Nuke Premult Explained: Why It’s Essential in Compositing (+file)

Comp Lab Compositing Basics Series ① Nuke Premult
Written & Illustrated by @CompLab

If you’ve ever opened Nuke as a beginner, you’ve likely encountered a mysterious node called Premult.
And let’s be real—if you don’t understand how it works, your comp turns black… and so does your soul. 😅

Today, let’s demystify what Premult is, why you need it, and how to use it correctly in real nuke projects.
You’ll also get a downloadable practice file to test it yourself.

🔍 What is Premult in Nuke?

Nuke Premult means multiplying your RGB values by your alpha channel.
In simpler terms: it makes transparent areas actually transparent by blacking them out.

When alpha is 1 → RGB is untouched.
When alpha is 0 → RGB is multiplied by 0 → becomes black.

This is critical when compositing keyed images or elements with transparency.

✅ Nuke Premult Quick Example

Say we have a grey constant with values (1, 1, 1, 0.5).
When we apply Premult, it becomes:

→ (1×0.5, 1×0.5, 1×0.5, 0.5) = (0.5, 0.5, 0.5, 0.5)
Half the brightness due to the alpha value of 0.5.

🎯 Why Use Premult?

Let’s say you have a tree image with an alpha channel.
You want to composite it onto a sky background.

• The tree is visible (alpha = 1)
• The white background should disappear (alpha = 0)

When you Premult this image:

• Tree remains as-is (R×1, G×1, B×1)
• Background becomes black (R×0, G×0, B×0)

Now it’s ready for clean compositing.

⚠️ What Happens If You Skip Premult?

• You’ll see unwanted black in transparent areas.
• Your composite may look washed out.
• You’ll get dark edges (a.k.a. “dark fringe” issue).

Especially if you Unpremult before adjusting and forget to Premult again—you’ll definitely see issues.

🛠️ Real-World Application – Nuke Premult

🔹 Case 1: PNG or alpha-included images

→ You can apply Premult directly.

🔹 Case 2: Rendered EXR in Linear Color Space

→ Follow the correct workflow:
Unpremult → Color Correction / Blur → Premult

💡 So… What is Unpremult?

Unpremult reverses Premult:
It divides RGB by alpha, restoring original edge brightness and color.

Unpremult = RGB ÷ Alpha

Use this when working with rendered EXRs or when you need to correct edge colors before Premult again.

🤯 Why EXRs Need This Workflow

EXRs use Linear Color Space. That means:

• RGB stores physically accurate lighting data.
• If you color correct after Premult, your edge data (already darkened) gets altered.
• Result: muddy or overly bright edges = bad comp.

So what’s the problem, exactly?

If you skip the correct order:

Unpremult → Grade / Blur → Premult

and instead apply color corrections directly on an image that’s already in a Premult state (where RGB is already multiplied by alpha), then…

👉 The RGB values near the edges are already darkened (close to 0)
👉 When you increase gamma or gain, the edges also get brighter
👉 Result: messy edges — black halos, white fringing, weird artifacts

🔄 Comparison of Workflows:

💡 So why do you need to Unpremult before grading?

Simple:

“To modify the true RGB values without distorting the edges.”

Once you’ve applied Premult, the RGB data near the edges is already compromised by alpha.
Any grading after that can damage those pixels even more.

So always Unpremult first, do your color corrections, and then Premult again to restore the alpha correctly.

🧠 Nuke Premult – Summary

• Premult: Multiply RGB by alpha → mask out transparency.
• Unpremult: Divide RGB by alpha → restore original color before color correction.
• Apply Unpremult before any grading, then Premult again after.

📁 Download the practice file (link)

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or, email : complab.vfx@gmail.com

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