Файл: ALGORITHM.pdf

with hypertrophy is a common case when the field size of the shot has to be doubled or tripled. It is important to remember that in this case the depth of the comfortably viewed space is reduced and the shot composition depth should be limited by the background beyond which the objects will be displayed with parallaxes exceeding the viewer's eye basis, causing the divergence of visual axes and double imaging.

In selecting the shooting basis we face a contradictory situation. As seen from the equation [5], the reduction of the shooting basis causes the reduction of the main ramp distance. When shooting lengthy compositions with a close foreground, the easiest solution seems to be a reduction of the value of the shooting basis, which is often used in modern 3D film production. This method makes it possible to get closer to the object and to shoot without limiting the background distance. But the reduction of the shooting basis results in the reduction of the stereoscopic effect. It is impossible to increase the depth of the comfortably viewed space by reducing the shooting basis, the same way we adjust the depth of field by using the aperture. This leads to “flattening” of the 3D composition viewed on the screen. The shooting basis value should be determined by the image scale rather than by an arbitrarily assigned depth of the comfortably viewed space. It is to be understood that the limitations of the range of the comfortably viewed space do not stem from the optical flaws of the 3D shooting equipment; they come from the physiology of stereoscopic vision, from the brain’s ability of simultaneous perception of unequally distant objects within a given location range. The DP encounters certain difficulties imposed by the limitations of the comfortably viewed space when he blindly transfers to 3D shooting the techniques of conventional planar picture composition with a prominent foreground set against an infinitely distant background. A long-term practice of Russian 3D film production has shown that shooting with a discrete shooting basis, most often equal to the average one (Bshooting=25 mm), makes the DP more disciplined in building 3D picture composition.

Our shooting experience shows that the formula “the larger is the scale of the object on the screen, the smaller should be the shooting basis” is not always true. For example, in transition from a medium shot to a close-up (especially when a longer-focus lens is used) the shooting should continue with the same average shooting basis. This will help render the natural stereo effect enhancement in zooming in on the object and compensate for the long-focus lens’s “flattening” tendency. Thus, if a medium shot with a following close-up are both shot with a gradual shooting basis reduction, the close up will appear flatter. At the same time, if the shooting is done in a limited indoor space with a wide-angle lens, the lens should be used with caution in zooming in. In this case, it would be preferable to use the shooting basis reduced to 75% of the average.

Please note that the stereoscopic image is sensitive to changes of the lenses and the shooting basis; so, in order to retain the visual unity of spatial compositions, it is recommended to shoot while keeping the same parameter (F and B) values within a unified sequence.

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The larger is the shooting basis, the greater is the angular disparity, and the more effective are binocular factors within the distant areas of the frame's composition. So, in shots of distant landscape-like scenes without foreground objects the shooting basis should be increased.

The shooting basis is a major factor affecting the display of a stereoscopic image. It is not recommended to change it during the shooting of a single shot and even the whole single sequence.

The Algorithm for Parameter Selection in 3d Shooting

In 3D shooting we have the following parameter set: Вview = 65 mm

Р∞screen = 1% of the image width

Wscreen = 6500 mm – the average screen width Bshooting = 25 mm – the average shooting basis

While building the image composition the DP makes a number of consecutive decisions:

•Selecting the scale of the object within the ramp plane

•Selecting the lens with necessary angle and focal distance;

•Determining the size of the shooting basis;

•Determining the location of and distance to the ramp;

•Determining the depth boundaries of the comfortably viewed space.

Photo 2. An example of a foreground composition with the background at infinity

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On the basis of the equations describing the relationships of 3D shooting parameters, cinematographer I. Pomorin compiled an interactive Microsoft Excel spreadsheet which helps the DP to efficiently define the limits of the comfortably viewed space with a given combination of the focal distance and the shooting basis, set in accordance with the artistic goals, or to select such combination using given space limits.

The reference parameters include:

the lens focal distance, F;

the 3D shooting basis, Bshooting

the infinity parallax on the sensor or the film, P∞film.

After entering the reference parameters the chart shows:

the ramp distance, Lramp;

the ramp index, H, equal to the space hypertrophy factor

the maximum allowable distance from the camera to the farthest object, Lbackground, calculated from the equation [7].

the minimum allowable distance from the camera to the nearest object, Lforeground, calculated from the equation [9].

The cells highlighted in blue contain the data unchangeable over the entire filmmaking span. Here, these values include the allowed infinity parallax on the screen (65 mm), the image width on the 3D camera sensor or film (25 mm) and the average screen width (6500 mm).

Having selected the appropriate lens, the DP enters its focal distance and the shooting basis. The chart then shows, line by line, the ranges of the comfortably

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viewed space shot with different hypertrophy indexes (the purple vertical row). The cyan and green vertical rows provide the variants of 3D composition ranges dependent on the hypertrophy index. The bottom horizontal row provides the limits of the comfortably viewed space based on the arbitrary ramp distance.

We installed a specialized electronic calculator into the lens’s remote control, and this calculator, like the above chart, shows the limits of the comfortably viewed space according to the initial 3D shooting parameters. (Photo 5)

. Photo 5

If we analyze the values of the ranges of the comfortably viewed space at different focal distances, we may notice a peculiar pattern within the Russian “Stereo-70” system, stemming from an apt combination of the shooting basis value of 26 mm and the frame width on the matrix of 24,8 mm. Such combination of values makes it possible to define the shooting parameters by a simple arithmetic calculation without any chart and with acceptable accuracy:

The main ramp distance in meters is determined by the first digit in the lens’s focal distance;

The farthest limit of the comfortably viewed space is equal to the main ramp distance with the previous hypertrophy index;

The nearest limit of the comfortably viewed space is equal to half of the ramp distance in use.

Thus, for example, in the “Stereo-70” lens array the main ramp distance values can be approximated as follows:

With these values the farthest limit of the comfortably viewed space is always at infinity

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When shooting with the ramp distance equal to a half of the basic ramp distance (hypertrophy index H=2), the background distance values are set to values of the main ramp distances for given lenses

With each change of the ramp distance by a multiplier the value of the previous ramp distance becomes the foreground distance.

Conclusion:

The above methodology allows the DP to use complex mathematical calculations in his daily work on the set, and now, the image creator can have an action algorithm independent from a 3D expert. Technical aspects of film making are important, but they should not stand in a way of the creative approach to selection of 3D shooting parameters in accordance with the artistic goals, and methods offered here help remove this obstacle.

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