Post Production

The Daunting Process of Stereoscopic Conversion

When you consider the vast library of films waiting in the studio vaults, it is clear that conversion will be a very important method of producing stereo films in the future. So let us take a look at the daunting process of conversion. Interestingly, it requires a fairly even mix of both 3D and compositing to convert a film. Some conversion processes use the original plate for the left eye and then synthesize the right eye image. Other processes will actually isolate all of the elements in a shot and reconstruct both eyes.

Consider the example in Figure 1. To make the right eye, the boy must be isolated, shifted left, then composited back over the background. You can see the offset in the boy’s position in Figure 2. Of course, to composite the right eye, the background will have to be painted or composited to replace the exposed pixels that were originally covered by the boy. The problem is that the tiniest variation between the pixels of the two backgrounds creates a ghosting effect that can be uncomfortable to the eye. It turns out that it is actually much better to isolate the boy, make one clean background plate, and then composite two slightly offset boys over the same background plate. There are no discrepancies in the background so there are no artifacts. But this is obviously more time-consuming and expensive.

Stereoscopic Conversion Compositing to convert film Figure 1 Stereo left eye

Figure 2 Stereo right eye

In the previous paragraph I glibly said, “the boy must be isolated,” which is obvious if an offset composite is to be made. Unfortunately, this is usually a Herculean task. Consider our boy in Figure 1. To start with, we will need a very tight roto around his outer edge. Since the wispy hairs cannot be rotoscoped, they will either have to be painted or keyed. Keying is a serious problem, because this is obviously not a greenscreen, so it is a very difficult key and painting is both time consuming and difficult to make natural- looking. Keep in mind that this would be considered an easy shot. How about a couple dancing the jitterbug with flying skirts and hair? How about a mob scene? The isolation of objects is one of the most labor-intensive aspects of stereo conversion.

Once the rotos, keys, and painted masks are completed, the next step is to layer the elements in depth. Figure 3 shows a high-angle view of the 3D arrangement to show how the shot is separated into layers. But wait – there’s more. If the layers are simply flat cardboard cutouts, that’s exactly what they will look like when viewed in stereo. Each object must be projected onto a three-dimensional mesh of some kind that can be contoured to give it a natural roundness like the illustration in Figure 4.

3-D Filming Digital Compositing Figure 3 3D depth view

Figure 4 With 3D mesh enabled

The mesh shown here is actually oversimplified for illustration purposes. The real mesh would have much more detail to reflect the different depths of the eyes, nose, neck, and shirt. And, of course, because the character would be moving around, this mesh must also be animated by hand to match – more labor-intensive work.

And we are still not done. A new artist, the “stereographer,” now joins the project to perform “depth grading,” by which the relative depths of the various objects in the scene are dialed in.

Excerpted from Digital Compositing for Film and Video by Steve Wright © 2010 Elsevier Inc. All rights Reserved

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1 Comment
   vidya said on July 24, 2012 at 8:16 am

thanks a lot for this information..but i need lot more information regarding how to match depth map in a video footage frame by frame??

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