6 Replies Latest reply on Feb 7, 2016 9:34 AM by Rick Gerard

    How to create an Alpha Channel for studio video with black backdrop

    Kens10 Level 1

      I am a basic after effects user and would like your input.  I have a studio shoot with a female model with a black backdrop.  I would like to use smoke video clip with alpha in post to add behind the subject. How would I achieve this.  I am trying to avoid a green screen as I will be using a fan aimed at the model's hair.  With a small studio I am concerned I would have a difficult time keying the hair.  Any tips on how to? Thanks

        • 1. Re: How to create an Alpha Channel for studio video with black backdrop
          Mylenium Most Valuable Participant

          You can use a Luma Key or channel operations based on Luminance. Check the respective effects like Shift Channels, Channel Combiner and so on. You may of course need multiple such effects with additional Levels and Curves adjustments and then combine them into a matte to be applied to your imagery.

           

          Mylenium

          • 2. Re: How to create an Alpha Channel for studio video with black backdrop
            Rick Gerard Adobe Community Professional & MVP

            Pulling a decent key from a black backdrop is extremely difficult because it's very difficult to make the background perfectly black and it's extremely difficult go get a good key just from luminance values. If you have blowing hair and it's blond Green is also not going to be a good choice. I'd use Blue. A small studio is also going to be tough regardless of the color of the background unless you have a professional video camera that will shoot without compression artifacts and a better color sampling than 4:2:2 or 4:2:0. You also need really good lights and technique so you can minimize the spill and separate the subject from the background.

             

            Without knowing what your model will look like and what she will be waring and what kind of gear you are going to be using it's pretty difficult to suggest the "best" setup. If you are simulating smoke it might be better to rent a smoke machine and just shoot it for real. It is a lot easier to add more layers of smoke if there is already some in the shot. Another possibility would be to do some roto, expand the mask and layer the shot using blend modes and opacity and then supplement the edges with Extract or Linear Color Key. I would shoot some tests and make sure that you are on the right track before committing to the studio session. 

            • 3. Re: How to create an Alpha Channel for studio video with black backdrop
              Kens10 Level 1

              Thanks guys for the input.  I am shooting on a small budget with a Canon 70D dslr, it is 4:2:0.   This is a home studio and have shot successfully decent green screen with the setup before as I use Kino Flo  fluorescents for lighting my backdrop.  However, I have struggled keying the subject's hands when they are in motion as the fingers get blurred and the key is not clean for the hand if they have motion.  Because of that I know the motion with the head and hair will be difficult too.  Why do you suggest to use blue instead of green for chromaky? I inserted a photo of the subject for reference, concerning outfit I can ask her to avoid certain colors such as green or blue. I would like to approach both options and shoot against a black backdrop with a haze machine (fog machines seem to output too cloudy of a smoke)  and as a backup and for learning purposes I want to tackle the green or blue screen chromakey option and use my stock alpha footage of smoke.

               

               

              Hair Smoke.jpg

              • 4. Re: How to create an Alpha Channel for studio video with black backdrop
                Rick Gerard Adobe Community Professional & MVP

                If you can get that good of a black background and you can get some light on her hair and maybe even over expose it a bit you might be close. There's quite a bit of noise in the background. Check the info panel as you move your mouse over the black. I tried two techniques. One used Extract, the other used Difference matte. I created a black solid sampling the color of the background. Then I used the nice but it takes a long time to render Refine Soft Matte at the default settings to get these comps. You'll probably have to do some roto around the face to hold back shadows. Don't be afraid to over expose a bit and set your camera to bring up the shadows. You will probably also have better luck if you transcode the camera original to a 16 bit Image sequence or at least a 10 bit or better production video format that has been carefully graded to open up the shadows as much as possible and remove as much noise as you can from the background. Here's what I get from your JPEGS in a 16 bit comp. Neither of the keying effects I used are 32bit. The Magenta solid was added to show the effectiveness of the key.

                Screen Shot 2016-02-07 at 7.40.34 AM.png

                If you look very carefully around the edge of the face and the ears you'll see the need for some roto. If the hair was blowing around you probably would need some more roto. With your DSLR this is going to require a lot of very careful work on every part of the production.

                • 5. Re: How to create an Alpha Channel for studio video with black backdrop
                  Kens10 Level 1

                  The difference key looks better than the extract, I think the challenge will be even much bigger with a fan aimed at the hair blowing in the air.  I am going to do it live with a haze machine to be sure I get the shot I need.  But will also do the chromakey for test purposes to see how close I can get it to look good in my small home studio restricted environment (studio is 16 feet wide by 18 feet long).  I will be doing close up, medium and medium long shots up to the knees.  Is there an advantage to using either green or blue screens in getting a better key for her hair?

                  • 6. Re: How to create an Alpha Channel for studio video with black backdrop
                    Rick Gerard Adobe Community Professional & MVP

                    Blue is generally better for blone, green for darker hair color. Run some tests. I think extract works best with your footage.

                     

                    You can buy smoke in a spray can. It will probably work better than a smoke machine in a small space.