Why chromakey background?

Discussion in 'Digital Darkroom' started by melissa_eiselein, Jul 14, 2004.

  1. Is there something special about a chromakey background? I
    understand that it's like the blue screen that movie studios use,
    but I'd like to know why it's better than, say, a solid painted
    surface or a solid color muslin.

    Do software programs such as Photoshop pick up the chromakey blue or
    green better than they do other shades of blue or green? I've read
    a lot about using chromakey paint/backgrounds, etc. But nothing that
    explained why chromakey is better than other backgrounds.
     
  2. I think the best answer is that most software written to work with chromakey is limited to a few colors that tend to not show up naturally and there is a definitive line between those blues and greens and the foreground. I think if you used a black or other more natural colors, you would have issues with replacing the background. I would think it would make it look like it does on TV weather boards where you have an electric noise on the edges of the person and background. I do believe that if you used a tan background and the foreground did not share any of the tones that you could effectively do the same thing.
     
  3. on principle: a "blue screen" or espacially a "green screen" is choosen becouse of the background color should not be in the foreground, if you would take "white screen" and apply key on white maybe your talent looses his shirt (if it is white). So I dont think that there is a special "chromakey background", chromakeying is just the name for the trick to set parts of the picture (one color) to transparent, other ways were lumakeying or difference keying or alpha keying.
     
  4. And BTW, Even with chroma key background ensure that the forground does not have the same color. If not those portions are also replaced with the background. I have seen this live in the weather room in CNN head quarters in Atlanta. They have a green cloth that you can wrap around your body and then only the head comes up in the display with the maps behind you! Cool for a floating head look but not good otherwise.
     
  5. In addition to being a color not resembling any color in the foreground, the background should be evenly lit to ensure as good a separation as possible. Thus, the background color and tone should be as uniform as possible and lighting should be shadowless. Of course, modern software are much better at doing the separation than some old solutions were...
     
  6. The historical reason for the Chromakey blue and green colors is: Chromakey was originally done with analog video. Special hardware was used to pull the matte, and it was done based on color. It works in real-time.

    As others have mentioned, when it came time to choose the color (which was pretty much "hard-wired" into the hardware), the inventors wanted something that 1) was unlikely to occur on the set, and 2) would be very intense and saturated, and therefore easy to detect and extract.

    In Photoshop, the color is not such an important issue. You can pull a matte using any nice, even colored area - as long as it is sufficiently different from the foreground. You can also easily "clean up" a matte that is not perfect.

    Of course, if you use Chromakey blue or green, it will be easy to pull a matte. You may even find a plugin that does it (but Chromakey, Primatte, etc. are patented processes).
     
  7. Thank you to everyone for your answers. I now have a better understanding of what I should look for when the time comes to play with putting people (ie: my son) onto various digital backgrounds.

    And thanks for the tip of using a background that won't be confused with foreground colors. I knew it would apply to a basic background, but wasn't sure if chromakey was different in that respect.
     
  8. Here's some good background information to answer your question on why chromakey at www.chroma-key.com
    Also information on using Photoshop and how to remove the background, light the subject, etc. www.chroma-key.com/chroma-key_faq
    From my experience - you want to choose a color for the backdrop or screen (chromakey) that isn't in the photograph anywhere else. For example, if you're shooting a person wearing a blue shirt choose a chromakey green backdrop or screen (not chromakey blue). Use the same technique if the person has green clothes, choose the blue chromakey. You want to be able to easily seperate the background from the subject! Also make sure you light it evenly (the background) and don't get any spill or color from the backdrop on the subject! For example, a blue cast or spill on someones hair will cause that part of the hair to be matted out during the process!

    Like anything else - you can get some awesome results - but you have to learn by experience!
     

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