green halo when touching the green screen

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    • #70952
      AvatarUncle Phil

      I have looked all over Youtube for the answer to this video question and cannot find the answer, Please help.

      I am starting to work with green screens and I understand completely the green halo effect that you get when the talent is too close to the green screen. They need to be 8 or more feet from the screen. I use Premiere and After Effects CS 4 and understand how hard it is to get rid of the green halo when the actor is even 5 feet from the screen. I get it and I understand how to fix it.

      BUT my question is about situations where the talent is actually touching the green screen. I have seen the following examples where the talent is actually touching the green screen:

      • A photo of a completely green screened bridge of the star ship Enterprise, including a green screen chair where the captain will be sitting.
      • Virtual sets where actors will be sitting and working at green screen tables.
      • Talent standing on green screen floors.

      In all of these situations they controlled and/or removed the green halo. How did they do it? How do you get rid of or eliminate the green halo in these situations where the talent is actually touching the green screen? Is there some kind of special light set up that you use in those situations or special software, etc? THIS LOOKS LIKE A GREAT OPPORTUNITY FOR A ONE OF A KIND TRAINING VIDEO! Because no one is explaining this!


      Phil Ebert

    • #208882

      There are no shortage of great books and videos out there, you're probably just not looking in the right places.


       The Green Screen Handbook: Real-World Production Techniques / Edition 1 by Jeff Foster | 9780470521076 | Paperback | Barnes & Noble


      Visual Effects for Film and Television :: Post-Production :: The DP's Bookshelf :: Books & Videos :: The ASC Store


      Basic Blue Screen and Green Screen Photography :: Cinematography :: Instructional Videos :: Books & Videos :: The ASC Store


      GreenScreen Made Easy:Keying and Compositing for Indie Filmmakers :: Production :: The DP's Bookshelf :: Books & Videos :: The ASC Store


      The big thing to know is that all keyers and all source material are not created equal.  For good composites, start with propperly exposed, uncompressed footage.  No HDV, DV, MPEG if you want a really nice, national level composite.  If it is compressed, make it lossless.  If you can get 4:4:4 color space, even better.  Not cheap, not small files, but it makes a world of difference.  Also understand that a motion picture composite is more than a guy with After Effects.  A team of artists may work on just one shot for weeks or months.


      Regarding lighting… generally a nice rim or backlight helps provide a better key.  You can backlight the green in the situaiton you describe but that can create another set of problems.

      Most programs have simple keyers built in that do a fine job of handling straight keys but an effects shot in a film and even many of the composites we run into in commercial production are far from simple and require more powerful tools and a more complicated workflow.  Subtle changes in the purity of the green can drive a simple keyer crazy especially if your working with a crappo format like DV, HDV.  I've pulled more insane keys in my career than I will bore you with.  Firstly, no two are the same.  The workflow that makes one key total cake is worthless on the next.  It's a matter of having a full bag of tricks at your disposal. Most of my high-end work is done with Autodesk Flame which is an amazing and amazingly expensive toolset.  The workflow will be different in your package and plugins but the ideas are the same.


      Use a better keyer:
      The standard keyer is probably an eyedropper tool, a softness selecter and thats about it. More powerful keyers have dozens more parameters to do all manner of cool things to the matte source, fill source, background plate, etc.  Secondary keyers that essentially remove a second range of colors or add that second range of colors back in.  Key in different color models RGB vs HSV, etc.  The ability to slightly defocus the matte input to supress noise.  The ability to grow, shrink or erode mattes.  Color spill supression is also a huge factor that can hide green cast on footage.  Forget the built in keyer and look at plugins from Primatte, Keylight, etc.  Buy a couple of plugins and get to know them in and out.  

      Garbage masks and rotoscoping are a HUGE part of complicated composites.  I've done the type shot you mentioned where talent touches a green screen.  It usually involves cutting out every finger, hand, arms, strands of hair, etc frame by frame to generate a matte that would be impossible any other way.  Got a problem area in a moving shot?  Use a garbage mask and motion track it.  Get comfy with the idea of sometimes using dozens of garbage masks to break down your shot into smaller areas.   


      Stack up layers each tackling more dificult elements.  Don't just use your keyer just to remove green, use it inverted to add flesh colors, etc.  I've done keys where the talent was wearing bluejeans on a blue screen.  Set the threshold to make everything besides the pants look right and the pants are gone, right?  Duplicate the layer and use a second keyer with a very tight threshold just on the blue in the pants. Invert the keyer and you have suddenly added a problem color back in.  Personally, I prefer this approach to using a stack of secondary keyers.  Talent has blonde frizzy hair?  Do a key just on general color of the hair, invert the key, tweek the edges, aditive mode, etc and that problem hair is suddently looking great.


      To answer your question more directly, the reason there is no video that explains how to key every shot in the world is that each and every shot is different. Typing in all caps doesn't help the fact that green screens are some of the most technically demanding shots to do.  The secret to the most complicated shots is problem solving with good keyers, garbage masks to isolate and break the shot into smaller areas, rotoscoping and lastly color correction to finess everything into the same look.






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