Position Keys on Even Pixels

Posted on by Larry

[ This article was first published in the February, 2006, issue of
Larry’s Final Cut Pro Newsletter. Click here to subscribe. ]

Here’s an interesting problem I ran into recently for a broadcast client. They were doing a lot of chroma-key work, using DVCPro-50 as their source video, and all their keys had a very soft focus.

In looking at the problem, I realized they were the victim of interlacing. The fix was simple, but not intuitive.

Here’s the issue. All Standard Definition (SD) video is interlaced for broadcast. And, unless you are specifically shooting DV footage in progressive mode, all DV footage is interlaced as well.

Interlacing means that first all the even lines are displayed on the screen, then all the odd lines. Final Cut hides this from you by displaying all lines at the same time — which is why, sometimes, it looks like there are thin horizontal lines running out from the edges of moving objects when you view fast-moving images on your computer screen.

However, even though the computer displays video progressively, the interlacing is still there.

The problem my client experienced happened when they repositioned the key source on V2 (the anchor on a blue screen) against a highly animated background on V1. In doing so, they put all the even scan lines on top of the odd scan lines. This created a weird interlacing conflict that caused the image on V2 to get blurry.

The solution was simple. Make sure, when you are positioning your key source on an upper video track, that you only position it in increments of 2 vertical pixels. You can move images horizontally however you wish. But vertical position needs to move by even pixels.

In the Motion tab, there are two data boxes for the Center parameter. The left hand box controls horizontal position and the right hand box controls vertical position.

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