F1 Channels and alpha · Lab

1 The idea

Every pixel stores four numbers: r, g, b and a (alpha). Alpha says how much of the pixel is covered: 1 = fully, 0 = empty, anything between is an edge, hair or motion blur.

This CG robot was rendered in Blender with its shadow on the ground. Press "a" on the Viewer and point at pixels: the shadow lives in alpha too, while its colour is 0.

2 Hands-on in the mini compositor

Task: use the Shuffle to send alpha into r, g and b so the matte shows as a black-and-white image

Click the Shuffle node and set out r, g, b to a

SkyVFX mini compositor · F1640×360 · float · 1001

3 Deep dive

Four numbers, any value

In Nuke every channel is a floating-point number, not locked to 0–1. A hot light such as a spark core can reach tens, and some steps can even push values below 0.

Alpha is different from the other three. It should normally stay between 0 and 1, because it is the share of the pixel the object covers.

Where in the image	rgb	alpha	What it means
Middle of the robot	Full colour	1	Hides the background fully
Robot edge	Part of the colour	0.5	Hides half
Shadow on the ground	0	Between 0–1	Only darkens the plate
Spark core	Far above 1	0	Pure light, hides nothing
Empty area	0	0	Nothing there

Alpha is coverage, not brightness

The robot’s shadow has colour 0 but alpha above 0. Through Merge over the result is 0 + B·(1−a), so the plate darkens by the alpha and no colour is added.

Example: a shadow with alpha 0.4 over ground at 0.5 gives 0.5 × 0.6 = 0.3. The ground keeps 60% of its light.

To see what an element blocks, look at the a channel before the colour

More than rgba: layers

One EXR file can carry many layers: depth, normals, motion vectors, separate light groups. A layer is a named group of channels, such as rgba.red.

Shuffle moves channels around. Copy is handy for one channel, such as alpha from a second input. Remove drops layers you are done with, so the script stays light.

1Read EXR
2Shuffle a layer
3Use it
4Remove (keep rgba)

Reading channels properly

Press R, G, B or A on the Viewer to see one channel at a time. A matte that looks fine at normal exposure can hide small holes that only show with the gain pushed up.

Another quick test: put the element over a white Constant. Holes in the alpha and dirty edges jump out at once.

4 In real work

In Nuke

Read (EXR) → Viewer: press A for alpha, R/G/B for each colour
Shuffle: wire a into the r, g and b outputs to see the matte as a black-and-white image
Copy: bring alpha from a Roto or key branch into the element
Remove (operation keep, rgba) once the extra layers are used
Merge over a white Constant to check the alpha for holes

Common mistakes → the fix

✗ Painting a shadow black in rgb with alpha 0✓ Over with alpha 0 just adds 0, so nothing changes. The shadow has to live in alpha.
✗ Shuffling a light-group layer into rgba and losing the alpha✓ Such layers usually have no alpha of their own; route the original rgba.alpha across too.
✗ Combining two mattes with plus until alpha goes above 1✓ Combine mattes with max or screen so the value stays at or below 1.
✗ Letting every layer flow through the whole script✓ Add a Remove (keep) once you are done; the script gets faster and easier to read.

Before you send the shot

Alpha viewed with A and the gain pushed up
No holes or dirty edges over a white Constant
Alpha stays within 0–1
You know whether this element is premultiplied
Unused layers are removed

5 Check yourself

One question to check yourself

A pixel on the robot’s edge has alpha = 0.5 in a premultiplied image. What does its colour look like?

What you learned

🔒 Answer the question above and this card opens