2. Mounting Rule

Why should the mounting rule be adhered to?

With stereo projection (and when looking into a hand-held stereo viewer) the spatial image is usually seen behind an apparent window. This window is called the stereo window. This stereo window is actually the spatial image of the stereo frame. (The left image is the left boundary, the right image is the right boundery.) Whether the image scenery can be seen in front of or behind the stereo window depends entirely on the position of the individual photos with each other.

The stereo window itself can be imagined as a solid wall. When the mounting rule is violated, the spatial image is often too far in front of the stereo window. Parts of the picture seem to penetrate or "impinge on" the wall. This "erroneous image" clearly causes reduced enjoyment when looking at spatial images. However, the mounting rule is a can-rule, i.e. it should be kept, but it doesn't have to be kept under all circumstances.

What limits and tolerances are allowed when applying the mounting rule?

The mounting rule is an aesthetic requirement. Stereo images violating this rule can nevertheless be viewed without fatigue. Adherence to the mounting rule causes the entire picture to be seen behind the stereo window. Commercial 3-D movies usually concentrate on spectacular effects rather than aesthetics, therefore violations of the mounting rule are most often found here, as the spatial image is set deliberately in front of the stereo window.

Only objects completely set within the frame and usually located in the foreground part of the scenery - for example, a shot of an elephant's trunk taken frontally - may "violate" the mounting rule and can be set in front of the stereo window (i.e. the trunk is seen coming out of the stereo window). More equally popular examples are: The pump lever over the well whose water stream splashes the audience, the animal stretching its head through the stereo window, the bird or the insect flying in front of the stereo window and others.

How do I avoid breaking the mounting rule?

If you cut the left image on the left side and the right image on the right side,(slides are drawn apart relative to the slide frames), the entire space shifts backwards, in the opposite case forwards. With digital stereo images, the positionof the stereo window usually will be positioned with a mounting software either manually or fully automatically. The optimal window position is achieved, when the spatial image is as far forward as possible, but as far back as necessary according to the mounting rule. Then the nearest point just touches the stereo window. The spatial scenery starts directly behind the window and no space between the stereo window and the near point is wasted. (This type of mounting is often called "near point mounting".) Remember - violations of the mounting rule cannot be corrected during projection.

Mounting rule: Mount only freestanding parts of the picture in front
of the window!

Note: For 3D video with frequently changing motifs and scenes, near point framing can lead to an undesirable disturbance at the far point, in the worst case the farthest point jumps from scene to scene forth and back. To avoid this, notwithstanding the recommendationformulated above, the farthest point will be assigned constant disparity, for example, 1/30 or 1/40 of the picture width. This form of image mounting is called far point mounting. Using this kind of framing, the mounting rule is fullfilled automatically (no stereo window violation!), if the shooting rule was met. For problematic images with a bit too much depth floating stereo windows within the image may help.

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