Using Nests to Clean Up Dissolves Between Chroma-keys

Posted on by Larry

[ This article was first published in the December, 2008, issue of
Larry’s Final Cut Pro Newsletter. Click here to subscribe. ]

Steve Gagne sent in a question that sparked this tutorial. He was dissolving between chroma-key shots and in the middle of each dissolve, the foreground clip color-shifted to green. He thought this could be fixed using nests, but needed some help in figuring out how.

Actually, nests are an excellent way to fix this problem and Steve gave me a great reason to talk about it in this issue.

First, let’s define a term: Nesting is the process of putting one sequence into another sequence. You can do this either by dragging a sequence from the Browser into the Timeline, or by selecting the clips you want to create a sequence from in the Timeline and applying a menu choice.

Regardless of how you create it, a nest is always a sequence contained inside another sequence.


Let’s start by illustrating the problem. Here’s a wide shot where Katie is standing up and stretching in front of a green screen.


Next, we cut to a close up. Both the foreground and background images changed between shots.


If we leave this transition as a cut, everything works great. The two shots will edit together seamlessly. However, if we dissolve between them, watch what happens.


In the middle of the dissolve, the green background starts to show through Katie – look specifically at her raised elbow and arm in the upper right corner of the frame. Notice how it has gone from blue to green. This is caused by the dissolve altering the background, which changes the results of the key.


As you can see here, I’m dissolving both the foreground and the background clips. However, even if the dissolve is only in the foreground with the background remaining the same, the results are identical – the foreground goes green.


We can solve this problem by creating a pair of nests – sequences of clips contained in other sequences – where one pair contains the clips before the dissolve and the second pair contains the clips after the dissolve.

To create a nest, select the clips you want to group – in this case, the first clip on both V1 and V2 – and choose Sequence > Nest items. (The keyboard shortcut is Option+C).


Create a nest for each set of clips you want to dissolve between. You’ll need two nests, one for the clips before the dissolve and one for the clips after the dissolve.

You only need to create nests for clips you want to dissolve between. You don’t need to nest clips when you are cutting between shots. There is no quality difference between nested and unnested clips.

Note: Remember that dissolves need extra video before the In and after the Out. So, be sure to include extra video at your transition point inside the nest.

Here’s what it looks like when you have nested both sides of the dissolve. Since each nest is a sequence, each can have its own tab in the Timeline.


Note: Nests are dynamic. That means that any change you make to the clips inside the nest are instantly reflected back to the nest itself. To open a nest to make changes, double-click it. Once a nest is open in the Timeline, you switch between it and any other sequence the same way as always – by clicking the sequence tab at the top of the Timeline.

Now, when you dissolve between nests, the video doesn’t color shift.



One last thought. While I don’t use nests very often, when I need to treat a group of clips as though it was a single clip, nests are perfect.

UPDATE – Dec. 17

Tom Hollingsworth had a followup question:

I read your piece on using Nesting to improve green-screens.


Any idea why nesting works better?

Larry replies: Tom, nesting doesn’t help with the key, it helps when dissolving between keys. And the reason is that when you dissolve between nests, you are dissolving between two fully-composited images – the green background no longer exists. When you dissolve between individual clips, you are changing the relationship between the foreground and the background which causes the color shifts to occur.

UPDATE – Jan. 1, 2009

Graham Taylor, a UK-based editor, sent this comment in:

I would add that such so-called ‘dirty dissolves’ can occur on all types of composites involving keyed images on a higher video layer. It is particularly common when fading a composite title+background[s] to black. It’s good practice to nest the last frame and then add a fade to black to the nest. It’s a pity that FCP doesn’t offer a macro for doing this, however!

Larry replies: Thanks, Graham, for adding this.


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