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Am I doing this right?
I am trying to teach myself how to use expressions (with a lot of help from this forum I should add).
One thing I’ve needed to do several times in my projects is to work out the direction that something is traveling at any point in time so I can adjust and other property or object accordingly. In the case bellow I have set up a particle emitter to move around the screen and I wanted the angle of the emitter to change so that it looks like the tail of a firework or something. I haven’t come across a straight forward way of doing this so I’ve done the following:
old_pos=effect(“Particle Playground”)(“Position”).valueAtTime(time-0.2);
new_pos=effect(“Particle Playground”)(“Position”);
y_was=old_pos[1];
x_was=old_pos[0];
y_is=new_pos[1];
x_is=new_pos[0];
y_dif=(y_was-y_is);
x_dif=(x_was-x_is);
angle=Math.atan2(y_dif, x_dif);
deg_angle=radiansToDegrees(angle);
[deg_angle+90];This works well but is it the simplest way of doing this or is there an easier way?
Thanks
Conrad
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6 Replies
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Looks okay by me ;O). One could probably concatenate a few things into single lines, but the code is clean and understandable. As an alternative instead of calculating the angle yourself, you could use lookAt(new_pos,old_pos), but it’s basically meant for working in 3D psace, so results may sometimes be wrong.
Mylenium
[Pour Myl
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Cool, thanks. I’m glad I’m doing it right.
I’m a little comfussed about the lookAt command and how it works. I’ve just read about it in the manual but it doesn’t make sense to me. How would I use that instead?
Cheers
Conrad
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lookAt() generates a set of orientation angles – you give it a source position from which to look, then a target at which to look. The angles are generated as an array, so you would extract any angle using something like this:
pos_new=position;
pos_old=position.valueAtTime(time-0.2);angles=lookAt(pos_old,pos_new);
angle=angles[0];
[angle]
Mylenium
[Pour Myl
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This should work:
v = effect(“Particle Playground”)(“Position”).velocity;
a = radiansToDegrees(Math.atan2(v[1],v[0]));Dan
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I thought there might be a solution using the velocity property but I could find anywhere in the manual that told me what it returns. What values are in the array that it generates? Are v[1] and v[0] the vertical and horizontal speeds of the motion?
Thanks
Conrad
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I’ve just got a version of the porject working with your solution Dan (I’m still only guessing what the array values from the velocity are) but the angle seems to be more jumpy as the particle emmiter moves around. Is that because the velocity property is more accurate than the was I was doing it? I’m guessing the inaccuracy in my method smoothed out the change in motion at sharp corners in the motion path. Would that happen?
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