Before I can talk about the stereo window, firstly, let me explain about the Z plane and how 3D depth is created along its axis. If we look at a gradient depth map, we can see that the foreground is represented by white and the background is represented by black with all the in-between gradients appearing in various shades of grey. In order to use the full Z plane depth (I will use the greyscale values of 0 to 255 as an example) then 0 would be infinity and 255 would be directly in front of your eyes. If we imagine the Z plane running along this axis, then the following diagram can be seen to be true.
With this in mind the Z plane allows us to make use of the full 3D depth that is available along its axis. This is where the positioning of the stereo window is of importance. As an example, if we place the stereo window in the middle of the gradient then we will automatically create a 3D scene that has the potential to not only use the full depth of the Z plane but also display positive parallax (depth) and negative parallax (pop-out) simultaneously.
With everything in position, it is now down to us where we position the camera and objects to create the stereo window. Firstly, I will simply position two characters in front of the camera and the lenses will be aligned to create an imagined stereo window just behind the characters. The stereo window is where the two pairs will converge and be rougly in alignment with each other.
In the above diagram, as the two characters are in front of the stereo window, they will have a slight pop-out quality and roundness to them because they will be affected by a negative parallax. Immediately behind them is the stereo window followed by a deep 3D which should theoretically stretch to infinity. This is my preferred set up for characters standing in front of the camera. Below is an example of this setup from Friday the 13th part 3. The stereo window lies somewhere in between the three foreground characters and the one background character. Not only do the foreground characters have a nice pop and roundness to them but we are also treated to a full 3D depth stretching right back into the distance.
Below is an alternate positioning of the stereo window and characters. In this example the point of convergence (and thus the stereo window) is placed directly in alignment with the characters themselves. Because the stereo window has moved forward, the negative parallax zone has diminished but the positive parallax zone has increased. The result will be that although the scene will have a good depth to it, the foreground characters will look quite flat because they are positioned right at the point of convergence.
This type of stereo window positioning was evident in several scenes throughout Final Destination 5. Here is an example. The two foreground characters look quite flat compared to the three foreground characters in Friday the 13th.
In the next diagram we see the same set up as in diagram 1 but this time showing all the key features of the 3D setup. Convergence is set in the middle (these objects will have their stereo pairs roughly in alignment and will appear flat) and we also have both positive and negative parallax available along the full length of the Z plane.
In the movie Hugo, although several different setups where utilized, I noticed that the Blu-ray 3D was primarily presented using negative parallax only. The stereo window was frequently set right back at the background so that the 3D depth was actually only a forward depth with some coming forward enough to pop out. An example would be during a flashback sequence when the young George Melies was performing magic on stage. There is a long shot, from the stage, looking at the theatre audience applauding. In this shot, the stereo window is set right at the back of the theatre with the audience all being in the negative parallax zone. The following diagram is one example of how this scene might have been set up.
As the back of the theatre was positioned in the stereo window and everything was in negative parallax, although the 3D effect looked very nice, with all the rows of people appearing layered, in essence a set up like this is potentially only using half of the Z plane and thus half of the 3D depth available. Obviously, I am only using 50% as an example as in theory the stereo window could be positioned anywhere along along the Z plane. In this case, if the background wall is set as the stereo window, then we are not seeing any positive parallax at all. That's why I feel this scene, although it looks good, doesn't appear to have that much depth to it. Personally, I would have set the stereo window a few seats back from the front row, which would have created not only some negative parallax, but also a 3D depth that stretched right to the back of the theatre. Although the 3D in Hugo looked very nice and had a pop to it, the overuse of negative parallax meant that it missed out on some 3D depth in many scenes.
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