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Blue Screen vs Green Screen for DV formats

by Nigel Cooper

Nigel Cooper clears up the myths behind Blue vs Green Screen when shooting on compressed DV formats such as MiniDV, DVCAM and HDV.

“Should I use a Green Screen or a Blue Screen when shooting on DV?” This is a question that has been floating around for ages and is one that I get asked quite often. There will always be the next generation of filmmakers, media students and the like who will continue to ask this question, so I decided to write this article to expel some of the myths between green vs blue screen when shooting on DV formats such as MiniDV and DVCAM for example.

Most of the pictures in this article (except the Robert De Niro shot from the movie Heat, and the single shot of a crew of five people on the green screen set, which was taken from the Lastolite website) are actual screen shots taken from a training DVD that I wrote and produced late in 2004 entitled Lighting for Digital Video The gentleman pictured carrying out the demonstrations in this training DVD is professional Lighting Cameraman Eric Huyton who is the Lighting Cameraman on the new Dr Who Confidential series on BBC.

Chomokeying in Hollywood
deniroThe Green/Blue screen process (also called Choma Key) is used often in feature films and dramas. Common scenarios include: somebody being pushed off the platform into an oncoming train, somebody being pushed off a balcony 3 floors up, somebody running towards the camera away from an exploding building or even situations where it would have been just too expensive to shoot at the actual location. On such instance that springs to mind is the 1995 Michael Mann film “Heat” staring Robert De Nero and Al Pachino. It was the scene near the end where Robert De Niro’s character got shot by Al Pachino’s character. De Niro and Pachino were shot against a green screen then the airport scene was dropped into the composite later. In fact there was some extra work for the compositor during this scene, as the green screen didn’t reach the edge of the frame during shooting. As De Niros squib (small packet of fake blood over a small explosive device) exploded, the fake blood shot across the screen into the area that was not backed by the green screen. Still, this extra work for the compositor was far cheaper than actually shooting the scene at an airport. As for the moving lights coming from the airplanes, these were also composited in later with clever lighting on the green screen set for De Niro and Pachino.

Green or Blue – that is the question
Okay, so should you use a green screen or a blue screen when you are shooting on DV? My answer to that question would have to be green. Here’s why:

It all boils down to the in-camera compression codec (Compression - Decompression) that DV camcorders use. DV camcorders are not an uncompressed format like film or high end HD (Thomson Viper for example), hence the codec throws away vital colour information; this lost information makes keying in post production more difficult.

green screen studioIf you are shooting on an uncompressed format such as Super16 or 35mm film or even high end HD (not the consumer Sony HDR-FX1 or HVR-Z1E HD models, as they are heavily compressed as they both use the MPEG2 codec, what I mean is the high-end professional uncompressed HD cameras such as the Thomson Viper or the models used by George Lucas for the later Star Wars movie), it doesn't make any difference whatsoever which colour you use for your screen.

In the case of these high-end formats, the colour used for the screen is based more on the subject matter being filmed – a scene with a lot of greens in it will sometimes get keyed using red as the screen colour. I've seen many different colours get used in higher end formats. But in the world of DV, green is by far the safest choice.

If it is vital that your talent has to wear blue in the scene, you would use a green screen, alternatively, if your lovely young female presenter had light green eyes, you would use a blue screen, otherwise she could end up getting her eyes keyed out in post production which would end up looking like she had two holes straight through her head – in the completed composite you would see your new “dropped-in” background through her head where her green eyes used to be.

For DV formats such as MiniDV or DVCAM I always shoot using a green screen and if my talent is wearing green, have them change it to a different colour. It is always better to have your talent avoid wearing anything green and to use a green screen for compressed DV work than to use a blue screen and not get such a clean key in postproduction.

The technical bit
green screenBlue was originally used in film because it is the one colour that does not appear in skin tones; skin tones are made up of red and green. In film the blue layer of the film (as in Super16 and 35mm film) is the sharpest, however it is also the grainiest, with DV on the other hand the blue channel is the grainiest, or noisiest.

The DV compression method used in-camera throws away a lot of important colour that is needed when you are trying to achieve a clean key in postproduction. Green is a lot more forgiving than blue simply because more blue gets thrown away by the in-camera compression codec, hence blue doesn't have as much information to work with in postproduction.

DV formats compress colour in such a way that for every 4 pixels, colour is stored only once, while luma is stored for every single pixel. Green, being the brightest of Red/Green/Blue, benefits from the most in pixel resolution. Single CCD chip DV cameras also retain more green pixels than red or blue ones.

Some people say, “The decision to shoot on blue screen or green screen is better left to the Compositor rather than the DOP or Director”. Personally I disagree with this; DOPs and Directors are well aware of the Blue/Green screen process and everything that the postproduction keying process entails.

It is also a scientific fact that green requires less light to illuminate it, which is always a good thing as far as Gaffers (lighting guys) are concerned.

A note on lighting your foreground subject
maraYou would be amazed at how many filmmakers and video producers give absolutely no thought to lighting their foreground subject on the green screen set – so you should pay maximum attention to the following:

I’m talking about “Shadow Direction”. If you want to achieve a convincing effect in y our final composite, it is absolutely vital that the direction of the shadows go the same way on both your foreground subject and your “to be dropped in” background footage. For example: if you were shooting an astronaut waking on the moon’s rocky surface you would start by shooting your astronaut walking around on a green screen set, this would probably be green on the floor as well as the walls and background. You would then light your astronaut by lighting him from the same direction that the light is coming from in your moon surface footage that you will drop in later. If you don’t do this you will end up with a very unconvincing composite, here’s why: if you lit your astronaut with your key light at 45° left off the camera axis and your original moon surface footage was lit with the key light at 45° right off the camera axis you will end up with two different shadow directions in the final composite. When people watch your final movie (although they might not notice the two different shadow directions) they will notice that something is obviously not right with the scene. It will look unnatural as the astronaut has shadows on his body that are coming from left to right, whilst the moon’s rocky surface has shadows coming from right to left. This would make the final scene look like there were two suns lighting it from two totally different directions, which leads to a very unnatural and unconvincing scene; think about how unnaturally lit footballers look on the pitch during evening games that are flood lit by four huge flood lights in each corner of the ground, they have four shadow going in four different directions, you see my point.

maraRemember, you don’t want your final scene to look like a composite of two separate images. It is a special effect, but you don’t want your viewers to see it that way. As far as they are concerned it should look like a regular clip. Some of the best Hollywood films are packed with special effects, yet you don’t notice them – one such film is Castaway with Tom Hanks, this film is packed with special effects, but after watching the film you aren’t left thinking that, unlike say The Matrix, where the special effects are obvious. It’s a compositor’s job to make the scene look as natural as possible, even with several clips composited together. If you can get your composite to look 100% natural, you can consider yourself a professional.

Pre Production Check list
Whenever you are planning a green screen shoot it is vital that you use a pre-production checklist. Below I have outlined some of the most common ones that I always use:

Costumes & Clothing
You should always consult your actor/presenter or costume designer and director about clothing. You want to try and avoid green coloured clothing (for reasons already explained), shiny clothing, as this can pick up green reflections that might be bouncing around the set.

Gaffer (Lighting man)
Your lighting man should already know this. He should be aiming to light the green screen evenly with the lights out to the front and not in at the sides. Having the lights out to the front will avoid creating shadows off any tiny creases that could be present in your green screen; if you have shadows on your green screen it will create complications in postproduction and make getting a clean key more difficult.

Sources of green screen material
studioThe best material for green screen is the cotton variety with a slightly silk-like sheen, available from Lastolite in the UK

These screens are primary chroma green and are available in 10’x12’ or 10’x24’ and are specially designed to reduce creases with a surface finish that is easy to illuminate. They require a Lastolite background support stand. Another company worth investigating is Colorama

Lighting Gels
Quite often you can get “Spill Light” that reflects off the green screen and onto the back of your foreground subject, this green spill light can make keying more difficult in postproduction. If you are concerned about the separating ability of your foreground subject and the green screen, you can augment the separation by adding an opposing colour gel to your subjects backlight. If you are lighting a green screen, a 1/4 or 1/2 stop minus green (magenta) will help give better separation by neutralizing any green spill that could hit the back of your subject. Minus green gels are available form Lee Filters in the UK

Procedures
dedoliteYour subject should be as far from the screen as possible to prevent green spill hitting the back of your subject. Aim for a minimum distance of 12 feet or so.

The cameraman should NOT use any kind of diffusion, frost or soft-focus filters on the cameras lens when shooting green screen. This will make keying much more difficult in postproduction due to the soft edges created by the soft-focus filter. Any soft focus FX can be added during the editing stage.

You should check all foreground objects with glossy or shiny surfaces i.e. a glass table or a wine glass as they can pick up reflections from the green screen and therefore become unintentionally keyed out during postproduction. Shiny objects can be fixed at the shooting stage by applying an accent light to the offending area, or by using a matt dulling spray.

Once you have lit your green screen you should check several different point of the surface using a spot meter to check for uniformity in the lighting – it should be as even as possible across the entire area of the screen, preferably within 1/4 of a stop tolerance.

Blue was originally chosen for its radial (vector) colour distance from human skin tones. Green on the other hand was chosen because it is a good compromise between skin tones, blue eyes and clothing, notably jeans.

It is well worth remembering that any subtle video signal advantage arising from green vs blue for DV is totally overwhelmed in the real world by how well the screen is lit at the shooting stage; Period. The main criteria for a well lit green screen is colour separation, a nice saturated colour, correct brightness levels, and above all, evenness of lighting across the screen.

So, which colour should you use?
lighting diagramSince DV stores luma at four times the resolution of colour, it could be argued that you should choose a screen colour that has the most brightness contrast with your foreground i.e. light coloured subjects (blond hair, pale skin etc) are best keyed using a darker blue screen, whilst dark objects (black hair, darker skin etc.) are best keyed using a brighter green screen. However, as long as you light the green screen evenly and it is a nice saturated green you should be able to achieve a good key no matter how lightly coloured your foreground subjects are.

Summing up
Green is revered as the best colour choice for shooting on DV formats simply because in the world of DV and its 4:1:1 colour-space, the green channel is where the luma values are drawn from and it is sampled more than blue, and therefore has more data to work with in postproduction. The blue channel in DV is sampled far less and is usually a poorer choice to use for keying purposes.

Compared to blue, red and green do not suffer this perceptual anomaly quite so easily – again, this refers to the DV 4:1:1 colour-space. You will get smoother edges when you key out the colour in postproduction if you use a green screen at the shooting stage. There are three things you need to get right for a decent key – Lighting, Lighting and Lighting. Good luck!

©2005 Nigel Cooper

Date: 10-11-2005

 
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