THETHE BASICSBASICS OFOF VIDEOVIDEO KEYINGKEYING by Brad Weston robably the biggest step in integrating video into your services is moving from a single video signal to multiple video signals. Generally, this means upgrading from a computer hooked directly into projectors to display words and graphics, to a video switcher or switcher scaler with Pone or more computers, cameras, playback devices, etc. as inputs, with the output going to the projectors. Usually when this step is made, it is done with the desire to combine, or “composite” the words generated from the computer to the other video signal inputs using a technology called “keying”. When you make this leap, you will be well served to understand how to “key” the words gener- ated from the computer over the other inputs, and the pitfalls that can occur with the keying process. Side Note: Many worship software applications allow you to take a live video input into the computer, which could result in better results than using a keyer, depending on the quality of the switcher, but it is gener- ally not recommended to put a computer as the last “stop” on your signal fl ow to the projectors; if the computer should go down for any reason, your video production is completely shut down. If you decide to use this method, it is good to have a simple VGA switch at the ready to switch between the computer output signal and the “clean feed” from your video switcher... just in case!

July/August 2009 tfwm.com 23 VIDEO/Keying The Basics of Keying Chrominance Keyer (Chroma Key) Think of keying as you would The easiest way stencils. Do you remember those to understand pieces of cardboard that had Chrominance key- holes cut out of them in the ing is to think about shapes of letters so that you the weatherman could spray paint words onto standing in front of signs? Keying involves three dif- his weather maps. ferent pieces: the “key” signal, If you’ve ever which is analogous to the stencil; seen a TV studio, the fill signal, which is analogous you would notice to the spray paint; and the back- that he’s actually ground, which is the cardboard standing in front The keyable image we are spraying the paint onto. of a blue or green Keying is more sophisticated than screen. Special key- this, though, as you also have ing equipment takes ways of limiting the amount of out this green or spray paint, or fill transparency, blue color to reveal that you see on the image. the background In video, our stencil, or key behind him. So, to channel, is visually shown as a continue our stencil grayscale image... every part of analogy, the three the image that is pure white is a pieces are: big hole that lets the fill image • Stencil/Key The mask created by the keying device (our paint) show through. Every Image - created area that is pure black is opaque dynamically the and lets no part of the fill image weatherman stand- show through. Every shade of ing in front of the gray is a level of transparency of green screen -- if the fill image. As such, if a part you were to look at of the key image is 50% gray, this key image you then the fill is 50% transparent. would see a white When we talk about the three silhouette of the most common types of keying weatherman over a (Chroma keying, Luminance key- black background. ing, and Linear Keying) we are • Spray Paint/Fill The combined image basically talking about how this Signal - This actually comes from the same image used to calculate the key grayscale key image is created so image... it is the weatherman standing in front of the green screen. that we know how much of the • Cardboard/Background - This is the weather map that we want to project fill signal to let through. the weatherman on to. Luminance and Chrominance When the keying processor combines these signals, the resulting image is keying use some kind of video what you see every night on your local news. As these news stations use source and create the key dy- sophisticated (read: expensive) keying equipment, it is smart enough to keep namically based on how bright track of shadows within the keyer, so it appears that the weather anchor’s the image is (Luminance), or a arm is casting a shadow on the map. specific color (chrominance). For In the sample images, we have a figure against a green background. The the example graphics, we are go- keying mechanism ignores all the green information and lets anything that ing to use a stock photo taken on is NOT green show through, thus creating a composited image of the figure the moon. standing on the moon. Pros of Chroma-keying: Most every video switcher and switcher scaler that has “keying” capability is usually some form of a Chroma key. Most of the switcher scalers that say they also have Luminance keying are basically using a black color signal for the key. Cons of Chroma-Keying: You are limited in that you cannot use the key-color anywhere within the graphic that you want to show up on the screen be- cause it will appear transparent when keyed out. Also, note the ghosting ef- fect issues that exist... explained in the sidebar, "The Ghosting Effect", pg.25.

24 tfwm.com July/August 2009 VIDEO/Keying THE GHOSTING Luminance Keyer (Luma Key) EFFECT Luminance keyers work best when what you want to show One of the biggest problems with on the screen is brighter than luminance and chrominance keyers the background you are using, is the fact that you are usually using such as white words. Usually, the same signal for both the key and you use this kind of a key with computer generated graphics. fi ll. This becomes a problem when Using the example of white you are dissolving between two The image we use as the keying input words on a black background, different text slides. you specify the tolerance level For example, let’s say we set the of brightness from which you tolerance on a luminance keyer to wish to create your key. It’s about 50% with white words on a important to note that in video, black background. As such, you can 100% bright is pure white, so that is 100%. Any color or shade say 50% gray means 50% transpar- of gray would be less than ent so that as the text fades to black, it gets more and more transparent. The fi ll we are specifying for the key 100%. So, if you specifi ed 100% white as your tolerance, then The problem is, if your fi ll signal is any shade of gray under 100% the same as that which is generat- would not be shown over the ing your key, then as white text background. turns to black, it goes through 255 In the examples (left) we’ve specifi ed a matte white color different shades of gray. The result as the “fi ll” for the key, which is that although the text is becoming means that the graphic will transparent through the keyer, it is The mask created by the keying device be composited with the back- also turning darker. This creates ground using various transpar- a “ghosting” effect where dark encies of white. To demonstrate residue is briefl y seen where white this, the image that is defi ning words used to be as you transition the key signal has color in it, from one set of words to another. but because a luminance keyer pays no attention to color, the If your switcher has the ability to mask created by the keying specify a matte white fi ll, then this device is made up of various will work very well with the keyer, The combined image levels of gray. Because we’ve because regardless of what level of specifi ed the fi ll signal to be pure white, the circles and squares appear at transparency is specifi ed by the key various levels of a transparent white over the background image. If we had channel going from white to black, used the same image as both the key signal and the fi ll signal, the circles the text will simply appear as some would be their respective colors, with the same transparency levels as what type of transparent white text. is shown on the composited white image. Pros of Luminance Keyers: If you are showing white or single color lyrics over a camera signal, luminance keyers will generally provide better results than a chroma-keyer. This is because the tolerances of the transparency make for cleaner keys, particularly in lower-end switchers that don’t give much control over the tolerance of colors that you want to key out in a chrominance key. Also note that high-end switchers will often have a bor- der or shadow option that will generate borders/shadows internally from the same key signal using a color you specify. There is no “ghosting effect” if you use a separate color-fi ll from the key signal. Cons of Luminance Keyers: They are usually only available in higher end switchers (as noted earlier, the low-end switchers and switcher scalers often will call their keying devices “luminance key” capable, but they’re really just doing a chroma-key of black which is not the same thing); You cannot have dark colors as part of your visible image as it will show through as transparent. The “ghosting effect” is also present in luminance keyers (see above) if you use the same signal for both your key and fi ll.

July/August 2009 tfwm.com 25 VIDEO/Keying

Linear (Alpha) such as PNG or TIFF. Keyer Video codecs that support The problem with keys are Quicktime PNG the Chrominance and and Animation codecs. Luminance keys is that In the example im- they limit the number of ages, notice that the colors that appear as part key channel has some of your key. For example, transparency on the trail if you do a chrominance of smoke coming out of key on blue, then noth- the airplane, and that the ing blue can appear as red title at the bottom of part of your image that The fill channel the screen has a gradient you want to show on that fades to nothing. The screen. If you do a lumi- composited image shows nance key, nothing black the clean results of being can show up on your able to explicitly specify key. An “Alpha” or “Lin- this level of transparency. ear” key sends a separate Pros of Linear Keys: It is video signal to the keying the gold-standard of key- device that contains the ing systems as it allows key information. This the mask to be explicitly video signal just looks specified and thus gives like a grayscale image you the full range of col- and defines exactly the The Key Channel ors and transparency of transparency of the way one signal over another. the fill signal is supposed As such, you can create to be overlayed on the feathered, transparent background. shadows on your comput- In the case of ProPre- er that will look beautiful senter’s alpha keyer, you when composited over a can either send two sepa- background. rate SD or HD-SDI signals, Cons of Linear Keys: or you can let the video Requires a more expen- card of the computer do sive computer system the overlay for you by and switching equipment, feeding your background The combined image as well as two separate into the computer, and video signals. u then letting the video card overlay the key and fill infor- mation onto the background in real time. ProPresenter Brad Weston is President of Renewed Vision, makers of creates key channels for generated text automatically, but ProPresenter, ProVideoPlayer, and ProVideoSync. He has if you wanted to use your own graphics, you can create a long history of enhancing church and ministry produc- them in graphics formats that support Alpha channels, tions. www.renewedvision.com

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