FIELD OF THE INVENTION

This invention relates generally to a method of producing and displaying a motion picture, and to an apparatus for use in the method. BACKGROUND OF THE INVENTION

Conventional practice in the motion picture art is to record or "shoot" a motion picture at a constant film speed and to then project the film at the same or a related constant speed. For example, scenes are normally filmed at a rate of 24 frames per second and are then projected at 24 frames per second. However, it is also conventional to show each frame twice during projection to avoid objectionable flicker. The resulting image projection rate is 48 images per second, but with only 24 different or unique images per second.

An image projection rate of 48 frames per second is usually sufficient to overcome objectionable flicker but there remain some other objectionable artifacts due to the fact that only 24 unique frames are shown per second. One of these artifacts is an objectionable "doubling" of images when an observer follows an object in motion across the screen. The severity of this artifact depends upon the velocity of the motion depicted in the scene. Cinematographers can usually avoid this artifact by planning their shots to minimize high speed motion within scenes. Another artifact is the blurring of the edges of an object in motion. This is due to the fact that when a scene is filmed at a low frame rate (e.g. 24 frames per second) the shutter in the camera is open for a long enough time for an object in motion to be recorded on the same film frame in different positions resulting in an indefinite position of the object on the film frame. When projected the edges of the object are blurred with the amount of blurring proportional to the speed of the object

in motion.

It is well known in the industry that shooting and projecting film at a higher frame rate can effectively reduce these artifacts resulting in a higher quality projected image. For example, U.S. Patents Nos. 4,447,160 and 4,560,260 (both to Trumbull) describe the "SHOWSCAN" (TM) motion picture process which includes using a frame rate of 60 frames per second to create a motion picture that conveys a vivid impression of realism. In addition to overcoming the motion related artifacts mentioned above, a higher frame rate also reduces the appearance of "grain" or noise in an image through a process of temporal averaging. Although a higher frame rate is desirable, it is often not economically feasible due to the greater amount of film required in. filming and projection. For example, to film and project a motion picture at a frame rate of 48 unique frames per second twice as much film is required compared to the conventional practice of filming at 24 frames per second. In addition to the double cost of film stock, there are other inconveniences such as the reduction by half of the time capacity of camera film magazines and the requirement for larger film projection reels to hold the double amount of film. These disadvantages are especially problematic when using a large format film such as the IMAX (TM) format, in which a single frame is about ten times the size of a conventional 35 mm film frame.

The invention has been devised primarily, though not exclusively, in the context of film projectors which have a film transport mechanism of the type which has become known as the "rolling loop" mechanism. U.S. Patent No. 3,494,524 to Jones discloses the principle of the rolling loop film transport mechanism. A number of improvements in the original Jones mechanism are disclosed in U.S. Patent No. 3,600,073 to Shaw.

In a rolling loop film transport mechanism, the film is transported through a film passage defined between

a curved stator and a rotor. The rotor has gaps in which loops of film are continuously formed as the rotor rotates. The loops are developed as the respective gaps travel from a film inlet location to an aperture. The loops then decay progressively as the gaps travels from the aperture to a driven output sprocket. As each loop passes the aperture, the film is laid onto registration pins at the aperture and an image is projected from a frame which is stationary at the aperture. The next loop lifts the film from the registration pins and positions the next frame for projection.

Each gap in the rotor is provided with an angled plate that acts as a shutter to block light each time the gap passes the aperture. The plate is curved to accommodate the film loops that form in the gap. In a typical rolling loop projector, the rotor has eight gaps and rotates at 180 rp , which translates to a frame rate of 24 fps. However, in order to provide an acceptable flicker rate, each frame is in fact projected twice by providing so-called "flicker shutters" between each two successive primary shutters (in the rotor gaps). This provides an effective image projection rate of 48 fps.

It is technically feasible to build rolling loop projectors that are capable of projecting 48 different frames per second or more. However, as noted previously, twice as much film is used if the projector is run at 48 fps as compared with 24 fps. Also, for many scenes in a typical film, the improved resolution achieved at 48 fps as compared with 24 fps is not perceptible to the average viewer.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to provide an improved method and apparatus for producing and displaying a motion picture, which addresses these considerations.

The method of the invention is applicable to the production and display of a motion picture which includes

a plurality of scenes. The method includes the steps of selecting at least two different film frame rates for recording those scenes, predetermining for each scene one of the selected frame rates and recording each scene on film at the frame rate that was predetermined for that scene. The motion picture is subsequently displayed by projecting the film using at least one projector which is capable of being operated at at least two different projection frame rates corresponding to the frame rates used for recording. The projector is controlled so that the film is continuously projected while varying the film projection rate to cause each scene to be projected at a frame rate related to the frame rate at which the scene was recorded. Normally, the "related" projection frame rate will be the same as the rate at which the scene was shot. However, the projection rates could vary with respect to the shooting rates, for example, by a predetermined constant amount (e.g. 5%). Where special effects such as slow motion or fast motion are required, ^* greater variations are possible.

In accordance with the method of the invention, a fast action scene in a motion picture can be shot at a high frame rate (e.g. 48 fps) while the other scenes are shot at a lower rate (e.g. 24 fps). When the• film is subsequently projected, the fast action scene is projected at the higher frame rate while the lower rate is used for the remaining scenes. In this way, the desirable higher resolution is achieved for the fast action scene and the additional amount of film used corresponds only to the portion of the film for which higher resolution is necessary.

It will of course be understood that the actual ilm on which the scenes are recorded will not be the same piece of film as that which is projected to the ultimate ' audience. Normal film editing printing and copying tech¬ niques will be used to produce a final print for showing.

Also, the term "scene" as used herein is intended to mean a defined episode or sequence within the motion picture. That episode or sequence need not be recorded using the same camera or on a continuous piece of film but will involve a defined continuous segment of the motion picture.

While the method of the invention may be practised using conventional motion picture cameras and projectors, the method is preferably performed in the context of a projector having a rolling loop film transport mechanism. More preferably, the film is large format film of the type available under the trade-marks IMAX and OMNIMAX. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be more clearly understood, reference will now be made to the accompanying drawings which illustrate a particular preferred embodiment of the invention by way of example, and in which: Fig. 1 is a somewhat diagrammatic view as seen from above illustrating a conventional rolling loop motion picture projector;

Fig. 2 is a view generally in the same direction as Fig. 1 but showing only the film path and associated elements of a rolling loop projector in accordance with the present invention;

Fig. 3 comprises views denoted 3(a) and 3(b) illustrating operation of the projector at 48 and 24 fps respectively; Fig. 4 is an enlarged detail view of part of

Fig. 2, showing a film sensor, and Fig. 4(a) is a detail view of part of the film shown in Fig. 4; and.

Figs. 5(a) and 5(b) illustrate segments of two film strips showing representative frames as shot, for example, at 48 and 24 fps respectively. DESCRIPTION OF PREFFJMrar. T ^g UQnTMRWT

Referring first to Fig. 1, the principal

components of a conventional rolling loop film projector comprise a rotor 20 and a stator 22 that together define a gap 24 forming a film passage. Film 26 travels from a supply reel 28 to a take-up reel 30. Film is fed into the gap 24 at a controlled rate by an input sprocket 32 and is withdrawn from the film gap by an output sprocket 34. An aperture block 36 is provided between the two sprockets and has associated therewith a projection lens assembly 38 and, within the rotor, a projection lamphouse 40. Immediately in advance of the aperture block 36 in the direction of film travel is a cam unit 42 for decelerating the film as it approaches film registration pins, (not shown) at the position of the aperture block.

Reference may be made to the '073 Shaw patent discussed previously for full details of the rolling loop film transport mechanism. The disclosure of the Shaw patent is incorporated herein by reference.

For present purposes, it is sufficient to note that, as the rotor 20 rotates, the input sprocket 32 feeds film into successive gaps in the rotor, forming film loops which progressively develop as the film approaches the aperture block 36 and then decay towards the sprocket 34. At the position of the aperture block, successive frames of film are laid onto a lens element (not shown) and the film is registered with the registration pins for projection.

Fig. 3 shows part of the rotor 20. Gaps in the rotor are indicated at 44 and shutters extending across those gaps are denoted 46. In Fig. 3(a), loops 26 of film are shown in each of the rotor gaps 44 while in Fig. 3(b) loops are shown in every alternate gap only.

In a practical embodiment, the rotor may be provided with 16 gaps each with its own individual shutter 46. If loops are allowed to form in each gap as shown in Fig. 3(a) a frame rate of 48 fps is achieved if the rotor rotates at 180 rpm. At the same rotor speed, if loops are allowed to form in every alternate gap only, the frame

rate will be 24 fps.

Fig. 2 illustrates one way in which it is possible to selectively control the film so as to achieve loop formation in every gap or only in every alternate gap. In Fig. 2, like reference numerals have been used to denote parts that correspond with parts shown in the other views. Again, the film travels from a supply reel 28 to a take-up reel 30 via an input sprocket 32 and an output sprocket 34. A diagrammatic representation of a lens assembly 38 indicates the position of the aperture block

36; a cam unit 42 is provided adjacent the aperture block.

In this embodiment, individual electric servo motors 48, 50 and 52 are provided for driving, respectively, the input sprocket 32, the cam unit 42 and the output sprocket 34. Similar servo motors 54 and 56 are also provided for controlling the supply reel 28 and the take-up reel 30. A controller for the servo motors 48, 50, 52, 54 and 56 is shown at 57.

Film "accumulators" generally indicated at 58 and 60 are provided respectively downstream of the supply reel 28 and upstream of the take-up reel 30. While constructional details have not been shown, each accumulator may comprise a set of three idler rolls 58a, 58b and 58c and 60a, 60b and 60c, arranged in a triangular configuration as shown in Fig. 2. The centre •roll is displaceable as indicated by the arrows 62 and 64 respectively to accommodate variations in the amount of film in the respective "runs" between the two reels and the rotor/stator combination. Suitable biassing means are provided for the centre roll to maintain appropriate tension in the film.

A film sensor assembly 66 is provided immediately downstream of the supply reel 28 and is shown in detail in Fig. 4. Referring to that view, the sensor assembly comprises a light source 68 and a photocell 70 positioned so that the light source 68 normally shines through the film onto the photocell except when

interrupted by so-called "witness" marks appropriately positioned on the film. One of those marks is shown at 72 in the form of an opaque square that will interrupt the light beam. The input sprocket 32 is the primary element that controls the frame rate at which the projector operates. The normal operating rate will be, say, 48 fps. Controller 57 is programmed to cause the servo motor 48 to drive input sprocket 32 at a constant rotational speed such that loops will form in each of the rotor gaps 44 as shown in Fig. 3(a) . The other servo motors will be driven at correspondingly related speeds. When a portion of the film is reached that is to be projected at 24 fps, controller 57 will cause the input sprocket 32 to rotate intermittently at a frequency and rate selected to feed film into alternate ones only of the rotor gaps 24 as shown in Fig. 3(b) . The film frame rate will then be half the rate achieved with the arrangement illustrated in Fig. 3(a). It may in fact be possible to control input sprocket 32 sufficiently precisely,that the input sprocket can also be used as a film deceleration means, in which case cam unit 42 may be omitted.

In any event, witness marks (as witness marks 72 — Fig. 4) are provided on the film between portions of the film that were shot at different frame rates. These marks are sensed by sensor 66 which then signals the controller 57 to appropriately change the speeds of the various servo motors and change servo motor 48 from continuous to intermittent operation or vice versa.

Sensor 66 is located a finite distance in advance of the input sprocket 32. Since the film will be travelling at a constant speed immediately before each change in frame rate, controller 57 is programmed to change the frame rate at exactly the appropriate time; that is, as soon as the frames that were shot at the different rate begin to enter the gap between the rotor and the stator. This time lag

is necessary in order to accommodate the inertia in the supply reel 28 and take-up reel 30; controller 57 is programmed to begin changing the speeds of those reels sufficiently before the speed of the input sprocket changes to allow for this inertia.

As an alternative to using witness marks, servo motor control information could be stored in a programmable memory and read out in synchronism with the scenes in the motion picture. Figs. 5(a) and (b) illustrate respectively sections of film that were shot at 48 fps and 24 fps Referring first to Fig. 5(a), three frames are .shown representing a subject (a car) 76 travelling from left to right across the field of view (the film moving from right to left) . In the space of two frames, the subject has moved from a position in which it appears to be entering the frame from the left-hand side to a position in which it has almost reached the right-hand side of. the frame. In the film that was shot at 24 fps (Fig. 5(b)) on the other hand, the subject has moved the same distance in only one frame. The information represented by the intermediate frame in Fig. 5(a) is missing from Fig. 5(b). This illustrates the improved temporal resolution that is achieved at higher frame rates with fast action subjects. Motion blur is also reduced at higher frame rates because, during shooting of the film, each exposure is of shorter duration than at lower frame rates; accordingly a moving object travels less distance and blurring is reduced.

Figs. 5(a) and (b) also illustrate the steps in the method of the invention of selecting two different film frame rates for recording different scenes, predetermining for each scene one of the selected film rates and recording each scene on film at the film frame rate that was predetermined for that scene. For example, in an actual motion picture, the 48 fps frame rate of Fig. 5(a) would be selected for fast action scenes while the 24 fps frame rate of Fig. 5(b) would be selected for slower

scenes. Witness marks (as marks 72) would be placed on the final film print between the segments that were shot at different frame rates and those witness marks would then be used to control the projector in which the film is shown, as described previously in connection with Fig. 2. It should be noted that the preceding description relates to a particular preferred embodiment of the invention and that various modifications may be made within the broad scope of the invention. For example, while reference has been made to two different frame rates, more than two frame rates could be used. Where a higher rate and a lower rate are used, the higher rate may be (but is not essentially) double the lower rate. The specific rates referred to herein are of course examples only and are not limiting.

Where a rolling loop projector is employed, methods other than those disclosed herein may be used for forming film loops or withholding film to prevent loop formation. Further, while the rotor of a rolling loop projector preferably rotates at constant speed and the frame rate is varied by feeding film into all or selected ones only of the gaps, different frame rates could be achieved by varying the rotor speed (and correspondingly varying the film input speed) . In the preferred method, reference has been made specifically to forming loops in every other gap only. However, other frequencies could be used. For example, loops could be injected into every third gap only if a one to three frame rate change was required. The rotor need not have an even number of gaps.

In a specific example of a nine gap rotor having a rotor speed of eight revolutions per second (480 rpm) a high frame rate of 36 frames per second could be achieved by feeding film into every second gap in the rotor and a lower frame rate of 24 frames per second could be achieved by feeding film into every third gap. Film need not be fed into the same gaps on successive revolutions of the

rotor .

Similar techniques to those described previously may also be used in 3-D motion picture projection systems. In that case, two films would normally be shot at rates that would be identical for the two films but that would vary from scene to scene. Similarly, during projection, the films would be projected at the same rates but the rates would vary from scene to scene. Where two projectors are used, they would have to be carefully synchronized to ensure proper projection. Alternatively, a 3-D projector could be used, for example as disclosed in U.S. Patent No. 4,966,454 (Toporkiewicz), the disclosure of which is incorporated herein by reference.

Finally, while the invention has been described in the context of a rolling loop projector, the method may also be practised in association with other projection systems. Whatever the projection system, the film may be fed intermittently through the projector using film feeding means that can be changed instantaneously (say within one frame period) between one intermittent film feeding rate and another such rate.