Green Screen and Chroma Keying

For my last little exercise I decided that in order for the afore mentioned shot to take place I was going to need to create some adverts to place into the scene. As this is the future and all adverts are projections or on some large screen beaming they're inane messages onto unsuspecting citizens below, I decided to do a little green screen work.

With the kind help of my multi talented pals Matt Nesbesth and Adam Lyduch, we spent a brief half hour in front of a green screen recording some uber cheese.

Green screen work in a project such as this, and especially advertising is the process of separating the foreground and background elements by what is known as Chroma Keying. Chroma keying essentially takes a sample of a single unnatural colour, usually green or blue, and replaces this colour with transparency leaving a cut out of your designated object, also known as a Matte.

The first thing to take into account when doing chroma key work it light. It's really important to have good light on the green screen and as important to have good light on your subject. I noticed when doing this exercise that shadows are no friend of the keyer. This means to try and keep your subjects as far away from the screen as possible and to also make sure there are no kinks in the screen that could also cast shadows.

As this was a completely impromptu improv advertising session we essentially had to try and sell whatever was on our persons. This made for an interesting shoot.

This first video is of an hugely passionate and expressive Nesbeth. What a great job he's done here. I really feel his extacy and love for the music he's listening too.

This key was pretty straight forward except for what could have been some potentially problematic reflections on the headphones themselves. Reflective surfaces can be a real bitch as they pick up the spill from the green screen and punch holes in your matte.

As this image of the Alpha channel shows the holes left in the matte by reflections and spill from the green screen. Luckily this was not a problem with this particular shot as you shall see in preceding posts.

The second shot that I worked on from this shoot was of Adam Lyduch selling a phone. This was also an exercise in cheesyness.

Here there is a similar potential issue with a reflective, almost mirror like object of an iphone. Thankfully the phone was never on the right angle to pick up the green screen so we kind of dodged a bullet there.

For this second key I employed a process known as Hard and Soft keying. This is a method taught to me by my man on Frankenweenie, Mr. Jon Brady. Jon is a veritable wealth of knowledge on all things Nuke and what he doesn't know he will find out.

Hard and soft matting works by combining a number of different keys together to help achieve a natural looking key without loosing to much edge detail.

As this first image of the Alpha channel demonstrates, the edges of the key when compared to the original footage are very harsh with very little detail around the hair. This is the hard matte and is intentionality keyed this way. The hard matte is a very basic matte, the green is keyed, in this case using the program Keylight within Nuke and the edges shrunk, or dialated to the negative. The alpha channel from this hard matte is then piped to a second instance of keylight where it is added to the inside matte, meaning the opaque part, or white part of this image is added to the second keyer.

This second keyer, with the opaque hard matte piped into it is then used to bring back the edge detail and creating a more believable key. This can be seen in the hair and stubble area's as the softness and detail is clearly visible.