Driver Glitch

Here we have a simple plane with a shape key that moves it up and down. The shape key is driven by a bone, so that moving the bone moves the plane along with it. Inexplicably though, when the driver gets close to its mid-point, it jumps off the F-Curve and snaps to 0:

Deleting the driver’s middle keyframe fixes this, but I need that keyframe to be there (it’s actually a stripped-down eyelid rig, which uses a Bezier curve instead of a Linear one).

The bone’s range of motion is quite small, so unless there’s a secret quantum physics feature that comes into play at smaller scales, I’m at a loss as to why this is happening. Here’s the file if anyone’s keen to poke around.

Driver Glitch 1a.blend (2.6 MB)

Thanks for any help!

(c:

That driver is weird all right. It’s probably some weird setting that I’ve never seen used, maybe some setting that was never intended to be used with driver f-curves or that was never intended to be used with three keyframes on a driver. Among other things, the line simply shouldn’t look like that for the f-curve values showing up in the sidebar.

Remaking it worked fine for me. If you want linear interpolation, use vector handles for the “keyframes”. I wouldn’t go messing with any keyframe interpolation modes for drivers, not unless my life was on the line. Aligned handles work fine too, if you’d prefer.

My revised version, using your values for keyframes to the third decimal. Notice the very different shape of the curve…

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Thanks for taking a look at it, the actual curve I need looks like this:

Driver F-Curve

It’s one in a series of shape keys that make up an eyelid blink, and I only changed it into a straight line for simplicity’s sake, and to see if it was anything to do with it being a curve (which it seems not to be). I made a demo file a while back which doesn’t exhibit this problem, and I can’t seem to reduce the file any further so perhaps I’ll report it as a bug and see what happens…

(c:

i dont know for you, but i think RBF drivers could help you more with that, for blending shapekeys and stuff…

Looks like my “Quantum Resolution” theory was right. I reduced the F-Curves on my other file to 1000th scale (which has also messed up the shape key transition, but never mind that), and now you can see the exaggerated effect of the driver jumping off the curve.

So in all likelihood this means the RBF drivers wouldn’t have worked either. I think the workaround in the case of my eye blinks is to stretch out the F-Curves x100 and then divide the driver expressions by 100.

Yeah it looks like a numerical precision problem but at the same time… if you click “update dependencies” in the driver tab it shows the correct value. As soon as you move the bone again though, it snaps to 0 once more. I don’t know that looks like grounds for a bug report to me.