Chroma-Key and DSLR image editing

Discussion in 'Digital SLR' started by Alan Browne, Apr 26, 2005.

  1. Alan Browne

    Alan Browne Guest

    Having spent over two hours today retouching images from a local fashion
    show with an atrocious background, I'm determined to avoid that in the
    future as well as be able to slip in some more interesting BG's.

    Has anyones used Chroma-key BG's (blue, green, red) and done post prod
    work this way in PS? Love to hear about your experience.

    In particular I'm concerned with how evenly lit the Chroma key BG has to
    be lit behind the subject.

    Cheers,
    Alan
     
    Alan Browne, Apr 26, 2005
    #1
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  2. Alan Browne

    Darrell Guest

    Chroma key has to be the exact colour value across the entire field. I
    played with it back in 1998-99 with video. Any shadow or hotspot will not
    drop out, much like the Photoshop magic wand tool when you get other values.
     
    Darrell, Apr 26, 2005
    #2
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  3. Alan Browne

    bH Guest

    It's not that difficult to do In Photoshop if the background colour
    doesn't clash TO much with the foreground. It doesn't even have to be
    perfectly evenly lit either. A simple colour selection (Select/Color
    Range) to get the initial selection works fine. You might need to play
    with the Fuzziness bar and maybe add some more colours to the selection
    if the background isn't that evenly lit (Shift/Click on colour).

    Once you have the selection, you can fine tune it by switching to Quick
    Mask mode. From that selection it's easy to drop in another background.
    Good luck!
     
    bH, Apr 26, 2005
    #3
  4. Alan Browne

    canongirly Guest

    Great and how would you like to do that to 200+ shots against the clock?
     
    canongirly, Apr 26, 2005
    #4
  5. Alan Browne

    Pete D Guest

    Trained monkey's will probably help here.
     
    Pete D, Apr 28, 2005
    #5
  6. Alan Browne

    Clyde Guest

    It's not as easy as it sounds. I have a green Chroma-key background that
    I use for portraits. I like to cut it out and create my own digital
    backgrounds. Here are the problems I have regularly encountered:

    You have to make sure the subject is far enough from the background so
    that no reflected green light bounces off the background onto the
    subject. This can give the sides and edges of your subject a green tint.
    That is hell to correct. Oddly the places you need this background the
    most never seem to have enough room to do this.

    Blond hair is no fun. It's light and wispy and lets lots of the
    background through. The worst is that it partially lets the green
    through. So, you have to have to get rid of part of the green without
    getting rid of the hair. Dark hair isn't nearly the same problem.

    I've tried a ton of ways to select the green for deletion. The best for
    me is to make a mask starting with a channel. I seem to pick the "a"
    channel of LAB most of the time. I then manually edit the mask. I crank
    up the contrast to separate the subject and background as much as
    possible. The key is to watch the hair parts so you get as much
    transparency as possible and still get the green. You will probably have
    to try a number of times to get it the best you can.

    Usually that doesn't get rid of all the green in the hair. I can either
    brush on the mask to make it more transparent or I can brush to tint the
    hair back to it original color. Neither is perfect and both are often used.

    I usually have to edit the mask to clean up the black and the white. The
    LAB "a" isn't perfect. If you don't clean it to pure black and white
    (except for the edges) you will have a lot of partially transparent
    spots in you picture. Part of the reason for this is that the green
    usually isn't evenly lit. If it's far enough away to not bounce light,
    it probably isn't inside your lighting setup. Actually, it shouldn't be
    or you will get green bounce.

    Another reason for this is that people's clothes aren't as monochromatic
    as your eye sees them. You tell them to not wear green and they don't.
    However, Photoshop sees a touch of green in brown, yellows, and other
    colors. This is a big reason for making the mask rather than other
    selection tools. [The first time I got this, I put my background back
    there and got a bunch of spots of that color popping through. Funny only
    for a few seconds.]

    When I get it, it looks pretty good. My clients have been happy with the
    results so far. I do get a bride that has had that during the engagement
    portrait and wants it at her wedding. I have talked them out of it. I
    can't imagine the trouble of cutting the green coming through a bridal
    veil. White dresses should pick up even faint green bounce.

    I don't use my green any more than I really have to. It's a lot of work
    to do all of this. I never use this technique on a whole batch of
    pictures. It would take forever. When you need it, it is very nice to have.

    I have never found anyone with a better way of doing it. Well, I have
    heard there are some very high end pro tools that make it much easier.
    You will pay through the nose for these, but if you will do a lot of
    these they may be worth it.

    If you are shooting a bunch of fashion shots I would seriously look at
    paying the money. A green background will be better than nothing, but
    you will still spend way more time on each picture than the high-speed
    turn-around the fashion industry demands. I wouldn't count on Photoshop
    being the answer here.

    The other problem with fashion is that you can't control what colors the
    clothes are. You are likely to get color the same as your background.

    Clyde
     
    Clyde, Apr 30, 2005
    #6
  7. Alan Browne

    Alan Browne Guest

    The place that gave me the most fits would have been difficult to light
    the BG evenly in the first place. OTOH, it was a fashion show and I had
    fairly flat lighting on the subjects so I think that would have
    clobbered the blue/green back reflection.

    I had that same problem in any case. The scene had a dark brown beam
    across the back that in some shots crossed the head line. PS'ing it was
    an iron clad b*****.
    Hmm. I'm less and less enthralled.

    I think I'm now convinced that it will be cheaper (in time) to rent a
    backdrop next time I'm stuck with a bad scene. The next show I'm doing
    (same client) will be outdoor, and hopefully I can control the scene
    better by allowing elements in it that are not distracting... that are
    natural for the event.

    Yep. And esp. this year cool greens and blues seem to be in everything.

    Thanks for much for your detailled reply.

    Cheers,
    Alan
     
    Alan Browne, Apr 30, 2005
    #7
  8. Alan Browne

    Bud Evers Guest

    The CS version of Photoshop (and presumably the new CS2 version) has
    an "Extract" feature which does an outstanding job of preserving the
    fine lines of hair and other wispies against the chroma background.
    In fact, I have had great results using a green screen made from some
    yard goods bought at the local super center.

    I use this approach to take shots of students at school events (proms,
    awards nights, etc) and then drop in a school logo as the background.
    I'm a long way from being a high-end photographer, but the results are
    pretty stunning.

    Using the feature in PS couldn't be easier: you just highlight the
    area of the picture that is the boundary between the subject and the
    BG, then highlight the area you want to preserve. PS does the work.

    Come to think of it, you might want to give this whole process a try
    with your "atrocious background" from your fashion show, and compare
    PS results with your hand-retouched version; I'd be interested to know
    how the two compare.

    HTH
     
    Bud Evers, May 1, 2005
    #8
  9. Alan Browne

    Alan Browne Guest

    First off I have Elements 3.0, and using things like the magic wand with
    the threshold carefully set cannot do a good job of separarion of the
    model from even a somewhat even BG. Where the model has shaddow tone,
    those tones get selected into the background. Two models had pretty
    loose hair, and getting this separated was a b****.

    I don't want to spend the dough on CS, if I can avoid it.

    I photographed the store's logo (a very nice relief, coarse gold/black
    on bold red) with the intention of putting behind the models. But
    separating the models from that BG was too tedious. So on some images
    the logo appears smaller and in the corner. I delivered the poster
    Friday to the store (24 x 36) and the proprietor was very happy with it
    as well as the other material. (As the poster is a mosaic of 30 someodd
    phots, the res is fantastic).

    I just have to find a way to spend less time with each project. I
    can't afford it. I'd rather control the scene than work PS.

    Cheers,
    Alan
     
    Alan Browne, May 1, 2005
    #9
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