With Kdenlive we can make a third eye appear on an actor’s forehead, using the two-dimensional tracking algorithm integrated into the program. And with a little trick we can perfectly control the position and alignment of the eye.
One of the most common and difficult things to want to insert in your own video is a modification to an actor’s body, typically an object to be pasted. The classic example is a third eye on the forehead, but often we also want to represent tattoos or holograms. The question is: how can you pretend the appearance of an eye on the forehead? We can do it with Kdenlive, and all we need are two video clips. One is obviously the one with a close-up of the object to be glued (in the case, the eye), and the other is the one with the actual scene, that is, the actor who plays. The basic idea is simple, just glue the clip with the eye over the other using rotoscope and chromatic key to cut it as much as possible and a composite transition to resize it and position it correctly on the actor’s forehead. But there is a problem: the actor moves, it would be unnatural to make him stand still only for this effect. So the eye must follow the movement of the actor’s forehead. There are basically two options for doing this: one is to move the glued eye frame by frame, but it is unnerving and boring. The other is to use Kdenlive’s tracking system to follow the actor’s movement. This is obviously the simplest and most effective solution. First, fix a point on the actor’s forehead, for example a square of black adhesive tape, and shoot in this way. The result is a video clip with a very clear point to follow. When importing the clip into Kdenlive you can use the effect Automatic mask to track the point. We don’t really need the effect, and we can also disable it, but it is essential because it takes care of tracking our point and memorizing the information on the movement also for the effects that we will apply later. Now, the apparently simplest solution could be to put a transition directly between the clip with the actor and the one with the eye, making them follow the tracking data. But this would not allow to control well the size and alignment of the eye. The solution is to create a transparent intermediate layer, a green color clip to which a chroma key can be applied works well. The transition Affine is positioned between the actor’s clip and the one that is transparent, applying the tracking data to the transition itself. Thus, it is the transparent level that moves following the actor’s forehead. Then, a transition applies Composite between the clip with the eye and the transparent one, using this transition to adjust the size and position of the eye. The result is that the eye has exactly the dimensions we want and is perfectly positioned where it must be, following the tracking point but without the need to have its own position, as it may not be perfectly centered. As always, you can see the example video at the following address: https://www.youtube.com/watch?v=LqwJr60_g0k
How to shoot the first floor
For the first floor of the eye, we need a particular setting. A telephoto lens can certainly be useful, but not too high, from 50 to 85mm. By using an average closed aperture, generally F / 8, you can get the whole eye in focus, blurring the rest. The lighting, then, must try to erase the shadows as much as possible, otherwise you would notice the presence of the nose and forehead too easily: we must make the skin disappear using a chromatic key, therefore ideally the skin should have the same color in each point. To do this, just use two diffused lights that point to the eye from opposite directions, so as to cancel each other’s shadow.
The basic track
Work begins on the clip containing the actor
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Track
We calculate the position of the tracking point and store the data
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Prepare the eye
We prepare the necessary to make the eye appear more
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Automatic positioning
We use tracking data to automatically place the eye on the forehead
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