Get Rid of Black Line Chromakey Green Screen

Most filmmakers, camera operators and DOPs know the basic principles of a green screen shoot – light for an even green background and avoid shadows. Here are a few common issues to think about that can help your production look even better.

Camera distance and perspective

Often people forget to think about what focal length to use or the cameras height. With a plain green screen background you have very little frame of reference, like the horizon, that would usually help guide your framing.

The focal length, camera height and angle of the shot, should be always chosen to match that of the background shot you'll be putting behind your keyed footage

Be careful of being close and wide. A lot of green screen presenter footage is used over graphics or abstract backgrounds. In these cases, it's best to get the camera far back from your subject. When close and wide, especially getting a full length shot, can have an odd look to the perspective of the image – the presenters feet and legs will taper away, making them look like an upside down triangle. Moving back and shooting on a longer lens, will straighten up perspective of the image, and will look much better when composited. For full length shots, having the camera slightly below eye line in height, will also produce a straighter image.

Have a look at the image above. In the example shot on the left, the camera is very close, leading to a slightly warped perspective – the legs look tapered, and notice how the wide but close angle means we're almost looking down on to the top of the shoes – this could look quite odd once overlaid on a backdrop. In the image on the right, the camera was further back and on a longer lens, producing a much straighter perspective which will look much better on most backdrops.

Reduce your Spill

Spill – unwanted tinge of green light thats caused from light bouncing off your green screen. It can be easy to forget to take take steps to reduce your spill, as it can be hard to spot in the monitor. But once keyed, a tinge of green on the side or underside of an actors face can be really noticeable. Keying software can do a good job in reducing the effect, but your post department will like you a lot more if you take a few extra steps to reduce unwanted spill

If shooting a straight to camera talking head, in a 3 way green cove, and you don't need to the side walls, then use black drapes either side, or block your subject in at the sides with polyboard.

Put black cloth down on the floor. If you don't see the floor in your shot, then you can but black cloth on the floor to stop any spill bouncing up from the floor.

Side lights. If spill is unavoidable – perhaps you are filming a wide shot, then consider adding a light either side of your subject. They don't need to be strong – Dimmed tungsten lamps work well here, as they get more orange when dimmed, which is opposite to green on the colour wheel so will help cancel out the spill.

Image 2 (above) shows an example of green screen spill. On the left, no attempt to reduce spill was made, and on the image on the right, the subject was stood on black carpet and black drapes were hung either side. At first glance it doesn't seem that noticeable, but look at the close ups and you can clearly see far more green on the side of the subjects face. Since the keying software tries to remove green, you could end up with bits of the face there getting keyed out too with the background – so do your best to remove spill whenever you can.

We've produced a short and insightful video showing you how to key your green screen footage properly for a professional, quality finish. Watch it here!

Do you really need to record a LOG image?

LOG recording modes in modern cameras are a great way to fit a large amount of dynamic range into a standard video signal. But they're not always the best mode for recording a green screen shoot.

You will generally get a better key if you shoot in a more standard video gamma. In a studio, you have full control of your dynamic range, so shouldn't need LOG. With LOG footage, lots of space in your video signal is left for this extra range, which wont be used, so you are wasting data that could be used to record more colour information. It's a trade off – more dynamic range means less data is used per level of brightness. Less dynamic range in a standard gamma means more data is used per level of colour. – And more colour data means a better key.

Thats not to say you shouldn't never use LOG. The times to shoot with LOG might be when you are going to put in a background that is already shot using a LOG, or if its going to be part of a larger edit that is primarily shot in a LOG mode, and you want the footage to match well. If you are using LOG for green screen, make sure you exposure it brightly enough. LOG recording modes can be noisy if under-exposed.

But if you are filming a talking head or full length shot – which will be graded to look like normal video footage to go over graphics or a virtual studio background – then get the footage looking as close to how you will finally want it in the camera, by using a manual white balance and a standard video gamma.

One final note on camera settings: If you have the hard drive space available, and time for backing up – then shooting on a camera that records RAW is the absolute best option for the best key. But it may not always need necessary – the data rates of raw footage are huge compared to normal video codecs, and may not be suitable for your workflow as they need to be handled properly in post, add on more cost in terms of data storage, and take a lot more time to backup on location. So always discuss your requirements with the editor / graphic artist who'll be doing the keying, as well as your DOP.

Are you filming the right way up?

If you are filming a full length shot of a person and they are static, consider rotating the camera 90 degrees to its side, and filling the frame with the person. This makes far more use of the amount of pixels you are recording, and when keyed and rotated, they will be much bigger in resolution, than if you'd filmed the with the camera in landscape. This little trick can also be useful as the extra resolution gained will give you the ability to crop in for a wide shot – so you've filmed your close up and full length wide, in one shot.

Image 3 (below) shows how if you film portrait then rotate the image in post 90 degrees, you will already have a close up when the image is scaled to 100%. Then you are able to scale down your full length shot to fit the height of your frame, to get a wide shot.

Don't compromise your lighting on your subject

A common mistake we've seen is people getting obsessed about getting rid of shadows on the green screen. You should be trying to reduce shadows, and get a perfectly evenly lit green, to aid the keying process. But this shouldn't be at the expense of changing the way your subject is lit.

Your subject should be lit to match the background you are using them on. So if you are keying them onto an outdoor sunny background, then need to be lit to look like it. Light to match your background, then try and reduce shadows on the green screen as best you can, without compromising how your subject is lit.

On wide full length shots, where the floor may be visible, there may be unavoidable shadow at times, especially around and in-between actors feet. Keying is not a magic one click process. The more complicated and wider the shot is, the more likely that in addition to keying, manual masking will have to be done in post – rotoscoping. In big budget productions and action movies, this happens a lot, as you'll have very complex scenes with dust and debris flying across the blue or green screen backdrop, that require rotoscoping in difficult areas in addition to using a colour key.

Read a little more in depth about Lighting your Green Screen studio. Read it here.

Avoid shooting with your lens wide open

It's a bit of a trend in recent years to have shallow depth of field in a lot of shots. Bit this is something best avoided in a green green shoot.

If a persons eyes are sharp, their shoulder soft and out of focus behind them, but then you have a sharp in focus backdrop – then then will look odd and unnatural.

It's also harder for software to key footage that doesn't have a sharp edge. So you want all of your subject in focus.

There is vignetting usually on lenses when wide open, best avoided for green green screen. The bigger range of colour and brightness the keying software has to remove, the harder it to to get a clean key. The image above shows the difference in vignetting between a lens wide open, and stopped down.

Lenses are usually a little softer wide open, so again stopping down with provide sharper footage

Also avoid fully stopping down. Once you go beyond F13, the iris becomes so small you get 'iris diffraction', where light curves off the edge of the iris and your footage wont look as sharp. Try and find the sweet spot for your lens, where it's sharpest and has least vignetting.

Stopping down a lens does mean you'll need to light your screen brightly enough. Don't put the ISO of your camera up. Ideally you want that as low as possible so your footage is as free of noise as possible.

Every shoot is slightly different, but if you keep all the above tips in mind, you should be able to avoid some of the common green screen mistakes and produce a much higher standard of footage.

Written by: Peter Ford
Director – DP – Filmmaker
Website: www.peterford.co.uk

We have dedicated an entire category to all things Green Screen and have written plenty of practical and informative articles on all sorts of related topics. Click here to see them all.

Get Rid of Black Line Chromakey Green Screen

Source: https://www.camberwellstudios.co.uk/blog/greenscreen/common-green-screen-mistakes.html

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