H.I.T or Horizontal Image Translation, in a Stereo 3D Scene:

Take a look at the image above (click for bigger). The image on the left half shows the scene captured using a parallel camera rig, and just a little bit of post processing used to adjust depth. The scene itself if viewed on a typical computer monitor or one of the new 3DTVs out there from Samsung, Sony, Panasonic and the rest, will show a nice deep 3D effect, very comfortable to view.

Looking at the imge, the right side version shows a modified version of the overall depth of the scene, using what’s called H.I.T or horizontal image translation.  In simple terms, it involves the stereographer suggesting to the editor or herself, sliding the left and right images horizontally to ‘re-converge’ the 3D image, in the case of the image above, to bring the left-right pair closer so that the house is more easily “fusable” by the audience and in keeping with a so-called ideal “depth budget” for the scene.

Notice the distance between the left and right images (binocular disparity) of the little white house in the left corner. The eyes can “fuse” this image with virtually no problem if this scene is viewed on a typical 3D display as mentioned, however if this were a cinema screen, the distance between the two images would be as expected… quite large! In a feature length 3D movie, if the eyes were to try and fuse such imagery a few number of times, it can overwork the senses and lead to mild headaches or much worse. This is why topics such as “converging the camera” on subject of interest, eye divergence etc. crop up in 3D movie-making.

Eye Convergence and Divergence:

Why do we sometimes get mild headaches when watching 3D movies? For a multitude of reasons, such as Cinematographers “thinking in 2D” and thus using 2D framing, is one example… but in context to this article it can be explained as making the eyes diverge or go “wall eyed” which is not how normal human vision operates. Converging is somewhat okay to an extent, an example of converging eyes is if you lift your index finger close to your nose and focus on it. It’s not so hard, but doing the opposite – diverging the eyes is un-natural and is exactly what would happen if the image above was projected on a cinema screen and someone tried to fuse the left and right images of the house together.

Floating stereo windows:

What is a floating stereo window? The next time you see a really well made stereoscopic 3D movie presented at the Cinema, take of your glasses. Chances are at some point you will notice it… It looks like the entire “screen” is floating a bit off the surface of the physical cinema screen, mostly a little in front, towards the audience. There are now creative tricks being employed to distort the stereo window in all sorts of ways, sloping it towards the audience, tilting it back etc. but that is for another article.

When H.I.T is employed, the edges of the images will not match and need to be cropped off (seen as the extra vertical Cyan colored strip on the left side of the right image).

However, depending on the subject matter of the scene – something else may occur as a result – foreground elements may then “break the stereo window” . The stereo-window being the overall frame of the scene. Also known as a Stereo window violation, in simple terms it would mean any object or person in the 3D scene that is in negative Z-space (seeming to pop out towards the audience, positive Z space is the opposite) AND near the left or right edges of the stereo window, would break the illusion by cutting the edges of the screen. This leads to Retinal Rivalry in the eyes.

A related and good explanation can be found here on creation of a floating stereo window.

The video above when viewed on the main Youtube website, can be seen in 3D with a pair of Anaglyph glasses. If set to anaglyph mode with the youtube 3D dropdown menu, and viewed without the glasses, you will notice half way through the video, the effect of H.I.T being applied. If this were instead, a floating stereo window scenario, this animated change would be what animated floating windows are.

In the video, the decision to change the depth of the scene was taken as a creative choice at the very moment it was “perceived” that the audiences would shift their gaze to the small white house as it came into view. The house becomes more comfortable to fuse, but the overall depth of the scene gets compressed as can be seen from comparing the start of the scene.

Directing a Stereo 3D Scene is part Science, part Art:

There are many questions to be asked when composing a Stereoscopic 3D scene, and no one scientific way to compose the ultimate scene, Some such questions:

• In the video above, there would be excessive divergence on the mountains – However, it is plain to see that there is nothing much of interest on the mountains and all the action is on the lake, the birds and the trees. At one point, the house grabs attention due to the stark contrast it makes with the rest of the nature scene. Would it then be better to animate the depth-budget during that part of the scene?

• Should a stereographer play safe and completely compress the entire scene by following a text-book approach to building the scene by keeping the separation of the mountains to a dictated “safe” percentage?

• What would happen to this scene if the Director wanted to “Converge-on-focus” or toe-in the Cameras on the trees in the foreground…or the birds? – How much divergence would then occur on the Dam and the mountains? How would this be corrected? For argument, if the scene was re-framed to get closer to the birds or the trees, what would the challenges be? Zoom lenses create a card-boarding effect in 3D.

• Would the use of a Stereoscopic 3D calculator help? – Can the scene be shot entirely by numbers? Would it cut down on the richness of the scene in 3D if stereo 3D movies used a connect-the-dots or paint-by-numbers approach?

These are some of the things to take into consideration when creating a visually pleasing and memorable experience in 3D Storytelling. Cinematographers are encouraged to explore Mise en Scene when creating 3D movies, and balance science with art for their next 3D epic.