GB2307819A - Image processing - Google Patents

Abstract

A telecine machine 2 is used to derive electrical signals from cinematographic film 10. The signals are processed and the processing of the signals provides an indication of the exposure level of the film. The resultant exposure level is preferably used, with a synchronisation signal, in the control of the printing of the film.

Description

IMAGE PROCESSING The present invention relates to image processing and is particularly applicable to the production and shooting of motion picture film.

The shooting and developing of film is a highly artistic process, with lots of adjustments being made along the way.

The variables that contribute to the lightness or darkness of exposed processed film include the following: The exposure speed of the film stock; The age and storage conditions of the film stock; The aperture selected on the film camera; The lighting conditions and/or weather, depending on whether the film is being shot indoors or outdoors; The condition of the film processing bath; and The time spent in the processing bath.

Traditionally, film shot one day is processed overnight and viewed first thing in the morning. The results of this viewing usually generate data for the fine tuning of the exposure of the film for that day.

Traditionally, the exposed film is viewed by projection through a cinema projector onto a screen.

It is often desirable to view the film in a video or television format. This is desired for a number of reasons. One reason is that it is common to shoot negative material, which is analogous to the negatives that one receives with prints shot in a stills camera.

To view negative material, it is necessary to print it onto positive material prior to viewing it. Thus this is an expensive and time consuming extra process, and also a stage adding another degree of variation.

Another reason for wanting to view the film on a television screen is that the material may be expressly intended for television distribution, and therefore this is the best way to judge its quality and content.

To view the film in a television or video format, it is usual to convert the film material into electrical signals using a telecine machine. Such a machine consists of a film transport, together with an optoelectric mechanism for sampling the requisite number of lines to make a television picture. Examples of such machines include the URSA Gold machine, manufactured by Cintel International Limited (formerly Rank Cintel Ltd) in Ware, England, and the FDL90 machine, manufactured by BTS in Darmstadt, Germany.

These machines are made to accept a very wide range of quality of film material, from extremely underexposed to extremely overexposed. This is accomplished by electrical signal processing within the electronics of the telecine. Further signal processing equipment that can affect the visual attributes of the picture include devices referred to as secondary colour correctors.

Whilst it is the primary function of such devices to alter certain colour attributes, they can also affect the overall signal level. One such example of a secondary colour corrector is the DCP manufactured by Pandora International Ltd. in Kent, England. In practice, such a device would be controlled in use by a Pandora POGLE controller also manufactured by Pandora International Ltd.

The electrical signal processing can be accomplished either in the analogue domain, via analogue 'characteristic' changing, with capacitive and inductive components, or preferably with digital lookup tables.

These tables are one-dimensional, and the input value is used as the address at which to look up the data that is used as the output value. This is a standard technique in the design of digital electronic circuitry.

In practice, all professional telecine control is accomplished from programmer/controller devices such as the Pandora POGLE or the Da Vinci RENAISSANCE. Such control signals may include, but are not limited to: Master Gain; Master Lift; Master Gamma; Differential Gain; Differential Lift; Differential Gamma; and Secondary corrections of: Each of the Red, Green or Blue content of each of the Red, Green, Blue, Cyan, Magenta and Yellow.

The control settings in both of the above devices are digital parameters, which are usually not interrogated by the operator, but are internally stored in conjunction with either a scene number or a timecode reference. This allows scene-by-scene programming, as taught in US 4,096,523, by Armand Belmares-Sarabia.

One problem with known systems is that it is possible to produce evenly balanced television images from film that is significantly under or over exposed.

In such cases there is no indication to the filmshooting crew that the exposure is incorrect.

According to a first aspect of the present invention, there is provided a telecine system in which digital signals generated from frames of cinematographic film are processed to provide an indication of the level of exposure of the film. In preferred embodiments the digital signals would be internal control signals from a telecine machine as such or from a secondary colour corrector.

The present invention also extends to a method of processing cinematographic film which may be used with the above-described system. Thus from a second aspect of the present invention, there is provided a method of processing cinematographic film, wherein the film is scanned by a telecine machine to produce video signals corresponding to the images stored on the film; the video signals are viewed as video images; the video signals are processed using correction signals to compensate for incorrect exposure of the film; and an indication of the level of exposure of the film is derived from the correction signals.

In practice these days, the control signals for the various signal processing functions either in the telecine or the secondary colour corrector are digital signals. It is therefore a simple task to one skilled in the art to utilise this signal, and display it either in a window on the telecine controller, or to route this signal to an external device. This external device could be an industry standard Personal Computer (PC) used to log the exposure level of various scenes of the film. This logging could take place together with the industry standard timecode signal, to build up a table of timecode versus exposure level.

One additional use of this signal of timecode versus exposure is to control a film printing machine.

Such a machine is necessary to convert negative film material into positive. This may be accomplished by 'contact printing', where the exposed negative film stock is placed in contact with unexposed positive film stock. This is then illuminated from a light source, through a series of electrically selectable filters to control the exposure level and colour balance.

Thus, according to a third aspect of the present invention, there is provided a system for printing positive cinematographic film from negative cinematographic film, in which a telecine system is used to provide digital signals generated from frames of the negative cinematographic film which signals are dependent upon its exposure, and these signals are processed and used, together with information establishing the positions of the frames with which respective signals are associated, to establish desired exposure of the frames during a subsequent printing of the positive film. In general the positive printing would be done with a standard type of optical film printer, provided with an interface to receive the exposure information.

An optical film printer may be utilised of the type originally manufactured by Bell & Howell Ltd. in the United Kingdom, or other types. The method of controlling such a device may be either a simple method of typing in the frame numbers of the start and end of each scene, and the corresponding red, green and blue filter values for that scene, or the automatic transfer of this data into the film printer via such serial line protocols as RS-232 or parallel protocols as RS-422.

In a further implementation of this system, a procedure may be initiated where the film camera crew shoot an industry standard grey card. During the telecine stage, the telecine transfer operator, when he recognises this card, presses a calibration button.

This process would initiate the telecine controller to disable all signal processing corrections, thus making the telecine become a linear device. Therefore, in this mode, the level of the video signal output from the telecine would produce an exposure index of the film.

Thus, according to a fourth aspect of the present invention, there is provided a method of determining the level of exposure of the frames of a cinematographic film, in which at least one of the frames contains a calibration image, a telecine machine is used to generate video signals from the film, and a calibration step is carried out on the frame containing the calibration image, whereby the video signals produced subsequently from other frames are used to determine the level of exposure.

In yet a further case, the derived video signal may be used in a feedback-type mechanism to control the video parameters on a telecine controller to produce automatic correction of material. In this mode, an operator selects the grey card image, and knows that for optimum reproduction a video level of, say, 200 millivolts is required. Via a feedback and comparator arrangement, the gain and/or lift controls are incremented or decremented until the video level is at the required level. For optimum control of multiple parameters, such as gain, lift, and gamma, a multilevel film image would be required, containing for example a 'white', 'grey', and 'black' steps. Of course, for any calibration step a colour chart or grey scale could be used instead of the grey card, although grey cards are more commonly available.

Thus according to a fifth aspect of the present invention, there is provided a method of automatically controlling a telecine machine wherein digital signals generated from frames of cinematographic film are fed back to the telecine machine to automatically control the generation of said signals, with reference to a calibration signal level.

In the case of the 'POGLE Platinum' telecine controller from Pandora the hardware of the controller may be one of the range of general purpose computer workstations from Silicon Graphics Incorporated, of Mountain View, California, USA. Such models from SGI that are suitable for this purpose are 8-bit graphics versions of the SGI 'Indy' or '02'. Such workstations have 32Mbytes of RAM, and a 1.2 GByte SCSI hard disc.

Software running on these workstations is typically the IRIX version 6.2 Operating system. Programming for applications such as the film exposure calibration may include compilers for the language 'C', and X-Windows functionality, together with the SGI provided 'OpenGL' graphics library.

Using the above workstation, the comparator and feedback process can be carried out digitally, which is preferable to the analog electrical methods. As an example, it may be that a mid grey card, when shot optimally onto photographic film, and processed optimally, should produce a consistent digital level, for example 128, in each of the three output channels (Red, Green and Blue) of a telecine with flat, ie.

linear, characteristics loaded. Now, because of poor lighting, this produces a video signal of 110, 110, 95.

Thus the telecine controller can compare the actual video level with the desired video level. On discovering that the actual is lower than the desired, the telecine controller can alter the gain tables to be downloaded into the telecine, whilst monitoring the video level. It is obvious in this example that the gain in blue needs to be increased more than that in red and green. Typical gain incremental parameters for this example may be +4 in Red, +4 in Green and +11 in Blue.

From these incremental gain parameters on the telecine it is possible to derive equivalent film printer filter values. This equivalence can be either theoretically derived, or derived by experimental calibration. For example, for film that needs +4, +4, +11 correction, values of filter on the film printer that are required to correct the print can be tried out experimentally, and the values that achieve the desired effect may be stored.

A preferred embodiment of the invention will now be described by way of example only and with reference to the accompanying figure.

The figure shows schematically the use of a correction signal and timecode to produce a film exposure list on a scene by scene basis according to the invention.

Referring to the figure, a telecine machine 2 is connected to a T.V. monitor 4, a telecine controller 6 and a film exposure logging system 8.

The telecine 2 is of a type such as the URSA or URSA Gold manufactured by Cintel International Limited in Ware, England or the FDL90 manufactured by BTS in Darmstadt, Germany. The telecine 2 converts images stored on cinematographic film, shown schematically at 10, to electrical video signals by means of optical transducers, not shown. The electrical signals produced, corresponding to the film images, are transmitted along channel 12 to monitor 4 for viewing by a user of the telecine 2 or by a director, for example.

The video signals may be, in addition or as an alternative to their transmission to the monitor 4, transmitted to a storage device, such as a video recorder, for storage on video tape or other such magnetic or optical storage media.

The telecine 2 is controlled by a controller 6 which sends control data, via channel 14, to the telecine 2 and sends synchronisation data in the form of timecode to the telecine 2 via channel 16. The film will generally contain its own synchronisation data or timecode (also known as filmcode or keycode) and the timecode for the entire system may be based on this data. Alternatively, the timecode for the system may be generated in the controller 6 and passed to the telecine 2. In either case, the timecode generates a unique identification value for each frame of the film which is consistent throughout the whole system.

The control signals passed to the telecine 2 from the controller 6 include the following parameters: Master Gain; Master Lift; Master Gamma; Differential Gain; Differential Lift; Differential Gamma; and Secondary corrections of: Each of the Red, Green or Blue content of each of the Red, Green, Blue, Cyan, Magenta and Yellow.

The adjustment of these various parameters by the operator of the controller 6 with reference to the representation of the film image displayed on the monitor 4 produces a resultant electrical image of optimum quality from the image stored on the film 10.

The control data from the controller 6 is also passed to a film exposure logging system 8, via channel 18. The timecode signal from the controller 6 is also passed to the logging system 8, via channel 20. The control data and timecode values for each frame are stored in the logging system 8. To reduce the amount of data stored in the logging system, only the control data and start and end timecode values for a whole scene, as defined by the operator, may be stored. This also considerably simplifies the further processing of the film exposure log.

The exposure log data stored in the logging system 8 is converted, by means of an empirically-derived relationship stored in a look-up table in the logging system 8, into settings for the compensated printing of the film 10. The conversion values in the look-up table are derived by reference to one frame of the film 10 which includes a shot of a standard grey card, such as is commercially available from Kodak. The grey card has predetermined levels of red, blue and green content and thus the control data for this frame provides a calibration of the required correction of the film 10, ie. the degree of parameter correction required to correct the red, blue and green levels of the image of the grey card stored on the film 10 to the known levels is a measure of the degree of under or over exposure of the film 10. Thus, the grey card calibration frame is identified in the exposure log and is used as a base level for the correction levels of the other frames or scenes of the film 10.

The printing data, comprising timecode and settings for printing of each frame, is passed via an appropriate interface and data protocol to a film printer (not shown). Under the control of the printing data, the print of the film is produced with compensation for the sub-optimal exposure levels.

Although there has been described above what is at present considered to be the preferred embodiment of the present invention, it will be understood by those skilled in the art that various modifications of and deviations from the disclosed exemplary system may be made without departing from the spirit or scope of the invention as defined by the appended claims.

Claims (12)

Claims

1. A telecine system in which digital signals generated from frames of cinematographic film are processed to provide an indication of the level of exposure of the film.

2. A system as claimed in claim 1, wherein the digital signals are correction control signals of a telecine machine.

3. A system as claimed in claim 2, wherein the correction control signals are produced by a telecine controller.

4. A system as claimed in any preceding claim, wherein the digital signals are processed to provide printing compensation signals for the reproduction of the film.

5. A system as claimed in any preceding claim, wherein synchronisation signals indicative of a particular frame of said film are associated with said digital signals.

6. A system as claimed in any preceding claim, wherein a calibration image is also provided, by reference to which the exposure level of said film is established.

7. A system for printing positive cinematographic film from negative cinematographic film, in which a telecine system is used to provide digital signals generated from frames of the negative cinematographic film which signals are dependent upon its exposure, and these signals are processed and used together with information establishing the positions of the frames with which respective signals are associated, to establish desired exposure of the frames during a subsequent printing of the positive film.

8. A method of determining the level of exposure of the frames of a cinematographic film, in which at least one of the frames contains a calibration image, a telecine machine is used to generate video signals from the film, and a calibration step is carried out on the frame containing the calibration image, whereby the video signals produced subsequently from other frames are used to determine the level of exposure.

9. A method of automatically controlling a telecine machine wherein digital signals generated from frames of cinematographic film are fed back to the telecine machine to automatically control the generation of said signals, with reference to a calibration signal level.

10. A method of processing cinematographic film, wherein the film is scanned by a telecine machine to produce video signals corresponding to the images stored on the film; the video signals are viewed as video images; the video signals are processed using correction signals to compensate for incorrect exposure of the film; and an indication of the level of exposure of the film is derived from the correction signals.

11. A system for processing cinematographic film substantially as hereinbefore described with reference to the figure.

12. A method of processing cinematographic film substantially as hereinbefore described with reference to the figure.