to the layer’s x position and ‘b’ will get added to the layer’s y position.

Hope that helps.

Dan

[Mike Clasby]

That’s a great expression for making a screw or spiral movement, but it get a little weird for my when I set keyframes from 1 to 0 (I added a slider for “orbits per sec”). The circular motion changes from Clockwise to Counterclockwise when the “orbits per sec” is decreasing, going from keyframes 1 to 0.

A slider for the “radius” does what you’d expect. I can get my spiral by decreasing the Radius to 0, but I can’t get it to slow to zero (by keyframing the “orbits” from 1 to 0). No biggie, I was just wondering what was going on.

[Dan Ebberts]

You run into that sometimes when you mess with the time component of an expression. What happens is, that at each frame the expression calculates its result as if the frequency had been at the current value since time zero. It doesn’t remember that it was at some other value on previous frames. So there’s this discontinuity. It takes more work to get it to act like you’d expect. For example, you could go frame-by-frame and calculate the accumulated angle at the current frame.

Dan

[Mike Clasby]

It’s still a great expression, it just needs a little adult supervision (like me). Thanks.

[Brian Mills]

thanks all…knew it was value somthing!!!