L5 Shutter angle and motion blur · Lab

1 The idea

Film cameras state the shutter as an angle. 180° means open for half of each frame’s time (1/48 s at 24 fps). Something moving 48 pixels per frame blurs over 24 pixels.

CG has to use the plate’s shutter, or things look choppy or over-smeared. Sparks that already carry blur must not get a second dose.

blur (px) = speed (px/frame) · angle / 360 · exposure = angle / (360 · fps)

2 Hands-on

Set the shutter angle so the blur length matches the plate

SkyVFX Lab · L5interactive · 1001

3 Deep dive

Degrees ↔ seconds

Exposure = angle ÷ (360 × fps). In Nuke the shutter is a fraction of a frame: 180° = 0.5. Blur length = speed × angle/360. A robot arm moving 40 px per frame blurs 20 px at 180° and only 5 px at 45°, the choppy action-film look.

fps	Angle	Open (s)	Nuke shutter
24	180°	1/48	0.5
25	180°	1/50	0.5
24	90°	1/96	0.25
24	45°	1/192	0.125

Read the shutter from the plate

No report? Measure it. Find something moving straight at a steady speed, measure how many px it moves per frame and how long its streak is (minus the object’s own size). Then angle = 360 × streak length ÷ motion per frame.

On set: note fps and shutter for every setup, and ask whether the DP changed the shutter for a look. Check the render’s shutter offset too (centred, start or end); a mismatch puts the streak ahead of or behind the object.

1Know the plate shutter
2Set CG to match
3Pick the blur method
4Compare streak lengths

Choosing how to add motion blur

Best is blur from the node that does the moving (Transform, CornerPin, Tracker, Roto, ScanlineRender), because it knows the real motion. If you see stepped copies, raise the samples, not the shutter. CG with rendered motion vectors can use VectorBlur; MotionBlur estimates motion from the image when nothing else is available (it may need NukeX / Nuke Studio, depending on your licence).

VectorBlur blurs in a straight line per pixel, so spinning or arcing things (wheels, rotors) get the wrong shape. Use a method that follows the curve, or render the blur in 3D.

When not to add it

A spark is a bright dot that is already blurred: its streak is its motion blur. Adding more makes a fat straight line. CG rendered with blur must not go through VectorBlur again. A flash shorter than the shutter (muzzle flash) records sharp while the rest of the frame blurs, and that’s physically correct.

When in doubt, switch the blur off and look whether the element already has some.

4 In real work

In Nuke

Transform / CornerPin / Tracker: motionblur (samples) on, shutter 0.5 for 180°
Keep Transforms next to each other so they concatenate: one filter, one blur. Don’t put a Grade between them
ScanlineRender: samples and shutter matching the plate
CG with motion vectors → VectorBlur, with the blur matched to the plate’s shutter
No motion data → MotionBlur (estimates from the image; may need NukeX / Nuke Studio, depending on your licence)

Common mistakes → the fix

✗ Using 360° to look smooth✓ Use the plate’s shutter
✗ Adding blur to sparks or CG that already have it✓ Check first; blur once
✗ VectorBlur on a spinning wheel✓ Use a curve-aware method or render the blur in 3D
✗ Seeing stepped copies and raising the shutter✓ Raise the samples, keep the shutter

Before you send the shot

Plate fps and shutter known (from the report or measured)
CG and transforms use the plate’s shutter and offset
CG streak lengths match real ones in the plate
No double blur on elements that already have it
No stepping on the fastest frames

5 Check yourself

One question to check yourself

The plate is 24 fps at 180°. How do you set the CG shutter?

What you learned

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