A Minimal Stereo Rig

As an aid to users who may contemplate creating their own stereo rigs, and so that you may better understand the test footage used for the stereo shots, here is a description of our simplest stereo rig.

It consists of two Panasonic HDC-SD9 CCD cameras rigidly mounted on a metal plate. Accordingly, neither the vergence nor the inter-ocular distance can be changed during a shot.

The cameras record on a small SDHC card at 1920x1080 at 24 fps. Since the cameras are CCD, they do not suffer from the rolling shutter problem that plagues CMOS cameras. The shutter time should be kept short to minimize blur. In-camera stabilization must be turned off. The cameras are not synchronized (they should be), leading to offsets of +/- 1/2 of a frame, which you can see in many of the SynthEyes tracks: the cameras are not "next to" each other.

The HDC-SD9 cameras require that the LCD display be open in order to record, which causes problems for a close-spaced stereo pair, at first glance. The problem is circumvented because the display can be flipped into a position where the subject views the display (instead of the cameraperson), and the display can be folded flat against the camera body with the display showing outwards, and the camera will record successfully. You can see this in the picture below.

The drawback is that you can not access the menus once the rig is assembled.

While shooting, the left viewfinder is the primary guide. Most of the time, we run from battery power, and the cameras' HDMI outputs are not available. If AC power is available, the batteries can be removed, the AC power adapters plugged in, and HDMI outputs can be run to a dual-HDMI-input monitor. This permits more accurate framing.

Due to lack of foresight, the SDHC cards can not be removed once assembled either, this would have been easy to get right by not making the plate extend quite as far to the rear.

During shooting, both camera record buttons are pressed simultaneously. The simultaneous push is surprisingly accurate, but for actual sync a clap is recorded.

In postproduction, trim the clips in Final Cut Pro using the recorded claps as a reference; usually there is an offset of -1, 0, or +1, so once in and out points are determined they can be easily transferred to the other clip. The selected portions are then written to disk.

Since the cameras are not mechanically aligned in the rig, and in fact the cameras have large internal aiming offsets that vary for each camera, shots must be post-calibrated using SynthEyes.

To produce a good stereo effect in the ultimate viewer, a satisfactory field of view must be produced. Unfortunately the cameras have a limited field of view even in their widest position, and it is further compromised by lens and cross-calibration.

We also have two Panasonic wide-angle adaptors for the cameras (available only from specialty importers). These adaptors provide a wider initial field of view, but have additional distortion that must be removed, reducing the range increase. Note: when the adaptors are mounted to the cameras, there is some jiggle to it, they do not mount as rigidly as we would like. If we were shooting in a vibration-prone environment, some countermeasures would doubtless be necessary.

Note that in the stereo tutorials, we tend to want to use footage with as little post-processing work as possible: that means shooting with narrower fields of view than desirable, and without doing the detailed alignment between images. The resulting sequences are worse than would be expected from a commercial rig. As with most quick demos, in reality there would be substantially more cross-checking and cleanup than shown here, in order to produce the best possible results.

Here is first an un-aligned stereo pair from the rig (for red/cyan glasses). It will take your brain several seconds figure it out due to the misalignment. Hint: start by looking at one particular thing in the distance.

Here is an aligned version, you should see the stereo immediately. Not only is quicker better, but this will reduce eyestrain in a full production.

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