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# Logic between the value and speed curves

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 Logic between the value and speed curves on Aug 22, 2013 at 9:58:58 am

Hi!

I usually work with 3ds Max, so I have some difficulties understanding the graph editor in AE.

Here's an example. If the value keyframes are smooth continuous bezier's, how can there be sharp corners in the speed curve? If I have a value curve like this in a 3d program, I'll get smooth results. In AE, I ended up with a very faint nudge in the camera animation I was trying to make. I discovered that the sharp corner's in the speed graph made it happen.

Is this due to the mathematics AE uses with keyframes, or am I just not fully understanding what happens in the speed graph? I have a feeling that AE and max have very different ways of calculating positions, because in AE you have the ability to modify paths in the 3d-view, but in max this is not possible in any way.

Ideas?

 Re: Logic between the value and speed curveson Aug 22, 2013 at 1:41:46 pm

They do work quite the same, though I've been there thinking something was wrong! I believe Max has a single curve that will let you control value, never speed. If you want to make a faster movement at the start you'll have to alter its value at that time or make a curve in the graph with handles.

In AE you have them separated. But they influence each other. Increasing a keyframe outgoing speed lets you quickly set this parabolic curve behaviour(for the value) without the need to actually look at a curve graph.

To avoid the camera nudges I usually just set a keyframe to linear (after all keyframes are there). This way I can see it's in/out velocity. Then I'll lock outgoing to incoming (keyframe velocity tab), after that I can set whatever easy in/out influence I want and the movement will remain smooth.

It seems a pain at first, but I get better/faster results for camera movements in AE than I do in C4D (like MAX).

 Re: Logic between the value and speed curveson Aug 23, 2013 at 12:56:49 am

The speed graph shows the rate of change of the value graph. Put another way: the speed graph is the derivative of the value graph.

When the value is increasing linearly (as it is between the two keyframes in your first picture) then the speed graph will be flat because the rate of change is constant.

Wherever the speed graph is not flat there is either acceleration (if the line is sloping upward to the right) or deceleration (if the line is sloping downward to the right) occurring.

So the sharp transition simply shows the point at which the value has stopped accelerating (foot entirely off the gas) but is still moving at a constant rate.

Darby Edelen

 Re: Logic between the value and speed curveson Aug 23, 2013 at 6:34:49 am

Hi Darby, thanks for you insight. I understand what a speed curve is, but I still can't see logic in the sharp corner. I still think (and might be wrong), that if the value keyframe is truly smooth, so should the speed curve go smoothly to the constant state.

 Re: Logic between the value and speed curveson Aug 23, 2013 at 12:30:13 pm

In a case without inertia, all corner should be sharp. But still, by default if you smooth a keyframe (ctrl + click) the speed curve gets smoothed as well.

In your first example (first SS) you smoothed only the value curve, not the movement. One should be able to get both types of animation. There is logic to it. But I'm with you when I say that maybe AE defaults to least used one.

 Re: Logic between the value and speed curveson Aug 23, 2013 at 2:01:25 pm

Hi Cassius. Thank you for your comment.

[Cassius Marques] "In a case without inertia, all corner should be sharp."

I'm not sure what you mean by this. I made a little test, creating a similiar animation in 3ds max and then made a speed curve for it with a little trickery. The speed curve looks almost exactly same as in AE, but in close inspection there is no sharp corners, but instead the smooth transition that I wished for.

For now I just think that this is due to the mathematics used in AE. Somehow the curves from both sides of a keyframe are not perfectly put together.

 Re: Logic between the value and speed curveson Aug 23, 2013 at 6:13:36 pm

I meant an object can change speed (or go to a complete stop etc.) without accelerating/slowing down. It has to be sharp corners.

[Gabriel Valkama] "...creating a similiar animation in 3ds max and then made a speed curve..."
You're probably changing the VALUE curve, not speed. I don't recall max letting you change speed directly (or I never used that)

[Gabriel Valkama] "Somehow the curves from both sides of a keyframe are not perfectly put together." What curves? If you mean the "handles" that set the incoming and outgoing speed. They will be matched if you lock them together like I said on my previous post.

 Re: Logic between the value and speed curveson Aug 26, 2013 at 6:28:21 am

[Cassius Marques] "You're probably changing the VALUE curve, not speed. I don't recall max letting you change speed directly (or I never used that)"

Yes, you are correct in both cases. I just created a simulated speed curve to represent the value curve. The corner was sharp, but not absolutely sharp like in AE. A very small difference.

[Cassius Marques] "" What curves? If you mean the "handles" that set the incoming and outgoing speed. They will be matched if you lock them together like I said on my previous post."

I was talking about the speed curve keyframes. I thought they should be locked in the method I used, but I'll have a go with the method on your first post, thanks for that.

 Re: Logic between the value and speed curveson Aug 23, 2013 at 6:28:16 pm

[Gabriel Valkama] "I understand what a speed curve is, but I still can't see logic in the sharp corner."

Then as you know the speed graph must be flat between the two keyframes because the value is increasing linearly between them. The value on the left side of the first keyframe, however, is decelerating all the way up until the keyframe. Therefore, at the keyframe there is an inflection point where the value is no longer decelerating and is now increasing linearly.

I can't see the logic in not having a sharp corner in that case*.

You've already found an option for eliminating the sharp corner, but you may not need to eliminate it. A sharp corner in the speed graph shouldn't necessarily correspond to a hitch in the animation.

The only circumstance where I've ended up with a constant rate of change between Continuous Bezier keyframes is when I've converted them from a completely Linear animation (i.e. the speed graph was flat when I converted them). I could see how you might end up with a graph like yours if you had a flat speed graph, converted to Continuous Bezier, and then started changing the values. AE does not update the bezier handles of Continuous Bezier keyframes based on new values. Auto Bezier keyframe tangents will update based on changes to keyframe values.

Also remember that you can edit the Speed Graph directly in AE which is not possible in many other applications I've used.

*Note: If you pull the incoming tangent of the keyframe on the value graph out to the left you should notice that the speed graph ramps more into the flat section, but in order to get it to ramp perfectly into a flat line (constant rate of change) you'd have to draw that tangent out to an infinite length. So as you can see there is no way to not have a sharp corner if you're transitioning directly into a constant rate of change. The option you found for removing the sharp corner is altering the rate of change between the keyframes so that it is no longer linear which allows for the acceleration to transition smoothly.

Darby Edelen