How and why to use green screen
In this tutorial I am going to look at how and why to use a green or blue screen properly for composition I will be looking in to camerawork, lighting, tracking markers, setting up properly and finally there will be a video tutorial on how to use key light 2.1 in after effects. The reason I am going to be looking into this is because I know I had a few problems when first using this technology as well as giving a few hints and tips when using this method.
Ok first of all there are a few reasons why you would use this the main reason would be if u need to create either a different background as you are filming your actors and maybe a) because you are using a cg background and designing it or b) you already have your background and wish to put your actors in it. For me the initial reason was why, I have created a cg background ready for my piece and will show you how to use all the elements to let you create a successful project
Setting up
First of all you need to get yourself a good quality green/blue screen hopefully you will be able to get a hold of a studio, if not then get a hold of some good quality cloth no marks and the least amount of creases possible if there are a few you should try to iron these out, reason being is that the colours blue and green don’t have a very large amount of white pigment such as other colours if you use other colours such as red or pink the amount of other colours that are used in them and because of the amount of white then when processing the illusion of a different background it will try and cover other colours that u may not have accounted for, you need to arrange your camera so that you have all green in the shot if there is floor that is not green then I suggest you should put a green carpet or green cloth on the floor if u have a ground shot, also arrange the camera so that there is not a lot of movement on the camera the last thing you want is a jiggling camera especially when it comes to tracking so get yourself a stand if it’s a single still shot or a rolling stand if you want a moving shot o and by the way make sure the floor is clean if your using a rolling camera and have lumps all over the floor your going to have a lumpy movement on your camera.
Lighting
When lighting your scene you need to make sure your using a few lights. There are three points to any shot, the foreground, middle ground or main point and background all of these points need to be lit in there own way lets start with the background, the background has to be lit in its own way, keep the lights high as low lights may screw up other lighting for the other focal points have the light directing down towards the bottom of the screen but still facing the screen this will also try to eliminate shadows from your actors also have a few more lights at the side of the screen toward the back so it keeps the background evenly lit, the reason you need to keep it evenly lit is so that the computer can recognize which parts are green and which parts aren’t , if your using a screen then I recommend putting weights at the bottom of the screen to keep the screen from waving. Ok the main focus of your action needs to be specifically lit for the purpose of what you want to film e.g. in a club (DIM), on a beech (BRIGHT) so arrange the lighting for how you want to use it and finally the fore ground needs just enough light for the cameras sensor to pick up. On a Digital slr in my case a canon EOS 500D the iso needs to know how much light it is taking in, to bright and you will have a washed out film to low and u wont be able to see anything.
Tracking
I’m going to be talking a little about tracking but I’m not going to actually tack my shot because I have a still shot I am going to use for the purpose of this tutorial, tracking means using points on your shot to understand the movement of the camera programmes such as bojou, matchmover and after effects can analyze these marker points and transfer them into data, so u can add the movements to your cg camera, this will only work if you have enough markers for your movement and also you will need to know the focal point, aperture and iso settings to get a true movement to your camera also I recommend that you clean the floor because I know from experience you can get really bumpy shots when moving the camera.
And here now is my tutorial on after effects.
