Strap on your big boy pants and use Motionbuilder F-curves to fix bad mocap

A short guide to fixing mocap errors:
Bringing in Optitrack mocap data to Motionbuilder I notice that I get errors with a character if:

a.) there are any gaps. Use the Fill Everything in Motive to fix unseen gaps in the timeline after you have gap filled – even if the timeline is 100% green, tiny gaps may not show up.

b.) Some of the auxiliary markers e.g. the upper arm say, but not the elbow, has a section where it ‘peaks’ out – even thou’ the ‘segments’ body may look like it moves fine – This peak may cause an unwanted bone rotation in MotionBuilder. So turn on the bones view in Motive to see if any bones are twisting and fix.

Moving over the Motionbuilder, import & run the take on your characterised character.

If there is some error e.g. hand ‘flipping’, start by looking at bones higher up – in the hand’s case start at the elbow. Zoom in on the f-curves. If there is a section in the Rotations that looks like it was lifted out of position up or down, isolate each of X,Y or Z and move the ‘lifted’ section back into position.

Fcurve section ‘lifted up’ or down
Click-drag over keys to make a section and drag the green area on the left side to move it down to the correct position – repeat for the X and Z curves.

If a section of the curve seems like it’s inverted, like below…

Inverted section

…where the slope of the section appears to be going the wrong way i.e. sloping down not up, then select that section again – we’ll need to flip the section like this:

1. Grab bottom white arrow handle

Grab the bottom handle (white arrow) on the left side and drag it above the top white handle like so:

2. Drag handle up to match the original curve

..and move it up so that the top of the selected curve section meets the original curve  (here it’s on the right-hand side of the selection). Then get the bottom white arrow handle and drag it up so the  curve matches on the left side of the section like this:

3. bottom handle to match selection to curve on left hand side

..And these methods have worked for me. You will probably then need to go and fix the next limb down e.g. the hand.

For Gimbal issues this seems to affect hips, where there is a little ‘jog’, you might see it affect the character, as if the character flips out of position for a frame or too. Look at the Hips fcurve you may see something like this  section – it’s not a straight-up lifted section like the example above and that technique won’t fix it.

Gimbal errors

…Instead select over the affected keys for one section – I’m picking the bit on the left:

Apply the Gimbal killer filter from the filters list- it makes the selection shrink down to a tiny area then hit apply. Should be good to go.

The main thing is to work out if it’s a flipped section or a gimbal – flipped sections don’t have a curving ‘shoulder’ it is a clean break from 1 frame to the next.

Gimbal Killer filter applied – shrinks the selection to a small area

One more thing, if you can’t see Gimbal killer in the filters list change the dropdown to Selected Properties like shown