My Knowledge is conflicting with each other about expression and keyframes.

[Suraiya A]

Hello,

I am following one course, (Motion Secrets with Emanuele Colombo  from motion design school)

There the tutor applies bounce/rebound expression to create a follow-through effect on a certain parameter. All was good so far.

But I keep seeing two things.

1. To make this expression work you have to make the last keyframe linear (he made it all easy ease first, then applied the expression, then made the last frame linear)
2. He kept adding keyframes to create anticipation and apply his saved bounce expression on a particular keyframe after selecting it.

My question-

So far what I learned from other tutorials that expression overrides key frames. I mean when you apply expression to some parameter (like opacity or scale or position) and if it had key frames, all those key frame will become inactive after you apply the expression. If my learning is right then why did he apply expression and then make last keyframe linear? What difference does/will it make?

I respect everyone. I am still a student. He knows a lot more than me. But I am super confused.

I knew expression overrides key frames. Am I wrong?

Another thing,  He kept selecting particular key frame and  adding keyframes  one or after that (according to his need)and then take the middle keframe and add bounce expression from his KBAR,  I dnt have it , so I click alt click on stopwatch and add expression).

So my question-

Can you apply expression on a particular key frame?

Or when you apply expression it will affect the entire parameter according to the expression’s merit? (I mean what the expression is telling the parameter to do, it will do that ignoring the key frames. Right?)

I am repeating  He knows a lot. But as a beginner my knowledge is conflicting with each other.

Hence I am asking here.

I need these answers. Otherwise I can’t sleep tonight.

Thank you.  

[Mylenium]

No, expressions don't override keyframes. The evaluation orde in AE is always:

Base Property Value --> Keyframe Value/ Modified Value --> Expression

There are a few finer points to this, but let that be enough for the moment. Of course this can mean that the expression value can completely ignore the keyframe value and overwrite it, but it must not necessarily do so. Many expressions in fact reference base values or keyframe values using the "value" or "valueAtTime" keyword/ function, respectively. And that's why you can also reference keyframes explicitly in the code with things like valueAtTime(key(1)).time and similar, but you have to make provision for it and implement these functions. The rest I do not know since I don't know the tutorial/ course in question, but it all sounds a bit half-baked and not like I would teach people these things.

Mylenium

[Suraiya A]

Thank you so much for your kind reply. I feel like being lost.

[Rick Gerard]

Embed (don't attach) a screenshot of the problem layer with modified properties showing (press 'uu'), use the </> tool in the Toolbar to share the expression, and share the starting and ending values for the keyframes and maybe we can help.