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-   -   Sky replacement (https://www.dvinfo.net/forum/digital-compositing-effects/82223-sky-replacement.html)

Nic Smith December 21st, 2006 02:56 AM

Sky replacement
 
I've got some handheld footage of a house with trees around it, and the sky is basically completely white, with a tiny bit of blue.

I'm wondering what the best way to key out this white area and replace the sky with other footage is. I've tried a conventional keyer (Primatte Keyer), but it doesnt want to work with the white, Im just left with it keying out most of the image and it wont key out the whole sky because of the tiny bits of light blue.

After Effects' keyers are hopeless as well. Ive seen various sky replacement videos and they all look so clean where the new sky and footage meet. I cant get this level of cleanness with any keying technique I've tried. It would be easy if the camera wasnt moving, but it is.

I was thinking I might have to rotoscope it all, but I dont know how easy that would be seeing as Ive got trees covering over the white as well.

What would be the best way to do this? Am i going to need software other than what I already have (After Effects, Primatte)?

Martin Chab December 21st, 2006 03:37 AM

Hi Nik,
To do that you need to make in more than one pass.
First try to make a garbage matte (a mask) to left most of the image you want to preserve without being processed by the keyer (may be youŽll have to animate that mask but that is a fast work while you only need to make the mask loosely match the lines).
Use a first keyer. I recomend you to try the extract keyer since you are keying the white.
In this step youŽll want to try a choker to see if that gets rid of the spots that the key couldnt take care of. Other choice is to use an alpha cleaner (there are several plugins to do that).
If you have big areas not processed by the extract key you can get rid of them by making a very rough key around (again, some animation could be necessary).
Also you can try another keyer to isolate some specifical colours.
Finally you may need to make some rotoscopy to make some details look better.
Of course all this depends on your footage (is it DV? is it noisy? the colour of the sky is constant in all the footage?)
If your footage comes from DV also is a good idea to start softening the chroma as a previous step before the keying process.
I hope this helps you.

Nic Smith December 21st, 2006 04:01 AM

Thanks Martin,

I tried the extract keyer and that worked alright. It got rid of all the sky, but the edging was still dodgy. It wasn't clean edging even with the matte choker. The footage is DV, taken with a pretty cheap camera, which might be whats making it difficult, because its not all that sharp around the edges. Still I would expect it to work alright, the same camera keys a bluesreen out fine with Primatte Keyer. I dont know why it wont get rid of the white.

Would I be able to rotoscope it to get better results? Ive been looking at Silhouette Roto for rotoscoping. I reckon that would handle it, but I only wanted to use that as a last resort. Do you know of any good keyers that I could try that are good at extracting white and other non-standard colours? What's that alpha keyer you mentioned?

Thanks,
Nic

Conor Ryan December 21st, 2006 05:15 AM

Could you perhaps post a still from your footage?

Hart Boyd December 21st, 2006 07:09 AM

Take a look at this tutorial on "01. Basic Sky Replacement" at the bottom of the page.

http://www.videocopilot.net/tutorials.html

Martin Chab December 21st, 2006 01:14 PM

Nic,
It would be good if you can post a piece of your footage to take a look and give you a better advise.
Anyway, as i wrote before, its very difficult to get a good key in one pass (thats specially true with dv footage) so, before trying something more complicated try this:

Chroma softening
Garbage matte
Extract key (explore the feature that let you key with a slope, dont use a 90 degrees end). Be very conservative in this point and let some artifacts (or parts that are not keyed out).
Make a new layer. Make a rough mask to select the edeg to want to work with with some space around and put a new extract. Now you can work your setting for this area only. Mask out the rest of the layer (you already have that part in the other layer)
If necessary put a choker or alpha cleaner (a plugin to get rid of artifacts in the alpha channel)
and so on

If this doesnt work consider rotoscoping

Riley Harmon December 22nd, 2006 03:33 AM

try doing 3d match moving with sky replacements, thats when it gets fun...

and wtf is with having an assisted suicide thing....

Nic Smith August 19th, 2007 07:28 AM

Good Tutorial
 
I know this threads pretty old now, but I thought it'd be good to add to it, cos i never really found an answer until Andrew Kramer went and did a tutorial that matched my needs exactly.

Go to http://www.videocopilot.net/tutorials.html and click on the 42. Advanced Sky Replacement link.

The tutorial works for when the footage has a completely blown out sky. Just thought I'd post anyway. Could be useful for someone.

Nate Benson August 23rd, 2007 01:33 PM

Does anyone have any tips on how to do this in motion 3?

Dale Stoltzfus August 23rd, 2007 02:47 PM

Quote:

Originally Posted by Nic Smith (Post 731030)
I know this threads pretty old now, but I thought it'd be good to add to it, cos i never really found an answer until Andrew Kramer went and did a tutorial that matched my needs exactly.

Go to http://www.videocopilot.net/tutorials.html and click on the 42. Advanced Sky Replacement link.

The tutorial works for when the footage has a completely blown out sky. Just thought I'd post anyway. Could be useful for someone.

Yeah, that site is really great. I haven't worked my way through all the videos yet, but after every new video I watch I think "wow, that is so simple - 'whoda thunk it?'"

Nate Schmidt August 27th, 2007 07:24 PM

Nate, I only watched the tutorial once so I don't remember all the specifics, but if you use the gradient colorize filter you can do the same effect that he did, turning everything but the sky black.... that will get you started at least.


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