CF6 Case 6: the sparks vanished and the robot’s edge went dark · Lab

1 The idea

Symptom: the artist plussed sparks into the CG, but in the comp they have almost all gone; only the part over the robot survives. The robot’s edge has also turned into a dark line. Two symptoms that look unrelated, from one cause.

The sparks have alpha = 0. Premult multiplies colour by alpha, so everything outside the robot’s alpha becomes 0. And the robot render was already premultiplied, so multiplying again darkens its edge. The same thing happens on real jobs to extra CG passes that aren’t inside the alpha: after a Copy + Premult they silently vanish.

Premult: rgb · a → a = 0 gives 0

2 Hands-on in the mini compositor

Task: find the one node behind both symptoms and fix it to match the target

SkyVFX mini compositor · CF6640×360 · float · 1001

3 Deep dive

Premult is a multiply by alpha

Premult only does rgb × a, so anything with alpha = 0 becomes 0. Light, fire, emission and extra passes, often rendered with alpha 0 or outside the object’s alpha, vanish the moment they pass a Premult.

This case has two symptoms from one cause: the sparks vanish (alpha 0) and the robot’s edge goes dark (already premultiplied, then multiplied again). When several symptoms appear together, look first for the one node that explains them all.

Pixel	Before Premult	After Premult
Spark outside the robot	rgb 5, a 0	rgb 0
Robot edge (already premult)	rgb 0.15, a 0.5	rgb 0.075
Robot centre	rgb 0.3, a 1	rgb 0.3

The same thing in real CG work

On a real job, an extra pass, liquid drips on a character, vanished even though it was rendered. Someone had copied alpha in from another pass and then premultiplied, so everything outside the new alpha was silently cut. No error warns you.

Before any Premult, ask: is anything outside this alpha that must survive?

A safe order

Finish the parts with alpha (the solid object) first, then add the light parts after the Premult or after the Merge over. Light doesn’t need premultiplying because it hides nothing. This order is for light that belongs to the object itself or sits in front of it. Light from something behind a foreground still needs the holdout from Case 5 (CF5): stencil it by the foreground’s alpha, or plus it onto the plate before the foreground goes over.

1Object: grade unpremultiplied
2Premult
3Merge over
4Light/emission: plus after

4 In real work

In Nuke

Before adding a Premult, look at the Read where a solid object’s edge turns semi-transparent, in both rgb and alpha. If the surface colour fades out together with the alpha, it is already premultiplied (most CG renders are). Don’t use “rgb ≤ alpha” as the test: highlights and emission brighter than the alpha, even at alpha 0, are normal in a premultiplied render
Merge (plus) for emission, fire and sparks, after the Premult or after the Merge over (if the light sits behind a foreground, hold it out first; see CF5)
Copy alpha then Premult: check every pass still fits inside the new alpha
Disable nodes one at a time (D) and see whether the symptom goes away (as in Case 2)

Common mistakes → the fix

✗ Premultiplying everything "to be safe" before the merge✓ Premult only straight images, and never alpha-0 light.
✗ Adding emission into the CG before the Premult✓ Add it after the Premult or after the over (if the light sits behind a foreground, hold it out first; see CF5).
✗ Fixing each symptom separately✓ Look for the node that explains every symptom first.

Before you send the shot

No second Premult on an already premultiplied image
Alpha-0 light and emission still present in the comp
Every extra pass survives any Copy + Premult
No dark line on CG edges (viewer gain up)

5 Check yourself

One question to check yourself

Which node made the sparks disappear?

What you learned

🔒 Answer the question above and this card opens