Motion Field

Normalise a rendered velocity buffer into the same flow field the Optical Flow node produces.

What it does

Motion Field normalises the Render node's velocity buffer into the flow map Optical Flow produces. A rendered scene already knows how everything moved, so nothing is estimated: there is no reference frame and no search.

It emits the same four outputs as the estimators, a flow map, a colour visualisation, a magnitude field and the frame's peak motion. That shared representation is what lets a rendered scene and a piece of footage drive Displace and the other motion tools the same way.

When to use it

ParameterTypeDefaultWhat it does
velocityRaster—Rendered velocity buffer (wire the Render node's velocity pin). Unconnected = no motion.
enabledBooleantrueEnable motion-field computation (off = passthrough no-motion)
flow_scaleNumber1Multiply flow vectors (amplify or reduce the detected motion)
opacityNumber1Layer opacity (0 = transparent, 1 = opaque). Editable, wirable, keyframable.

Gotchas

Wire the Render node's velocity output into Velocity. That connection is what turns the velocity pass on, so nothing else needs enabling first. With Velocity unconnected the node reports no motion rather than the previous frame.

Flow Scale multiplies the vectors and is the only shaping control here. There is no threshold and no smoothing.

The flow map encodes motion in its red and green channels around a midpoint of 0.5, at a range of 128 pixels each way. Set Displace to Vector mode to warp on both axes from it, and an Amount of 256 reproduces the motion pixel for pixel.

Worked example

  1. Add a Render node and wire its velocity output into Velocity.

  2. Wire Flow Map into Displace's displacement pin.

  3. Set Displace's Mode to Vector and raise Amount until the smear reads.

  4. Raise Flow Scale to exaggerate the motion, or lower it to damp it.

See also

Optical Flow · Structure Tensor · Displace · Render