By Florian Schäfer, P+S Technik
When it comes to the practical part of a stereoscopic 3D (S3D) shoot, one of the first questions on set is ‘How do we calibrate the 3D mirror rig?’
In a perfect world, no calibration would be necessary – the two cameras with sensors, lens mounts and lenses would be absolutely the same. But we all know that nothing is perfect, and just tenths of millimetres of difference in a camera setting can equate to many pixels on the screen. Thus a professional 3D rig requires adjustment mechanisms for calibration purposes. At the end of the calibration, both images should look exactly alike, resulting in a homogenous grey image were they to be displayed using a difference map.
Which rig adjustments can be made?
There are six adjustments that can be made to a rig. This is due to the fact that in a three-dimensional world there can be X, Y and Z rotation and X, Y and Z translation (shift). One camera should be adjusted against the other, so normally, each correction knob only exists for one camera.
The way that adjustments are made is to choose one object in the foreground and one in the background, and calibrate the rig for each object.
The two most popular horizontal adjustments for S3D are known as convergence and interaxial.
For vertical calibration, the adjustment is height and tilt.
Interaxial and convergence are used to calibrate horizontal offset. Height and tilt do the same for vertical offset. Use the angle (convergence or tilt) to correct the offset in the background. Use the shift (interaxial or height) to correct the offset in the foreground. It is very important not to mix this up!
For example, imagine something far away in the background, such as a mountain. A shift of a few millimetres or even centimetres will not affect the positioning of the mountain at all, whereas a foreground one metre away will be affected by such a shift. With convergence or tilt on the other hand, a change in angle will change the positioning of the mountain, no matter how far it is away. So always use the angle functions to correct offsets in the background.
For rotation around the Z axis the adjustment is roll, and it is Z shift for moving the cameras back and forth.
Some people use the Z shift to correct an offset in focal length, but this is not advisable, since a Z shift affects the foreground but not the background. It is not the same as zooming, rather, it will result in a change in the perspective. The result is a difference in scaling between foreground and background, and such an error is very difficult to correct in post, requiring much more sophisticated processing than the simple scaling of one image. It is therefore best to leave both cameras in the same Z position.
If calibration charts are available this can be very useful. One can be clipped to a stand in the foreground, and one to a stand on the background. A simple calibration chart will help to make alignment adjustments, although for shooting in 3D, an additional colour chart may be required.
To begin the calibration process, first check that the cameras can move freely in the complete interaxial range, and then mount the cameras in the rig’s zero position for interaxial, convergence, tilt, height and roll. Place one chart in the foreground, and a second one in the background, with both charts visible at the same time (note that instead of a chart in the background it is also possible to look at horizontal and vertical structures in the background of the scene). Check that both cameras are flipped in the correct position (left-right, top-bottom), and display both cameras in anaglyph, difference or overlay.
Then complete the following steps:
It is always good practice to hand over the best quality source material possible, so while some misalignment problems can be fixed in post, investing in a couple of basic charts and stands, and the relatively little amount of time it takes to calibrate a rig, can save a lot of time and cost in the long run.
Calibrating a 3D rig is a straightforward process, as demonstrated in the following video:
3D Mirror Rig Calibration Tutorial.
Florian Schäfer is the 3D Specialist for P+S Technik, manufacturer of top-end film equipment. P+S Technik also offers a free download version of the calibration chart shown above, including a step by step calibration procedure for mirror rigs.