FOLLOW PATH CONSTRAINT- keyframing Evaluation Time

Blender Manual 2.6 says in regard to the follow path constraint:
“(We) need to keyframe Evaluation Time for the Path. Select the path, go to the path properties, set the overall frame to the first frame of the path (eg frame 1), set the value of Evaluation time to the first frame of the path (eg 1), right click on Evaluation time (slot), select create keyframe” and likewise for the end frame.

But I have found that keyframing fails. I want a rigged character to walk along a curved path - either Bezier or Nurbs. I apply the Follow Path constraint to the armature - it is accepted. But when I try to keyframe the ‘Evaluation Time’ slot, a cryptic Error message turns up:
“F-curve with path = evaluation time (0) cannot be keyframed. Ensure it is not locked or sample. Also remove F-modifiers.”

Could someone kindly explain what on earth this message means?

It looks like for some reason you already have an Evaluation Time F-curve for your curve that has a modifier on it. With the curve selected, go to the F-curve editor and see if there is already a curve; if so, press N to see the panel on the right that shows the modifiers and delete the modifier.

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I can’t thank you enough, Alan. I have been struggling with this error message for nearly 10 days and it was frustrating progress on my path animations.
But your suggestion did the trick. I had kooked at the F-curve before but didn’t where to find the modifiers. I pressed N for the transform panel and sure enough, the ‘generator’ modifier was already there (I think by default). I deleted it as you said and VOILA! I could keyframe the Eval Time slot.
Phew - I am grateful to you, Alan, as this issue was bothering me quite a bit.

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Thanks from me too. I’d also been stuck on this. An addition to the manual might help others.
It seems that when an onject is given a follow path constraint, a generator modifier is added to the path. Removing it makes the eval speed keyframing control work properly.
What is the purpose of the generator modifier being added automatically ?

I had the same problem. I suppose the modifier is added automatically because we clicked on animate path when we created the following path constraint. If you don’t do so and just set the evaluation time key frames for example 1 for the first frame and 100 for the last frame everything works fine.

Thanks for the handy info. Saved me a lot of time and frustration. I included a link of this response in the video description of an animation I just uploaded on YouTube (chilingomations) to hopefully help others. :+1: