Am I misunderstanding the Difference Matte?

[Moof1904]

Do I understand the function of the Difference Matte (Effects > Key > Difference Matte) in FCP correctly?

1) Take video clip of static scene (a room, for example) and put it on the timeline
2) In a track above the first clip, insert a clip of the same exact scene (shot using a tripod without having moved the camera between clips 1 and 2) with a person walking
3) Apply the Difference Matte key effect to the top clip (the one with the motion that I want to isolate)
4) The result will be that everything that's the same between the two clips will be matted out and I'll be left with only the object(s) that are different between the two clips.

This isn't what's happening when I try to apply the difference matte to a pair of clips. I end up with a ghosty image of the person in motion superimposed over the static background.

Can someone please set me straight on this?

TIA.

[amiga]

Hi there, I don't have FCP but no, I don't believe you are understanding the principal correctly.

What your trying to do is key somebody over some footage? A bit like for example a space ship in ,say, Star Wars?

This is called chromakey and the process involves shooting the back ground separately then filming the object in front of a coloured screen usually green or blue but what ever colour it must be as far from the colour of the object as possible.

The computer treats the green colour as colour 0 and removes everything with that colour, leaving only the object (or person), who is then keyed over the background image...

Is that the effect you're trying to create?

[evil_santa]

Yep, thats what a difference matt should do, nothing to do with croma key as Amiga suggests. I have used difference matts (not on FCP) when we want to key something & its not been shot as croma key.

Im not sure what the result of the Difference matt should be, on the Quantel system i use at work, it will produce a Matt/Alpha channel that you then apply to the original footage, to cut it out. FCP might be different. I have used it for a while but i remember that it was quite difficult to use.

[amiga]

Oh, sorry...

[Moof1904]

Yes, what I'm hoping to achieve in the end would be the same effect as a chroma key, only using a difference matte instead. Because I can film my scene with a tripid and a single individual walking, if I understand the difference matte correctly, I should be able to matte out everything that's stationary in the scene and end up with just the person walking and essentially a transparent alpha channel that I can them superimpose on some other backdrop.

I'd use chroma for this, but it seems that using a difference matte would be easier in this circumstance. If I'm understanding how to use it correctly, that is.

Reading the various texts on the issue, it seems that all I need to do is layer the video thus:

---scene with person moving---
---same exact scene with no person in it---

Run the difference matte effect and end up with a video channel that has only: person moving through empty space.

Is it this simple? Then what am I doing incorrectly?

One more thought: from the WWDC keynote, it looks like the new iChat is running a basic Difference Matte when employing the feature whereby one can add different still or motion backdrops on a video chat session. It seems to me that when [what's his name?] stepped out of the frame that defined the static portion of the frame and when he returned, iChat calculated the difference and removed the static background. If iChate can do it that easily, I have to believe that I can get it to work on FCP. Has anyone done this?

[chameleon]

This may be a very old thread, but I'd like to revive it.

I too would like to do this as a poor man's green screen. As Moof1904 said, if iChat can do this, surely FCE or FCP can.

Can anyone shed light on the right way to make it work?

[LethalWolfe]

As I understand it difference mattes rarely work because the margin of error is so ridiculously small. Video compression or film grain can be enough to torpedo a difference matte.

Also, the iChat bg effect doesn't exactly look stellar.


Lethal