Nov. 07, 2003 - At least once a week in one of the many user groups out there, someone inevitably asks, “What is the best background for my chroma key shoot – blue or green?” If you are shooting on DV, then I say go green, you’ll have better luck.
Way back in the day, television cameras used tubes to capture an image. Nearly all color cameras were made up of three tubes, each representing one of the three color channels (red, blue, and green). Actually, way back in the day is a bit misleading - as a student of the 80’s our station’s TV cameras still used tubes with all the fun and joy that came along with them. [an error occurred while processing this directive]By turning off one of the color channels, the camera could be made virtually blind and unable to see that particular color – like blue, for example. This created a “hole” or empty space that allowed the Engineer or Technical Director to fill that blank area with other information (weather graphic, video channel, etc.). Today we call this transparent area the alpha channel.
Obviously making the camera blind to everything that had blue would cause all sorts of problems; anyone wearing blue (or green) clothes would key out, and the viewer at home would see floating heads on the screen – not so good. To solve this problem, engineers decided to set the key color at a one hundred percent pure hue of that color. This resulted in chroma key blue and chroma key green paints that were used to color sets that would later be keyed out.
For what seemed like a very long time, keying through a switcher or isolated keying system (Ultimatte for example) was the only way to pull off a successful key in video. Today, with digital technology, the process is much easier, and you don’t even have to use chroma key blue or green paint, as any solid color will work. You can pull a key from the color purple just as easily as you could pull a matte from chroma key blue or chroma key green screens.
So why do the people still use chroma key green and chroma key blue paint for the background? Chroma key green and chroma key blue are colors that will more than likely not show up on your foreground subject. Blue was selected because we do not have a lot of blue in our complexions. Green was used as an alternative because many people wore blue suits, jeans, etc. Because the chroma key colors are so brilliant and pure, they also help contrast the foreground subject with the background. This makes it easier for you to eyeball a key and spot problems during post.
The other reason why chroma key green and chroma key blue paint is still used extensively is because the collective “we” still base our video signal on the original RGB components. If you were keying out the color purple you would be delving into the red and blue channels instead of just a single color channel. This could cause problems with other colors in your image and should be avoided.
I mentioned RGB color space a moment ago, but there is a video space that is used quite a bit and actually stemmed from the early days of color television when color video had to be backward compatible with the black and white signal.
YUV is the result of this 1950’s compatibility mandate and is still used to describe video space today (you will see why this is important in a moment). YUV is a signal that contains Luminance information (Y), and two channels of chrominance (color) information (U and V). The Luminance channel is made up of the brightness - or intensity - values from the RGB signal (represented in grayscale), and the U and V channels contain actual color information for the Red and Blue channels (U=Red-Luminance, V=Blue-Luminance). Green channel information is contained in the Luminance signal only.
Because the “raw” RGB video signal is bandwidth intensive, YUV is preferred because it uses less bandwidth and thus saves precious space by not sending or transmitting the luminance signal three times.
Of the three channels, Luminance is the least compressed (the Chroma channels are always subsampled at either the 4:2:2, 4:2:0 or 4:1:1 compression schemes depending on the format being used). Since the color green is derived from the Luminance channel and makes up nearly 60% of the video signal, it tends to key best.
Want to see this for yourself? Open Adobe Photoshop or other paint program. Create a new canvas and divide the area up into three areas. Fill each of these sections with 100% of Red, Blue, and Green.
To simulate the Luminance channel, change this color image to greyscale.
As you can be seen, the Green channel is “brighter” than the other two. Blue at 100% luminance contains approximately 1/6th the luminance of Green. This is not only a technical consideration, but our mind also perceives the color blue as being darker than the other two color channels.
While there are other considerations to take into account when shooting chroma key footage (foreground subject, video format compression scheme, lighting, etc.), by understanding how your video information is being processed, and where most of the information is contained, you will be on your way to having a great key, and using the color green is a good place to start.
When not working deep in the labs of the DMN Central Division testing the latest and greatest software/hardware products Stephen Schleicher can be found at the local university teaching a few courses on video and web production. He can be reached at [email protected]. You can also visit him on the web at www.mindspring.com/~schleicher
Source: Digital Media Online, Inc.
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