Green Blue
Posted on Thursday, November 19th, 2009 at 3:21 pm
Where Do You Get a Blue Or Green Screen?
Before you can use any chroma keying software, you must prepare either a blue or green background. The color to choose depends on what your actors are wearing. If they are wearing blue, then use a green screen. If they are wearing green, then choose blue.
There are several ways of getting a chroma key background. If you are setting up a home studio, you can simply paint a wall green or blue. There is special chroma key paint – which costs $40 a bucket – but you can get similar results with cheaper paint from Home Depot. Use either pure green (255 green) or pure blue (255 blue) paint. First sand and primer coat the wall. After the primer dries, evenly paint the color on the wall.
Another, cheap way to get a green background is with green colored posterboard, which is sold at Wal-Mart or any art supply store. At about $1 a sheet, you can affordably cover a lot of area. The downside of this method is that the edges of each sheet may show in the final video. It is also a pain to tape up many small pieces of posterboard to a wall.
Many vendors sell a foldable chroma key backdrop that is blue on one side and green on the other. The folding backdrop collapses small enough to be portable, which is great for doing interviews on the run. The drawback is that the screen may not be wide enough to do certain action shots like two people fighting each other.
Chroma key fabric is the last way of getting a background for the Ultra software. True chroma key green or blue cloth will be very expensive, but your local fabric store probably has a similar color. Buy a small piece and test it out with the software.
Picking Key points
The next step is very quick, as you simply pick a bunch of points on your green or blue background. You are supposed to pick points that are both in the light and dark areas. This gives the Ultra algorithm the information it needs on what shades to chroma key. In most cases it takes less than five seconds to pick all your points.
Seldom will you have a smooth surface on your green screen fabric or painted wall, but Ultra will easily key these out. As you select your points, put a couple clicks on where the creases or folds are, and when you apply the Chroma key everything should look great.
If you want a more realistic looking video, you can opt to keep shadows in the final render. Simply select points around the shadows and Ultra is smart enough to chroma key around them. You generally don’t have to select too many points; three or four seem to work well.
Lets say that you have an actress standing in front of a huge green screen. There will be a lot of empty green around her. Usually, you only want to Chroma key the area right around the actor, as any extra chroma keying will just waste computer cycles during the render phase. This is where Input Cropping in the Input tab comes into play. This will chop off the sides or the top and bottom, leaving you with just enough area around your subject. As you adjust this setting, you can see the results in real time.
How to Apply the Points
Clicking on Apply Points in the Keyer tab will turn the green/blue color into the background that you selected. You will probably notice that some areas on the actor/actress will be transparent. The checkerboard pattern really helps you see the transparent areas. This transparency is normal and can be easily fixed by moving the transparency slider to the right until you don’t see the checkerboard any more.
Transparent Objects
Chroma keying has traditionally had trouble with keying out hair and semi-transparent objects such as veils or water bottles. Ultra, however, has no problem with these situations. You can see in our wedding picture that the veil is very thin and that you should be able to see through it in real life. On the right side is the final render where the ocean and sky colors can be seen through the veil.
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