The masterclass regarding green-screen and composting helped me greatly with my post work as I already had some knowledge of composting and green-screen but over the time forgot the fundamental rules of green screen marking and also the reasons behind the traditional way of marking and composting because before this class I only knew what the standard distance and colour of markers are but never really knew the reason why they are there.
First of all for any green screen object to be tracked during the post work it needs to be perfectly marked and for that following factors are to be kept in mind. First is what color of the markers should be used. Steve told us that why it is always better to use black and white markers as the softwares that we use to track markers find it easier to find these colours due to the contrast but sometimes even this can get tricky as if we are using the black colour it might be hard to extract it when their are hair involved in a shot. Like in our exercise Steve put a marker right behind our actors hair which in a real world scenario you would try to avoid as it can be tedious and time consuming to extract the marker behind the hair due to the same colour but keeping that in mind color markers can also be a problem if the circumstances are not correct for example using a blue marker will require two keys which can be a problem for the wardrobe and props and also some cameras dont work well with such colour transitions and leave behind a visible difference in the keying of the shots.
Another key factor that I learnt was how many markers should be used for example in a moving shot Steve advised to have at least 3 markers making a triangle reason behind this is that even if we move if one of the markers move out of the filming frame another is added and having a triangle shape or a rectangular one allows the camera tracker to track points horizontally and vertically.
Also while putting the markers make sure you have a good knowledge about the positioning cause in our case we were given the challenge to remove the markers from behind the gnome’s head which in an ideal situation should-be been avoided because if you are putting a marker behind an object you will need to rotoscope it and mask it out step by step.
With markers being set the other key factors are the lighting, the camera that is being used and also the distance between the green screen and the object being filmed.
LIGHTING & DISTANCE
First we come to the lighting of the green screen for that we need to make sure that we use to lights to light the green screen at equal distances and get them centered so they can light the green screen evenly because if the light is not being emitted evenly on the green screen it will leave behind shadows which will appear in your backdrop making it difficult to key.
It’s always a good idea to have some distance from the actor and the green screen preferably around 2 meters as this allows you to leave more space for you to set up the light and evenly disturbute it as this will help you avoid any shadow being cast by the actors and also assisting in easier access to setting the green screen lights.
Most importantly once you have eliminated all the shadows and evenly lit the green screen you need to make sure your primary light is in the same direction as the plate that you are going to apply, in our case our primary light for the plate was the sun in a diagnoal direction coming from the window. So we had to replicate it in a way that it matches our plate if we fail to do that you will have lights coming from a direction which might not look natural taking away the realistic feel you are going for.
CAMERA SETTING & Distance of Camera from Green Screen
The other important thing that needs to be considered while filming something on a green screen is the camera. The camera that is being used should either film on 4k or ultra HD reason for this is that any camera that is having a codec that is compressing the footage will cause your green screen footage to have pixellated results which will be shown in the form of small boxes i.e pixels that will make your shot look unappealing to the eye another factor is that filming in 4k allows the post-production process easy as they have more room to play with effects resizing and also framing repositioning after we track the camera.
We need to make sure that the markers are in focus and are easy to track for that we adjust our depth of field and usually use an f-stop of 2.3 with a low gain setting as we don’t want to overexpose our footage or clip anything. We are also required to note down the lens we use for filming as it should be the same when you create a camera in After Effects otherwise your plate would not match your footage.
Secondly, we are also required to measure the distance of the camera and the end of the set and to the end of the green screen, and also the measurement of the camera lens from the ground. So when we get to the Post-production and are setting up the plate we have a reference point which matches your measurement.
POST PRODUCTION PHASE
After you have filmed the shot we take our footage into the software to key out the green screen and mask out the markers and use our desired backdrop, but for that, we need to have a proper understanding on which software can be used usually its one of the following
– Sony Vegas
– After Effects
In our case, we used After Effects, now before we get into the procedure on how to apply the backdrop and removing the green screen I’d like to address the problems that I faced while filming and working on the post with this particular scene. First of all was the distance between the green screen and our set up was not sufficient which caused us some trouble while lighting the set up as it is always safe to have some distance which can help when it comes to the Z axis of the tracking and giving depth to your shot, but more importantly giving you enough room to light your shot.
The other challenging task set by Steve to have a marker right behind the gnomes head now it isn’t a big problem in this case as the shot was fairly simple but having a complicated shot could’ve made this task fairly difficult and time-consuming as it takes time to extract a frame by masking out each frame. So going forward we were told to always make sure of where we position our markers.
On the minor difficulties was while filming the shot we had issues with the dolly which made the shot a bit shaky which going forward is something I would keep in mind as during post work it caused some issues when we applied the backdrop as it made it hard to track the shot and make the background more believable due to an unstable shot
Now when it comes to having your greenscreen removed we use a keying tool which is built in. In all the software that is mentioned above what that does is it extract the color that you choose from the shot and make it transparent. In our case its the color green, We usually extract color blue and green because these two colors are not found in our pigment and can be extracted completely.
Once you apply the keying plug in you have to play around with the whites and blacks on the shot with black showing the parts that are being shown in the color and white as the ones that have been extracted we need to mask out the tracking points while masking them out adjust the feather and the expansion of the mask in a way that it looks as natural as possible after the masking is done for the markers you need to remove any chatter or spill of the greenscreen to give your footage that clean natural look.
For that what i usually do is duplicate the keyed layer and use key cleaner and further using an advanced spill suppressor to get rid of any additional noise by now your footage should have most of the spill and noise removed the next step would be to track the camera what that does is that it track down all the motion analyse it and extract all the 3d movement in the form of keypoints. This 3d data helps you to put a 3d object or a background layer in 3d space over your 2d footage.
In our case we are applying a background to our green plate so by using the data collected by our camera tracker we can create a 3d Camera that allows us to track and parent our background layer to the exact position in 3d space by copying the position values and applying it on our background layer what this does is it creates a movement that is being generated by our tracking camera and apply the metadata on the background to make it move seemlessly with our camera.
Once the plate has been set on to the trackers we need to adjust the size to a place where it pleases the eyes and looks natural once we are happy with the position we can use different effects and fake lights and lens flares to make the added background look more natural in our case we used an adjustment layer and diffused it to give that dreamy look but we can also use optical flares and give that sunshining look and we can also change the blur level of the background for it to blend in with the footage finally once we are happy with the look and have applied a grade to our footage we can add a grain on to our footage and change the blending mood to give our footage that film look.