7 Replies Latest reply on Jan 21, 2011 12:23 PM by shooternz

    Subject  transparent  w/Blue  Screen  ??

    George D.D.

      I've been using Adobe CS5 now for a few months, and, of course, there is a lot still to learn.

       

      When doing blue-screen compositing, I have tried several exposure techniques on the screen;  over exposing one stop, underexposing one stop, and exposing it exactly the same as the subject in front of it. It appears to work best if it's over exposed by one stop.

       

      To composite the front and rear elements is pretty easy and straight forward with the Blue-screen Key, but I'm running into a problem that I just can't seem to solve.  No matter what I do, I can't seem to cancel out a slight transparency of the subject 100%.  I can always detect a little bleed-thru of the background.  I have tried viewing the foreground subject with the Mask Only option, and have made it completely white, and yet it still bleeds through when the mask is turned off again.  I have tried everything with the controls, until I'm blue-in-the-face....ha, ha, ha .

       

      Using Ultra Key instead of the Blue-Screen Key does not seem to help either.

       

      Could it be something weird, like the blue-screen material is the wrong color?  When I bought it, it was supposed to be for digital video, but if it is for film how much of a difference would that make?  Or, could it be something about the background scene I'm trying to composite into the shot?

       

      Any help would be greatly appreciated.

       

      Thanks.

       

      .

        • 1. Re: Subject  transparent  w/Blue  Screen  ??
          the_wine_snob Level 9

          Normally, if one is pulling a good Key, if there is some transparency in the subject, it is because of the presence of the bluescreen's color in the subject, or the clothing of the subject. That issue is what prompted many to go to a greenscreen, with a color that should not be present in most clothing (though some designers seem to like that color... ). My guess would be that there is enough of that blue in the subject, that it is being picked up in the key.

           

          As for exposure of the bluescreen, it is more about how it is lit, separate from the subject - basically two lighting kits with each flagged off of the other.

           

          Good luck,

           

          Hunt

          • 2. Re: Subject  transparent  w/Blue  Screen  ??
            Colin Brougham Level 6

            Keying takes some time to do properly, both during production and in post-production. Definitely don't bother with Blue Screen Key; that's a pretty worthless effect. Use Ultra, and flip into Alpha Channel view to check the purity of your matte. Typically, you want a very high-contrast matte with extreme white and extreme black. Even on a good shot, you'll have to play with it a bit.

             

            A good practice is to put a solid color matte behind your shot to check your matte as you create it. I'll typically use yellow or red, which will quickly highlight problem areas. Also, start with one of the Garbage Matte effects to limit the range of colors you're dealing with. This is essentially like roughly snipping out the parts you know you don't need, so you can further refine the edges of what you do need. On a wide shot, I'm typically only working with 10-20% of the total color field after using a garbage matte.

            • 3. Re: Subject  transparent  w/Blue  Screen  ??
              Todd_Kopriva Level 8

              Colin is right about the value of garbage mattes. Similarly----and perhaps more directly relevant to your issue---you can use a hold-out/hold-back matte. (Yes, the page that I pointed to is for After Effects, but you can do the same thing with Premiere Pro.)

               

              Do you have access to After Effects? After Effects has more features for color keying and compositing. Premiere Pro is fine for simple  keys, but sometimes you need to use a compositing application rather  than an NLE with some compositing features.

              • 4. Re: Subject  transparent  w/Blue  Screen  ??
                Colin Brougham Level 6

                Good point on the hold-back mattes... reminds me to submit a feature request for Ultra to have masks ala Keylight. I've had to do that with people who have intensely-colored irises, or are wearing jewelry of some sort.

                 

                Another trick, sort of akin to hold-back mattes, is to break your key into pieces. I had to do this recently: I had a poorly-lit green screen shot ranging from almost blown out on the screen to underexposed, and the subject was a doctor in a white jacket with blue-green scrubs and foofy Don Johnson hair! It was a nightmare, until I cut off the top of his head

                 

                Well, more specifically, when using the garbage matte effect, I adjust the matte to eliminate the top of the doc's head; this let me duplicate the clip, change the garbage matte so I was working with just the hair, and key that separately. I was able to get much finer control than I would have had I just used a single instance of the clip and the keying effect. By the way, this was all in Premiere with Ultra and the GB matte effects. What can I say? Hardware MPE has spoiled me!

                 

                @ Todd: seems to me you mentioned you were working on tutorial or something that explored the use of multiple instances of a keying effect to get a more refined matte. Am I recalling incorrectly?

                • 5. Re: Subject  transparent  w/Blue  Screen  ??
                  Todd_Kopriva Level 8
                  > @ Todd: seems to me you mentioned you were working on tutorial or something that explored the use of multiple instances of a keying effect to get a more refined matte. Am I recalling incorrectly?

                   

                  The "Color keying" tutorial on this DVD is exactly that:

                  https://www.video2brain.com/en/products-27.htm

                  I show how to use two instances of Keylight---one for the body and one for the head/hair---as well as a garbage matte and a couple of hold-back mattes for the eyes and the buttons on the subject shirt.

                   

                  If you don't want to get the DVD, here is the same course online:

                  http://www.creativeedge.com/9780132616430

                   

                  I try to not use this forum to actively advertise the non-free stuff that I make, so I hadn't mentioned this video before.

                   

                  For even better keying results, I like to use a somewhat more advanced technique that Mark Christiansen describes in his After Effects CS5 Studio Techniques book: He uses a super-tight garbage matte created with either manual rotoscoping or Roto Brush (or even Auto-trace), plus a super-tight hold-out matte, leaving a narrow edge in between; and then the keying effect is only applied to that narrow edge.

                  • 6. Re: Subject  transparent  w/Blue  Screen  ??
                    Colin Brougham Level 6

                    Well, if it eases your mind somewhat, you'll have small bit of royalties coming to you now

                     

                    Actually, this is exactly what I was looking for; I've got a decent handle on basic AE, but there are still parts that break my brain. For example, everytime I try to do something with 3D and comp cameras, I create an awful mess and I just give up. Considering the amount of AE stuff I'm actually doing these days, I need to change that

                     

                    Thanks for the links and references.

                    • 7. Re: Subject  transparent  w/Blue  Screen  ??
                      shooternz Level 6
                      I have tried several exposure techniques on the screen;  over exposing one stop, underexposing one stop, and exposing it exactly the same as the subject in front of it. It appears to work best if it's over exposed by one stop.

                       

                       

                      I have always under exposed by 1.5 to 2 stops and never had issue with any key.

                       

                      Exposure depends a bit on how you measure your exposure.  I use a spot meter in combination with a waveform monitor.

                       

                      An incident light meter may not give you the exposure difference you think you are getting. It's about reflectance