Chroma Keying Article
By Bill Schnarr
Chroma Keying and Jagged Edges
Ok, so youíve read up on how Chroma Keying works because you either wanted to add some excitement to your shots or you always wanted to see footage of your friends dancing on the wing of an airplane.
You collected foreground footage of your friends doing the Macarena and you collected some footage of a jumbo jet sailing the clear blue skies. Simple stuff, right? Then you Keyed up your camera to eliminate the blue screen in your foreground shot and dropped that big airliner in behind your friends dancing away.
And thatís when all hell broke loose.
Jagged edges! Why didnít anyone tell me there was going to be jagged edges!
Why You Have Jagged Edges in Your Chroma Keyed Shot
Jagged edges are a common occurrence in Chroma Keying. Nothing makes a shot look more unrealistic than ugly dark edges that separate your front shot from your back shot.
Jagged edges are most often caused by colour bleeding from your Keyed backdrop onto your subject. When colour reflects off of the blue screen in your background and "bleeds" onto your subject, the camera can become confused. Sometimes it knows that the reflected colour is the same as the backdrop, but since it isnít exactly the same it can cause problems. These problems are the jagged edges you see.
Dealing with jagged edges is a time consuming process, but like all aspects of the Chroma Key technique it is something that will come easier with time and practice. If you experience jagged lines in your footage, there are a few things you can try to help eliminate them.
Getting Rid of Jagged Edges
Removing jagged edges takes a little thought and a lot of patience. First of all, double check your lighting. There a lot of problems relating to film work that can be easily cleared up with proper lighting.
Start by moving your subject away from the backdrop and then re-shooting. You need a solid, continuous light on your blue screen in order for the Chroma Key to work properly. This is best achieved by using large bounce cards to reflect the light toward the screen.
Unfortunately, you also need to light your subject separately. These two separate types of lighting can interfere with each other, causing Chroma Key problems all over the place. You donít want your filler lights reflecting off of your screen any more than you want the ambient lighting created by the bounce cards used for backlighting to interfere with your subject.
The light reflected off of the screen can create a situation where your subject is side lit or backlit by the colour of your screen. This is what will create the jagged edges in your shot when it is time to composite the Chroma Key.
Another thing to do while your checking the lights is to check the shadows from your subject as well as the possibility of shadows on your backdrop screen. Shadows can cause many of the same problems that improper lighting does with regards to improper Keying. Ideally you want to have lights set up on both sides of the subject to eliminate the possibility of shadow problems.
Watch out for shiny objects when you are shooting your subjects. Shiny objects can be a source of great headaches due to the fact that they will happily reflect your light all over the place and cause colour bleeding like crazy.
The easiest recourse would be to do away with filming shiny any objects altogether. Of course, if you canít avoid them, try one of these suggestions instead.
First, you can try to manoeuvre the object so that the amount of reflected light is reduced. If it is something like a button or belt buckle, you can always have the subject position themselves so that the object wonít be reflecting light back at the cameras.
Another thing you can try to do is to move the object away from the light so that its effective surface area is reduced. The theory behind this is that the smaller the reflective area, the less amount of bleeding is likely to occur.
Finally, if that fails, or you simply canít avoid filming a reflective surface that is causing an unacceptable amount of bleeding (and believe us when we say that just about any amount of bleeding is unacceptable) you can pick up matte sprays at photography shops and art stores that should be able to help you to reduce the glare off of the object giving you problems.
As a safety concern, if you are having problems with jagged lines because your subject is bald and his (or her!) head is reflecting light back at you, please make sure your matte spray is non-toxic and there is nothing in it that could make your subject sick. Remember that some matte sprays were never intended for human contact, so you must be careful about chemicals that could prove dangerous to people.
Sometimes these jagged lines can also be fixed by adding filters to your lights. This is especially true when you front light your subjects and are experiencing "bounce".
If you are using a blue screen for your Chroma Key process, the situation can sometimes be remedied by adding an amber-coloured gel to your backdrop lights. Amber light will absorb much of the bleeding qualities of a blue screen, eliminating the need for a lot of complicated post-production work. Also, most amber gels are light enough that they wonít significantly interfere with your shooting light. Itís a simple matter to set up your backlights, apply the amber gel, and then configure your video camera to "Key" on the amber-lit blue screen.
It is a good idea to always use the same setup if you are going to do a lot of Chroma Keying. Try to stick with the same settings each and every time so that you can become comfortable with how your set up works and you can easily recognize possible problem areas before they cause you too many hassles.
If you use the same set up each and every time, you will also save yourself tons of work from resetting lights, angles, and camera settings.
Above all, stay relaxed, and try not to get too frustrated. Keep working at it, and before long your friends will be dancing the Macarena anywhere you want them to!
About The Author
Bill Schnarr is a successful website copywriter providing tips and advice about video chroma keying, digital cameras and photography. His numerous articles offer moneysaving tips and valuable insight on typically confusing topics.
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