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Best for Chromakey? 24P or 30P?


Frank Barrera

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shooting music video with SDX900 and DVX100 as B cam with some green screen. The dancers in front of screen will be completely silouetted (sp?) and moving pretty fast for the most part. someone suggested we shoot at 30P for "better resolution" and therefore a better key.

 

1- Do you get more res at 30P vs 24P?

 

2-I've never shot 30P. Does it sort of look like 24P? Could anyone tell just by looking?

 

Thanks for any advice

 

Frank B

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There's no difference in resolution, you'll get the same quality of key technically. What he means is the temporal resolution. There are more frames per second at 30P and it will look more realistic if that's what you want. With fast motion, they might appear a bit "strobe-y" at 24P. Having a lower shutter speed will help offset this, cuz you'll get more motion blur and less stuttering. I'd shoot it whatever frame rate the rest of your project uses. Yes, you can easily see a difference in the frame rates. Do a test first.

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1- Do you get more res at 30P vs 24P?

Frank B

Gordon said all.

But frame rate should be guided by the use of media.

 

Are you broadcasting in NTSC ? then 30p is better than 24p (which will lead to 3:2 pulldown).

 

Are you planning to do a film print, 24p is mandatory.

 

If you want to burn DVDs, I'm not shure, but both should be OK.

 

Hope this helps.

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A few more notes...

 

30p and 24p look a little different when played at 29.97 interlaced.

 

30p will have a little smoother motion than 24, but some of us like the look of 24p with a 3:2 pulldown. (It looks less "video" like)

If you are blending with other 24p elements that have a 3:2 pulldown, then maybe you should shoot 24p.

 

Be aware that motion blur will be more apparent at 30p and 24p, depending on your shutter angle. Your green/blue screen keying software (or hardware) hill be taxed to the limit at those blurred edges, where the green blends with your subject. Once again, a test may be in order. If the green blurs are too difficult to remove, consider a lower shutter angle (Faster shutter speed) to minimize the blur.

 

Shooting at 30p and playing back at 24p has a nice slight slo-mo effect, taking the hard edges off of fast motion. Your dancers would have to dance 20% faster to remain in sync after the speed change, but it is a nice effect. Definitely test this before betting the farm on it...

 

-Crow

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Your green/blue screen keying software (or hardware) hill be taxed to the limit at those blurred edges

 

Yes - make sure your green or blue screen is as flatly lit as possible. In other words, try to make it one shade of green / blue right across and top to bottom. The reason for this is so your compositor will have the best chance at making something useful from those blurred edges. Such edges to a computer are just grades from green (or blue) to the foreground colour. So the more shades you have in your background screen, the more shades of that blurred edge you are telling the computer you don't want. Take that process too far, and hands turn to sharp sticks!

 

If you have any locked off camera positions, depending on the software your compositors is using it can be helpful to shoot the green / blue screen without any foreground. The computer can then use this to understand what shades are really part of the background screen.

 

David Cox

Baraka Post Production Ltd

www.baraka.co.uk

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Here is a 30p frame we shot recently. Our green screen was not as flatly lit as it could have been with more time, but you play the cards you are dealt...

 

30p 1/48th sec sthutter speed (reduced from 1/60th)

Before: (Note the blurred hand)

05.jpg

 

After: (Note a slight white outline around the blurred hand)

05b.jpg

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Thanks to everyone for the respones. Here's some more info and a couple of more questions:

 

This is a music video. We will not be going to film.

 

My main concern is that the dancers in front of the green screen will be moving very fast. If the key becomes a problem it will be my fault because I'm selling them on the green screen in the first place.

 

I don't mind shooting everything in 24P and then swithcing to 30P just for the green screen. Afterall it is a music video...

 

Should I shoot the green screen with V.Res on PROGRESSIVE? Will this help the key? The key effect is simply solid color changes.

 

Crow T Robot: you mentioned in your first post that a faster shutter speed will help. But then in your frame grab example you don't seem to take your own advice. You stated that you DECREASED your shutter speed. Please explain.

 

Thanks again to all. And oh yeah- I don't have time to shoot test.

 

FB

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I planned to play the 30p aquisition at 24 fps in the edit to get a 20% slo-mo.

I shot this with the 1/48th shutter because I wanted the slowed-down frames to look

as if they were shot at 24 fps with a 180 degree shutter angle (1/48th sec)

 

Only in post did I see how the blurred edges of motion caused my mattes to be a little harsher than I would have liked. I didn't take my advice because I hadn't been bitten by the blurred edges yet. So you are getting the benefit of my pain and suffering. (And next green screen shoot, I'll take my advice, you betcha)

 

Most of the blur problem has to do with spill suppression issues. If you are going to pull a matte, and just fill with a solid color, the blurs may not be an issue for you.

 

-crow

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Regarding VRES Progressive or Interlaced,

ONLY use the Progressive setting if the final product will NEVER be shown on an interlaced display. In your case, if video tape or DVD displayed on an interlaced CRT is how the final will be viewed, shoot in the Interlaced mode.

 

The Progressive setting is intended for film out. I have been tempted to use it when the final distribution is planned to be internet video only, but the client always decides to release on DVD or vhs, and if that happens, the footage shot in the Progressive VRES mode will have unnacceptable interlace twitter. (I've tried it, it looks REALLY flickery)

 

-crow

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Another shot:

 

Note the green edges on her right arm:

 

Same shot filled with color, no blur worries:

 

Is this how you are going to treat your dancers?

 

Crow,

 

What are you keying with? AfterEffects? The edges are kinda nasty. :unsure:

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We are using "Keylight" in AE 6.0.

 

Yes, the edges did get out of hand a bit, because of the "hurried" lighting on the green screen. Most of the shots from this shoot keyed very nicely though. I just pulled this as an example of some of the pitfalls that await after a green screen shoot...

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Frank -

 

1. If the dancers are silhouetted, I would personally NOT do greenscreen but whitescreen and just pull a LUMA key rather than a chroma key. This has several advantages. First, no green spill (which will be VERY difficult to control on-set if your characters are silhouettes). Two, you can shoot on any white cyc without having to paint or hang a backing. Three, keying will be superior since your primarily using the Y' channel, which is not chroma-subsampled.

 

2. With a good compositor and good green screen hygiene, motion blur should not be a problem. If you're worried about how well the compositor can handle motion blur, you might just try to sell the director on a high-shutter speed look, which is quite common in music videos anyway. A shutter speed of 1/125 or 1/250 will do the trick.

 

3. You said you have a DVX-100A as B-cam. Don't use it for shots that will be composited, it won't key nearly as well at the DVCPRO50 shots.

 

4. Regarding Crow's advice on V.RES mode, I disagree, especially since you're shooting silhouettes, which have no small detail to cause twittering. I would use V.RES PROG for the greenscreen stuff. However, don't have the detail turned up too much or the edges will get nasty.

 

J

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...for those nasty highlighted or green edges, the software we use, Mistika (www.mistika.tv) has a neat feature called "edge build". It basically traces a border of 1 or 2 pixels around the outer edge of the foreground, but replicates the next (inside) pixel colour of that foreground for the border. Its kind of harder to explain in text, but it is very effective at getting rid of that outer edge, which is caused by the foreground image being antialiased against green, rather than the new background.

 

David Cox

Baraka Post Production

www.baraka.co.uk

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Mistika (www.mistika.tv) has a neat feature called "edge build"

Another trick is "color cancel" that removes the chroma-key color from the final program.

 

Luma key offers indeed better quality, I have been using it on the ground and on snow on outsite broadcast (virtual advertising).

 

Also, the background beeing uniform, you might select a range for the lumakey which does not need a saturated or dark signal to work (tuning of threshold, clip...).

 

In the end you could even try to build a key combining the luma and the chroma key.

 

But generally, you get acceptable results with the simple method if you got a good signal and a correct lightning :)

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