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Photography Question 
Linda Buchanan
 

Chroma Key Background


I have seen the term Chroma Key Background. Would someone please explain to me what this is? Also, is it possible to purchase digital backgrounds on CD to use in Photoshop? Thank you so much for all your help.


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2/9/2006 7:07:09 AM

 
Bob Fately   Chromakey is a term from the video world, actually - it's the process whereby a computer is used to replace some solid colored background with an image or scene of choice while the subject in the foreground can act or talk, etc in front of the camera.

That's how your local weatherman (or weather-chick...weather-person?) is beamed to your home TV. In the studio, 'Dallas' or 'Fritz' or 'Al' or whomever is actually standing in front of a blue (sometimes green) backdrop. That's it, just the weatherman in front of this green background. The weather monitors he looks at are placed to his left and right sides, out of camera range - but that's how he knows hwere to place his hands when he's showing the Santa Ana winds coming down 'this-a-way'.

Meanwhile, a real-time sophisticated specialized computer replaces the green background with the live weather maps, or the live scene from a rooftop camera, or the forecasted temperatures, or whatever. So, on the final TV broadcast, it looks like he's standing in front of the maps, but in reality he's just standing in front of the green backdrop and the computers place the maps where the backdrop green is found.

So, in still photography, the same thing can be accomplished. A ChromaKey background is simply one that is a solid blue or green color (those colors are used because they are generally not found in the live scenes - if Al wore the right color green shirt then his upper torso would become "invisible" too!). You shoot your subject (say, grandma) with the green background, then in Photoshop use the layers and replace the green background with another shot of TImes Square on New Year's Eve or whatever. It'll look just like grandma was there!

Well, that is, assuming you are careful with the lighting, etc., so it doesn't look faked.

But anyway, that's that Chromakey is about.


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2/9/2006 8:11:38 AM

 
Christopher A. Vedros
BetterPhoto Member Since: 3/14/2005
  Very good description, Bob!

Linda - I have seen lots of CDs with digital backgrounds for sale on eBay. You can use any digital image, really, as long as it is a good enough resolution for printing.

Chris


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2/9/2006 8:36:07 AM

 
Linda Buchanan   Thank you both so much. What resolution would I be looking for in a CD of backgrounds?


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2/9/2006 8:40:58 AM

 
Bob Fately   In general, Linda, the resolution for printing should be on the order of 300 dpi. This may change somewhat for a specific printer (for example, Epson printer drivers are optimized for 360dpi, Kodak dye subs for 301dpi, and some large format printers are designed to accept 240dpi). In any event, 300dpi is a good place to start.

The thing you don't want to do is obtain your background scene in something made to show on a monitor - usually at about 75dpi resolution. If you try to upsize that kind of a file then there may well be digital artifacts craed - jaggies, etc. - that will make the background look fake.


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2/9/2006 1:02:17 PM

 
Linda Buchanan   Thank you for your help. One more question...in digital photography a portrait can be physically taken in front of a chroma key backdrop, and then in PS the portrait can be "lifted" and put on a different backdrop taken from a CD? (I know there are more steps to it than this, but I'm sure get what I am asking) Thank you for your help.


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2/9/2006 1:17:43 PM

 
Bob Fately   Short answer, Linda: yes. That's exactly the deal - in PS (and other image editing programs) you can "knock out" the background to replace it with that Hawaian sunset.

If point of fact, you can do that without using the green screen background - the mono-colored backdrop just makes it a lot easier. Imagine taking a shot of a brunette against a backdrop of a town park, where the tree branches appear behind her. Well, the knocing out process essentailly works by telling the program to ignore everything with color X - so if you try to remove color X brown you may get rid of the trees, but also her hair. I think you get the idea...

ChromaKey was necessary for video production (particularly live video like the TV news) because software couldn't keep figuring out what to keep and what to discard otherwise. FOr still shots, though, there is a whole class of knock-out software (sometimes plug-ins for PS) that will allow you to specify the general thing you want to keep and then figure out what to remove, even when there are some close colors in the back.


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2/9/2006 2:01:12 PM

 
Justin G.
BetterPhoto Member Since: 7/13/2004
  Chromakey is extremely simple, quick and effective. Just learn the powers of blending and blending modes with your layers. Blending modes are simply mathematical calculations of how color values are compared. Just search for articles on advanced blending and similiar methods. I'll give you a hint to peachpit.com.


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2/9/2006 7:20:46 PM

 
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