Chroma Key 'Before and After' transition.
 

Hi,
I think I might half know the answer to this one, but would appreciate a 'yay' or 'nay' from you folks...my google 'skills' haven't brought me much luck this time.

I'm trying to get my head around carrying out a transition (a simple diagonal swipe) which shows a green screen background transforming into a 'green screened' clip, whilst the subject remains unchanged throughout (this is probably harder to explain than execute...!).
I've seen this a few times, and for show-reel purposes it's pretty demonstrative.
Would I be right in suggesting that this is merely a little trick...the greenscreen and subject is not in fact the original clip, but an already chroma-keyed clip with a new green screen background (either the green screen filmed solely without subject or a plain green background from the NLE) transitioned to the new background/movie of your choice.

Sound about right or might there be another way which returns better results?
I'll be giving this a whirl later on in the evening, but such is the amount of reels I need done by this friday...!!

Many thanks.

you could do it that way but surely it would be better to show the actual key?

As in the original green screen? Definitely, although a shot of the original blank screen (of which I have no footage...forward thinking I'm often not!!) would work quite well (spill would be easy for a start!).
However, much as I've thought about how I would do that, I can't seem to work out a way in both Motion or FCP to carry that out?

You don't have the original shot against the screen?

if you do try something like this:

In FCP you should be able to put both shots on top of each other, original green screen on top then draw a mask over it and animate it's keyframes so that over time the mask moves to reveal the image below.

It might be nice to go from subject against screen>subject keyed>subject in new environment>subject lit/coloured to fit new environment

I've not used motion but it maybe easier to do it in there.

Best of luck

Stick

Stick - I have the original footage of the subject against the screen, but not a shot of the screen on purely on it's own - no subject. Of course this might not matter - I was thinking of having the 'sole green screen' as a keyed background, making it appear to be the original green background (the 'trick') and then adding the transition. But if I key it the way you've suggested then that's great.
However if you're suggesting the 'two shots on top of each other' requires me having the blank screen/no subject the I guess I'll have to write that one off.
When you say 'subject keyed' - are you referring to the masked (black for example) background you achieve before adding the new environment?
I'll give this a whirl later this evening and let you know how I get on!

Many thanks.

This something that could be accomplished by having the keyed foreground footage on the second track and the transitioned backgrounds on the first track. Duplicate the foreground video on tracks 1 & 2 until the time for your transition. You will be keying over identical footage at first but it will be invisible.

F = foreground
B = background
t = transition

Track 2 - FFFFFFFFFFFFFFFFFFFFFFFFFFFFF
___________________________________
Track 1 - FFFFFFFFFFFtttttBBBBBBBBBBB


I not sure that's clear but I hope it helps.

I think it's clear William! Struggling to get my head around this without putting the principles into practice in FCP (which I'll be doing shortly...against time at the moment).
I now have the original green screen footage in the browser with the keyed footage in the timeline (I used the 8 point garbage matte, at least in FCP - I also keyed other footage in Motion)...so would I place the unkeyed footage anywhere at all on the timeline or just use the keyed footage (display mode set to source?) duplicated?
I'm sure it's easy enough once I get going?!?!
Cheers.

The funny thing is that right after I wrote that suggestion I ended up doing a variant of what I described on a client's project. The scene called for a title to rise up from behind a building yet be in front of the sky. I duplicated the building footage on tracks 1 and 3 and placed the title graphic on track two. After a little tweaking with the key settings on track 3, it worked out great.

That's a good effect William - purely using FCP or did you use AE (or Motion/effects progs etc)?

Something came up last night so I haven't, as yet, had chance to work through this properly. When I do I'll note down where I (most likely) went wrong!
As I currently see it, I'll be splicing a chromakeyed clip onto a 'pre chroma-keyed' clip on track 1 (connected via a transition) whilst on top (track 2) lies the full pre chromakeyed track...am I at least on the right track (pun not intended)?
I'm best giving it a whirl and reporting back.
Cheers.

I used FCP only. I try to avoid leaving the program whenever possible. AE would have been the next place I would have gone, especially if I needed to smooth out the graphic's movement.

You sound like you are on the right track.

It's been a while since I've had the chance to attempt this technique, but attempt again I have this morning. Unfortunately I'm not quite getting this to work as it should:
I've placed the original (non keyed) footage on track 2 with the same footage on track 1 - halfway through track one footage transitions to the background of my choice.
What I'm getting is a black background whether I choose 'Source' of 'Final' from the chroma keyer Display mode.
Same applies if I have the keyed green screens duplicates on top of one another - black background (which does transition very well into the new background image) - again turning both clips to 'source' in the chroma key display makes not one jot of difference.
I'm probably missing something fundamentally basic here...but I'm starting to grit my teeth whilst it continues to beat me!

Conversely using the mask, as suggested by Stick above, (a 4 way garbage matte on this case) keyed to slowly reveal the background image on track one works very well. I'd just like the choice of the array of transitions the other method offers.
Can't seem to find the method on the Ken Stone site, which might offer clues to where I'm going wrong.

Cheers.

Make sure that Track 1's video does not have the chroma key effect enabled.

I thought I'd tried all variations William...I'll try that again! So in effect, track one and two are both the keyed green screen (rather than using the original 'pre chroma keyed' footage) but with track one having the key disabled (rather than set to display source/final).
I'll give that a whirl and let you know if it fixes my headache, although it looks good using the garbage matte as a mask.

Many thanks.

Please delete

Sure that'll work but look at a screen grab and the result of what I did.