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Removing or Keying background or something better?

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Rik Trem
Removing or Keying background or something better?
on May 7, 2018 at 11:32:05 pm

Here under you can see two images that I made of two test videos. Sorry for the bad focus on the green screen. For the final video I will do this better.

I was wondering what is the best way to remove the background. With a black or white background or green screen?
The hand will move and the fire too.

If use a luma matte the hand disappears as well. Is there a solution?



Until now, I get the best result with a green screen. (Even with this bad focus and lighting). But it's not yet sufficient.
I think if I use a green screen, it's best to lighten the whole scene more. So the flame becomes less blown out. The background has to be be evenly lit. Therefore the flame must have a good distance from the wall.



Maybe there is another way, that don't know of. In that case ENLIGHTEN me please!


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Steve Bentley
Re: Removing or Keying background or something better?
on May 8, 2018 at 12:47:04 am

For flames that don't have a high temperature element (the blue near the lighter) we use blue screen or shoot on black. Even then we can add that blue element back in with some color correction, to what was a light yellow flame.
Yellow on a green screen is like pink on a blue screen - its not the best to key.

Distance from the screen is a huge factor for either blue or green screen (you can see the lighter flame is changing the color of the green screen).

Dont be afraid to use two approaches - perhaps a luma matte for the flame and a green/blue screen matte for the hand (from the same source footage). Or even just "screen" the flame over the background footage. A sure sign of a matted flame is the dark fringe around the edges because they used a matte instead of a more appropriate transfer function. And remember that out in the daylight you often can't see a flame, so the sure sign of an effects shot (and not a very good one) is a flame comped in that would normally be invisible - its kind of like a lightsaber where the core is dark and edges are bright - it just never looks right.

While this footage may be "raw" and therefore showing washed out in an 8bit jpg, if they aren't raw, both the black and green are pretty washed out and will not provide a good clean key in either luma or green.

You can also change the chemistry of your fuel in the lighter. Butane burns very clean and therefore has a high-temp blue flame component (the yellow flame indicates a lower temperature and incomplete combustion). Ironically what we're going for is a more incomplete combustion so you get an oranger flame which keys better on blue. But changing the fuel can be dangerous if you don't know what you are doing.



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Dave LaRonde
Re: Removing or Keying background or something better?
on May 8, 2018 at 2:01:22 pm

I want to add a couple of thoughts to Steve's -- with which I TOTALLY agree. You need more space, and you need more lights.

The second shot has a shadow from the arm on the background. That means the background is WAAAY too close! For a shot like this, you need six feet minimum between subject and background.

"Well, we only had one light, so we needed to have the background really close."

Hence the need for more lights. You need to have sufficient lighting to light the subject separately from the background -- which needs flat, featureless lighting with no hot spots that can be adjusted up & down in intensity.

If you do those two things, you give yourself a MUCH better chance at pulling a good key.

Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA


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Rik Trem
Re: Removing or Keying background or something better?
on May 8, 2018 at 3:03:49 pm
Last Edited By Rik Trem on May 8, 2018 at 3:51:23 pm

Dave de LaRonde and Steve Bentley,

Thank you for your advice. I'm gonna experiment with it.

Instead of holding a lighter, I will be holding a glass bucket. If I use another background will this come through? Do you know that?
By another background I mean, another then the keyed out blue screen background. Will you see the glass bucket?


@ Steve: Sorry, I reported your post instead of replying. I don't think it will harm you. Just a rookie mistake.


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Steve Bentley
Re: Removing or Keying background or something better?
on May 8, 2018 at 5:49:08 pm

No worries, I get a lot of those "reported posts" for some reason. I'm on a first name basis with the Cow moderators because of it.

Glass eh? You really are testing your green screen chops. But yes, glass is keyable and your background will show through. But its trickier than most foreground subjects. The key needs to be excellent and therefor the techniques used in the shoot need to be good too.

Green/blue screen is a personal preference thing based on what works and what has worked for each artist in the past. So you will get as many different views on the best way to approach it as there are VFX films in the Hollywood cannon. But here's my two cents for the best keys.

First, the reason we use green screen is that it's A) very different from skin tones and B) is the second least noisy record of the three RGB channels in an image (more so in film but this is still true for video/HD/4K). So for glass, green would be a better choice over blue (the noisiest record by a long shot). If you don't need to have your hand in there Red screen (red is the least noisy of the three channels) would be the best choice.

We also don't use the lights that are on the talent to light the screen. These talent lights tend to be of the yellow variety, or worse, gelled to get some effect or drama. Instead we use a separate bank of lights (in our case KinoFlo's) with green gels or green bulbs in them so that the only light (for the most part) that is hitting the green screen is, well, green. Any other color of light will contaminate the green color and reduce what you can do with the keyer. (remember white light is made of all colors so even that has some "anti" green components in it). This isn't always feasible (like outdoors with hundreds of yards of green screen billowing in the wind) But it can make a difference with a hard key like glass or smoke.

Even lighting across the screen is also important. If you have a hot spot in the middle of your screen that will be seen by the keyer as a separate color from the dim outer edge and you will have to pull separate keys for both areas or crush one key farther than would be ideal to accommodate both ranges of color in the one key. Use a spot meter to lens the screen and see where the hots spots are. This goes double for wrinkles. Either use a painted flat or stretch the screen fabric on a frame. Wrinkles cause shadows and with shadows you get another color of green to contend with.

Foreground colors to avoid: For Green screen - yellow and of course green. For Blue screen - Pink and of course blue ( a lighter blue is usually ok). These colors will not key properly on their respective screen colors. (pink is made of red and blue light and yellow is made of green and red light so there is a lot of the screen color hiding in these two bad colors)

Keep the talent/props away from the screen. A good rule of thumb for a minimum distance is the width of the green screen. We tend to use 20x20' screens as a minimum so we always make sure the talent is at least 20' in front of the screen. This also helps with keeping the shooting lights on the talent and off the screen and keeping the green lights on the screen and off the talent. But remember as you push that screen into the distance it will get smaller to the camera and cover less and less of the talent.

You want a rich green color. I prefer something in the 66FF00 range (hex) while others prefer something in the 00CC00 range. Saturation is the most important because today's keyers, while tuned for Blue and Green screen, can really key any color. There are some standards when you buy green screen paint and green screen fabric and tape and they all tend to be in the same family. But they will vary depending on where you are in the world and who is the biggest user/vendor in your area. You can also rent "proper" green screens with stretch frames.

There are a lot of myths out there for this type of effects work, spawned, unfortunately, by the bane of our existence, the "behind the scenes" featurettes on DVD's these days. They make it look like its a one button operation. It can be if every thing is just right, but that rarely happens. Watch the behind the scenes for Lord of the Rings. Almost every shot had rotoscoping in it - even with giant green screens. That's probably as close to the tedious reality as those featurettes get.

Don't be afraid of the green screen reflecting/refracting off/in the glass. You can use this element to inject what the glass would have been reflecting if shot in the environment that you are compositing it over.

Don't be afraid to mask of parts of your shot that are giving you bad keys and key them separately (in another layer) with different settings or a different technique. Don't be afraid if highlights or shadows don't come out clean in the key - you can add them as another layer with (for highlights) a screen or lighter color blend function or (for shadows) multiply or darker color blend function, without any color keying at all. FYI and in reference to your first post - shadows on a green screen will not key away, we use them to put shadows from the talent into the background scene. If you are having to crush the key to get rid of a shadow it means the edge of the key on the talent will suffer.

Many keys are not just one effect - adding in things like the matte choker and de-chatter effect or adding another layer with the exact same key and using it to clip the original's key matte just a smidge can give you that little bit extra. The edge of hair can be separate element so you don't get helmet head and you can rotoscope in a face so that the face itself is not keyed at all.

Work with as uncompressed footage as you can. Any lossy compression will add noise and remove color or information.
Work with the highest color space you can. 8 Bit can work but you can crush mattes and levels far better with 16 bit and 32 bit footage and it also means there are more divisions of the screen color to play with. (use a 16 or 32 bit comp in AE regardless of footage depth)

Try and shoot your footage at the size you need it in the comp. Blowing something up or scaling it down changes the grain structure of the footage and it won't match the background footage grain structure. It also softens the image (again not matching the sharp background) - a circle scaled waaay down becomes a square when its made of pixels.

Every body blurs their backgrounds too much thinking that it should be way out of focus. Reference footage shot on the day can get this right, but a rule of thumb we use here when we don't have that reference is to do what you think is right and then cut the blur in half. A real lens keeps things behind the subject in focus farther than objects in front. And use a camera blur or an after market camera blur like Frischluft to get the kind of bokeh and edge effects a real lens would impart. The eye knows a Gaussian blur or a box blur is not the same as out-of-focus.

Don't try and color correct your green screen before you key. The color you photographed will be pure whatever it is and have the widest latitude. As soon as you try and change the color or the brightness you start clipping information out and your key will suffer.

Color correct everything after the key.

Finally, rethink your shot - we have so many producers come in wanting to shoot green screen because.... well they don't really have a good reason. It's often better to do either as an in-camera effect or shoot it on black or white or shoot it in the real environment or a faked real environment.



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Rik Trem
Re: Removing or Keying background or something better?
on May 8, 2018 at 8:47:35 pm
Last Edited By Rik Trem on May 8, 2018 at 8:57:09 pm

Thanks a lot for all this information.

It's for an art piece that can be adjusted to different environments. I constantly rethinking and testing for the best way. This information is very helpful. I'm off doing some tests.


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Dave LaRonde
Re: Removing or Keying background or something better?
on May 8, 2018 at 9:00:26 pm

HOORAY!

Someone who actually runs tests before jumping into the deep end of the pool! Good for you!

Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA


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