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Shooting for Chromakeying

Shooting for Chromakeying



by Barend Onneweer, Gouda/Rotterdam, The Netherlands
©2001 by Barend Onneweer. All rights are reserved. Used at CreativeCow.net by kind permission of the author.

Barend Onneweer

ARTICLE FOCUS:
In addition to his tutorial on how to pull a usable matte from badly lit blue or green screens, Barend Onneweer has put together a list of things to consider when shooting for chromakeying.

 

INTRODUCTION

As many posts on various forums have shown here on the COW, the difficulty and complexity of decent chromakeying is often underestimated. Until the footage is taken into post-production that is… Badly shot screens often result in huge disappointments (and post-house bills…) and rotoscope-jobs.

I have also written a tutorial for the COW on 'procedural matte creation', a technique that allows for extensive control over the matte, using multiple layers of the footage to create a combined matte.

However, there are some things to consider when shooting for chromakeying, that will make post-production a lot less depressing, and in combination with my other tutorial, will allow you to produce high-quality results. The tips below have been collected over the years, from all kinds of sources. I've compiled and rewritten them for this article. Most of these are common knowledge for the seasoned professionals, but I thought it would be a valuable resource for the less experienced.

DV OR NOT DV - THAT IS THE QUESTION

First of all, DV is far from ideal for chromakeying. The DCT compression produces JPEG-like artefacts, and sometimes even motion blocking. Not good. The artefacts may seem almost invisible at first glance, but during the process of chromakeying the compression results in jagged fluttering edges around the foreground objects.

If at all possible, shoot on Betacam or Digibeta, if only for the shots that involve chromakeying. Make sure to capture the footage uncompressed, or with as little compression as possible, when using Betacam or Digibeta, otherwise you're not really better off than shooting DV. If you have to shoot with DV, but have the possibility to capture uncompressed from a good DV-player, it's recommended to capture from the analog (preferably component) output of the DV-deck. This may seem strange, but a good DV-deck features highly advanced filtering algorithms that will reduce the damage done by the in-camera compression.

I left High Definition Video and 35mm film out on purpose. Of course the higher resolution and better color depth of these formats will allow for extremely high quality compositing, but when working with these kinds of budgets, I suggest you hire an experienced Visual Effects supervisor…

It is possible however to create high quality mattes from DV footage, so if your budget won't allow for anything else, don't be afraid and read on.

SETTING UP THE CAMERA

Video cameras often feature image enhancing circuits that artificially increase image detail. This results in overly sharp edges between the foreground and the background (screen), which is often accentuated by the compression. This feature is often called 'contour' or 'aperture control' or just 'sharpness' or 'detail'. If possible on your camera, turn this feature down or even off.

Besides this, you can use depth of field to create a focussed image of the foreground, but leave the screen as blurry as possible. This will effectively hide the inevitable imperfections in the screen, making keying much simpler.

THE COLOR OF MONEY… UHM

Traditionally blue has been favored as the color for the screen. Blue is the complimentary color to (caucasian) skin tone, so is easiest to separate from actors. Film also used to be most sensitive to blue light, although this is less true today.

Basically with current video effects software any color can be keyed out. Blue, red or green, all channels can been used. Most important to the choice of the screen color is what you want to shoot in the foreground. Make sure your background screen is a color that is opposite to the colors in the foreground. And avoid the screen color in any foreground objects. It sounds simple, but if the wardrobe is not carefully selected, socks, shirts or even eyes can disappear in the key, as will anything that reflects the background color (especially whites and metallics).

GREEN GRASS TASTES THE BEST

That said, green screen is better for video. Green is represented in the luminance channel of the YUV signal. In 4:1:1 or 4:2:0 compression ratio's, green is sampled 4 times because most images have a large component of green. This means more information is used in the green channel then in blue or red, which makes it more suitable for extracting a high-quality matte.

The television and movie industry use special blue and green paint, if you use some other paint, use a vivid hue, that comes in the direction of pure green, blue or red. Check for instance your Photoshop color palette and look up pure green as (0,255,0). Make sure your screen material is not glossy or too reflective, this will cause the green light to contaminate the edges of the foreground to the point where you cannot extract a decent matte.

USE A BIG BARN AND GIVE YOUR COWS SOME SPACE

It's best to keep your talent or foreground objects as far away from the screen as possible. You may need a pretty big screen for this, although it's not always necessary to fill the entire frame with screen: you only have to place the screen behind your foreground, no more than necessary. Reducing the size of the screen will reduce the amount of reflection of the screen color on the foreground (spill). And it will prevent your subjects from casting shadows on the screen.

Just make sure none of the talent move out of the screen, or its rotoscoping time…

LIGHTING THE SCREEN

It's important to light the screen evenly. If you are using textile, make sure to get the wrinkles out of the background. If possible, give the screen surface a slight curve on the vertical axis. This will help soften the light and lessen visible highlights on the screen.

If at hand, use a video waveform monitor to measure the brightness and color of the screen to make sure the screen is lit consistent from corner to corner.

A professional lighting kit is best of course, but it's quite possible to use 500W worklights from the hardware store to light your screen. In that case you're best off bouncing the light onto the screen (using foamcore boards or styrofoam) , to achieve a more even and diffuse screen than when lighting the screen directly.

Don't over light the screen, though. You want an even, saturated color, but an over lit screen often results in serious spill of the background color into the foreground.

LIGHTING THE FOREGROUND

When lighting the talent or foreground, make sure the light doesn't hit the screen. This is achieved partly by keeping distance between the talent and the screen. You can also use 'flags' and, which can be a piece of cardboard on a stand, or the barn doors on the lights to keep the light away from the screen. If you add color your lighting, make sure to stay away from the keying color. So if you're shooting against a green screen, do never use a green gel on the foreground lighting!

It's common practice to use a color filter on the 'kicker' or backlight when shooting for keying. When shooting against green screen, use a magenta gel on the backlight. For blue, use an orange or amber gel. This effectively separates the foreground from the screen, and reduces spill: the reflection of background color you get around the edges of your subjects. This will make keying a lot easier and prevent the hair and fingers from disappearing in the key. Don't overdo it, though. If your backlight is too heavy, you'll end up with a white halo around your subject. Depending on the background you're going to place the actor in, this might look very odd. I always try to create my background so that there is a source for the backlight in them. Which brings us to the next issue…

MATCHING FOREGROUND AND THE BACKGROUND LIGHTING

No matter how successful your keying efforts are, if the foreground doesn't look as if it belongs to your digitally inserted background, the effect is going to look pretty bad. If the lighting setup of the foreground does not match the inserted background at all, even the cleanest composite will look fake. If possible, use a reference image of the background footage when setting the lights for the foreground. If your background footage is an exterior, consider to shoot the foreground in sunlight also, this will make the overall look much more consistent and convincing.

If the background is going to be a 3D animation, make sure to pass on a diagram of the lighting setup and camera setup to the animator.

 

Well, those were the tips and tricks that I wish I had known a lot earlier…happy keying! And remember, the grass is always greener…

Barend

 

Barend Onneweer is a leader in the Adobe After Effects Creative COW . Drop by and discuss this or other effects. Like to see who Barend is? Click here.

If you'd like to see Barend's Procedural Matte Creatiion tutorial which was mentioned above, click here. In that article, Barend Onneweer explains Procedural matte creation by combining multiple keying types into a single matte for extensive control over the final matte. This is essential for pulling those difficult mattes from badly lit greenscreens.

If you have a question regarding lighting, drop by the Pro Lighting Cow where we have two award winning lighting directors leading the forum.

This tutorial is for the use of Creative COW members and visitors and may not be reproduced without permission by the author and CreativeCOW.net



Visit Creative Cow's website and forums if you got here by direct link to this article...



Comments

Edges problems in chroma key
by mazin alani
Hi Barend;

I faced a problme when keying green as i see its so hard its a edge allover the person and sometime failong the project because that edge appear

We'v green chroma hanged from one side and released to make a slide slope on floor,applying four cool-lights distibuting as:two lights in corner as 45 angle and two lights mounted one on the front and another one in rear...besides we'r using SONY DX55 studio camera with FORE-A AV3800 vision mixer

So. please give your advice

Thanks

Mazin
IT- department
Questions
by fatima ismael
Hi Barend Onneweer,

We have a high-school project...a short movie..
and we are planning to do the CLIFF JUMP effect in the covenant but we can't decide on how to shoot the vid.Can you help us?

I have a little background in AE CS3

-Fatima-
need your reply ASAP...
Thank You Very Much!
Shooting for Chromakeying
by Alex English
Sorry Steve, but I'm pretty sure you're wrong. While YUV isn't equivalent to RGB, it is the case that most of the green information is derived from the Y channel, which is sampled for every pixel. That's not to say that green is sampled for every pixel, but that the information it's primarily derived from is.
Shooting for Chromakeying
by Barend Onneweer
Hi Steve,

Although I should probably update the article a bit, the general information in it still stands.

The bit about color space that you refer to is a bit oversimplified.

4:2:0 and 4:1:1 tells us the ratio between Luma and the two color channels in a YUV signal.

So in 4:2:0 for each 4 luma samples there are 2 chroma samples.

In the conversion from YUV to RGB the green channel tends to be most dependent on the luma records in the YUV image. Thus the green channel tends to hold more resolution than the blue channel and red tends to be worst.

Barend
Shooting for Chromakeying
by G2_Media
I have to take issue with a segment covered by the author.

Green is represented in the luminance channel of the YUV signal. In 4:1:1 or 4:2:0 compression ratio's, green is sampled 4 times because most images have a large component of green."

From my understanding green is only sampled in 1 out of 4 pixels. Hence the difficulty in pulling a green (or blue key) in DV. I have yet to try keying with footage off a Sony HVRVIU, which I think only uses a 4:2:0 color scheme.

Thoughts?

Steve



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