GB2287373A - Electronic video processing system - Google Patents

Abstract

An image processing apparatus 1, 23 comprises a first image store 12 for storing digital image data representing a first image, a second image store 16 for storing generated digital image data representing a generated image, and a stencil store 17 for storing stencil data. A drawing processor 18 is responsive to a stylus/touch tablet device 19 for producing digital stencil data for storage in the stencil store 17. A combiner 14 serves to combine the data from the first and second image stores under the control of stencil data from the stencil store and the combined data is output for display of the combined image represented thereby on a monitor 15. The generated image data may be derived from the first image data or it may be produced by the drawing processor 18 in response to manipulation of the stylus/touch tablet 19. The apparatus further comprises a transportable storage medium 22, such as a magneto-optical disc, for storing the stencil data from the stencil store together with data relating to the generated image, once the user is satisfied with the displayed combined image, for subsequent use in effecting corresponding processing of high quality data in order to produce a high quality version of the combined image. <IMAGE>

Description

AN IMAGE PROCESSING APPARATUS AND METHOD The invention relates to an image processing apparatus and method.

Image processing systems for processing digital video data are known. Such systems generally comprise a high capacity store or stores for storing video data representing up to several minutes of video, together with a user controllable editing facility by which stored video frames may be selected for manipulation and/or combination with other stored video frames in order to create an edited video clip.

In our British Patent Application Nos. 9205503.7 and 9316626.2, the teachings of which are incorporated herein by reference, we describe a video processing system which we manufacture and sell under the trade mark "HENRY". The HENRY system comprises a bulk store for storing data representing one or more video clips, a video disc store for storing video data representing at least one video clip from the bulk store. The HENRY video processing system further comprises an image processor which is operable under user control to edit video data from the video disc store to create data representing an edited video clip which also is stored in the video disc store. The video disc store may be similar to the store described in our aforementioned British Patent Application No. 9205503.7 or it may be similar to the store described in our British Patent Application No. 9226199.9, the teachings of which are incorporated herein by reference. The video disc store has a relatively high storage capacity for such a device, typically 20 GBytes (GB) and is thus able to store data representing up to about fifteen minutes of video for processing by the image processor. The video disc store may include two bidirectional data paths which together enable the video disc store to output and/or receive simultaneously data corresponding to two video clips each at video rate.

The high capacity and high data transfer rates provided by the above discussed video disc store enables on-line editing of lengthy video clips to be effected. That is to say if the initial video is of broadcast quality then the resulting edited video will also be of broadcast quality and can be encoded immediately, i.e. on-line, for broadcast transmission without the need for further processing.

Our aforementioned British Patent Application No.

9316626.3 also describes a second video processing system which we manufacture and sell under the trade mark "MICRO HENRY". In contrast to the HENRY video processing system, the MICRO HENRY video processing system comprises a disc storage device capable of storing around 4 GB of data, a significantly smaller capacity than the previously discussed video disc store. However, the disc storage device is connected to receive and to output data via a compression/ decompression circuit, which may be similar to the circuit described in our British Patent Application No. 9312039.2 the teachings of which are incorporated herein by reference. The compression/decompression circuit serves to compress incoming data to about 1/20 prior to the data being stored in the disc storage device. The use of data compression enables data representing up to about 60 minutes of video to be stored in a 4 GB storage device.

The processing and editing operations performed by the MICRO HENRY image processor cannot be executed on compressed video data and it is therefore necessary to decompress the compressed video data from the disc storage device before the data is output to the image processor. Also, the edited video data output from the processor must be compressed before it is stored in the storage device. If further processing or editing is subsequently to be performed on the edited video data, the same must again be decompressed prior to the processing and then be compressed once again.

The process of compression and decompression a number of times inevitably leads to some degradation in the video image. It is not our intention that our MICRO HENRY system should be used as an on-line editing facility and we therefore market our MICRO HENRY system as an off-line editor in which initial editing decisions are made and editing decision lists are created for subsequent use in an on-line facility.

It is not uncommon during editing for an editor to wish to retouch the image in one or more video frames or to paint or draw further features into the image. Our HENRY editing system therefore includes a drawing processor which is responsive to user manipulation of a stylus/touch tablet device or other user operable input device to simulate the painting and drawing of strokes and lines in the image. The drawing processor may be configured to effect processing in a manner similar to that described in our British patent application published as GB-A2089625 and corresponding US Patent 4514818, the teachings of which are incorporated herein by reference.

Our MICRO HENRY editing system also includes a drawing processor and stylus/touch tablet device by which strokes and lines can be painted and drawing electronically into a video frame. However, the drawing processor in MICRO HENRY is of only limited use in producing a final image of broadcast quality because the data representing the image being edited has been compressed and decompressed and therefore is itself not at broadcast quality. Even if the compressed/decompressed data were to represent an image of adequate quality for broadcast, the recompression of data representing the retouched image would introduce further artifacts into the retouched image which may well be unacceptable. Furthermore, under certain circumstances (for example the drawing of a graphic over a noisy picture such as a crowd scene) the data compression will cause the generation of unwanted artifacts to be most pronounced in the place where they will be most visible (for example in the drawn graphic). Clearly this is unacceptable.

The present invention aims to overcome at least some of the above discussed problems.

According to one aspect of the invention there is provided an image processing apparatus comprising: a first image store for storing digital image data representing a first image; a second image store for storing generated digital image data representing a generated image; a user operable input device; an image processor responsive to user manipulation of the input device for producing digital stencil data representing a stencil identifying data defining a portion or portions of said generated image to be combined with said first image data; a stencilostore for storing said stencil data; a combiner for continuously combining the image data from the first image store and the generated image data from the second image store depending on the stencil data from the stencil store to produce combined data representing a combined image; a monitor for displaying the combined image represented by the combined data to enable a user to view the effect of the manipulation of the user operable input device on the combined image without altering the data in the first image store; and a transportable storage medium for storing the stencil data from the stencil store together with data relating to the generated image data, once the user is satisfied with the combined image displayed on the monitor, for subsequent use in effecting processing of high quality image data providing a high quality representation of said first image in order to obtain image data defining a high quality representation of an image equivalent to the combined image displayed on the monitor.

According to another aspect of the invention there is provided a method of image processing comprising: storing digital image data representing a first image; storing generated digital image data representing a generated image; manipulating a user operable input device; producing digital stencil data representing a stencil identifying data defining a portion or portions of said generated image to be combined with said first image data and storing said stencil data in a stencil store; continuously combining the image data from the first image store and the generated image data from the second image store depending on the stencil data from the stencil store to produce combined data representing a combined image; continuously outputting the combined data to a monitor for display of the combined image, without altering the data in the first image store; and storing the stencil data from the stencil store together with data relating to the generated image data, once a satisfactory combined image is displayed on the monitor, for subsequent use in effecting processing of high quality image data providing a high quality representation of said first image in order to obtain image data defining a high quality representation of an image equivalent to the combined image displayed on the monitor.

The above and further features of the invention are set forth with particularity in the appended claims and together with advantages thereof will become clearer from consideration of the following detailed description of exemplary embodiments of the invention given with reference to the accompanying drawings in which: Figure 1 is a schematic block diagram of a first system embodying the invention; and Figure 2 is a schematic block diagram of a second system embodying the invention.

Turning now to Figure 1 there is shown an image processing system 1 comprising a source 2, which may for example be a video tape recorder, which provides digital data representing one or more video frames which together form one or more video clips. Video data representing each video clip is output from the source in a non-compressed digital component format, for example a format corresponding to the so-called D1 standard. A buffer 3 is connected to the source 2 via a data path 4 having sufficient bandwidth to enable data to be transferred thereover at video rate (e.g.

22 MBytes for a full colour picture according to the standard CCIR 601). The buffer 3 is also connected via two bidirectional busses 5, 6 to a compressor/ decompressor circuit 7, which may be any suitable known circuit or circuits, for example the circuit described in our British Patent Application No.

9312039.2. The compressor/decompressor circuit 7 has an associated SCSI (Small Computer System Interface) interface 8 which is connected via a SCSI bus 9 to a SCSI interface 10 associated with a disc store 11.

Compressed video data is transferred between the compressor/decompressor 7 and the disc store 11 via the SCSI interfaces 8, 10 and bus 9. The disc store 11 has a capacity of about 4 GB which enables the store to store compressed video data representing approximately one hour of video on a frame random access basis. That is to say the disc store 11 is a frame random access store and therefore the data defining any video frame can be selected at random from the disc store without the need to sequence through data relating to other frames during the accessing. The disc store 11 is used to store data from the source 2 defining video clips selected for editing.

The buffer 3 is also connected to an image store 12 having sufficient capacity to store data representing at least one video image frame. As shown in the drawing the image store 12 is formed as three separate parallel storage planes, one for each of the red, green and blue (RGB) colour separations of a full colour video image. The image store 12 may be a separate video framestore or random access store or it may simply be part of a larger random access store (depicted by the broken lines 13 in Figure 1) which provides several different storage areas for storing data representing several frames of video during editing. Our British Patent Application No. 9215949.0 and corresponding US Patent Application No. 08/097950 the teachings of which are incorporated herein by reference, describes a suitable large random access store configuration. The buffer 3 provides an interface between the disc store 11, via the compressor/decompressor 7 and the image store 12 and is used in the transfer of video data between the disc store 11 and the image store 5. Image data from the image store 12 is output via a combiner 14 for display of the image represented thereby on a monitor 15.

The system 1 also comprises a second image store 16 and a stencil store 17 which, like the image store 12, may be separate dedicated framestore devices, separate random access devices or part of a larger random access store 13. The second image store 16 also has sufficient capacity to store data representing at least one video frame and is formed as three separate storage planes connected in parallel, one for each of the red, green and blue colour separations of a full colour video image. of course, the data is not limited to defining the image in terms of RGB colour separation components, and for example may instead define the image in terms of luminance and chrominance components. In this case the two image stores 12, 16 would be configured for storage of luminance and chrominance data. The stencil is a monochrome image and therefore the stencil store 17 comprises a single storage plane.

A drawing processor 18 is connected to receive data from a stylus/touch tablet device 19. In response to user manipulation of the stylus on the touch tablet the stylus/touch tablet device 19 generates XY coordinate data corresponding to the position of the stylus on the touch tablet and pressure data corresponding to the instantaneous pressure applied by the stylus to the touch tablet.

Colour data representing a colour selected by a user by way of a menu (not shown) displayed on the monitor 15 is input to the drawing processor 18 from a colour data register 20. Similarly selected brush data representing the profile of a drawing implement is also input to the drawing processor 18 from a brush store 21.

It will be appreciated by those possessed of the appropriate skills that the system 1 further comprises means (not shown) for controlling modifications and processing applied to video data by the drawing processor 18 during editing and controls the transfer of video clip data between the disc store 11 (via SCSI interfaces 8, 10 and bus 9), the buffer 3 and the image store 12. Also, means would be provided for displaying a menu of options by which a user can control operation of the system. Such controlling means are not shown in the drawing for the sake of clarity.

Initially the second image store 16 and the stencil store 17 contain data representing a black image and an opaque (black) stencil. In response to XY data from the stylus/touch tablet 19 a patch of image data is read from the image store 16, modified by combining with it colour data from the colour register 20 depending on the pressure data from the stylus/touch tablet 19 and the brush profile data from the brush store 21, and is written back to the image store 16 replacing the patch of data previously stored therein. At the same time a corresponding patch of stencil data is read from the stencil store 17, is modified by combining with it data corresponding to the colour white (i.e. a transparent stencil) depending on the pressure data from the stylus/touch tablet 19 and the brush profile data from the brush store 21, and is written back to the stencil store 17 replacing the patch of stencil data previously stored therein. This read-modify-write process is substantially the same as the process described in our aforementioned British Patent Application published as GB-A-2089625, and is repeated at regular spatial or temporal intervals during manipulation of the stylus/touch tablet device. Thus, as data representing a colour image is created over data representing a black background in the second image store 16, corresponding data representing a monochrome image corresponding to the colour image is simultaneously created over data representing a black background in the stencil store 17.

Normally the colour black corresponds to data having a zero value. However, the standard CCIR 601 allows black to be defined by data having any value between zero and sixteen and therefore appropriate offsets should be added between the drawing processor 18 and the two stores 16, 17 if necessary.

In the following it will be assumed that black is represented by the value zero in both the second image store 16 and the stencil store 17. Consequently, the result of manipulation of the stylus on the touch tablet is to create in the second image store 16 data representing a picture Ic comprising foreground image data F created in accordance with keying data P derived from instantaneous pressure data and selected brush profile data (where 0 is S P < 1) over a black background represented by background data B. The picture data Ic can be expressed by the equation Ic = FP+B(1-P). (1) The manipulation of the stylus/touch tablet device also creates in the stencil store 17 data representing a stencil K comprising a transparent (white) portion T keyed in accordance with the keying data P over an opaque (black) background S. The stencil data K can be expressed by the equation K = TP+S(1-P). (2) This arrangement of simulating the painting of a colour onto a black background in the image store 16 and the painting of a white stencil onto a black background in the stencil store 17 facilitates the correction of errors made by the user during the painting operation. If an error is made which the user wishes to correct, simply changing the colour data in the colour register 20 to a value corresponding to black (and using black stencil data instead of white) allows the user to erase the unwanted error. Once black data has been selected further manipulation of the stylus/touch tablet 19 will cause patches of data corresponding to the black of the background to be written to selected areas in the two stores 16, 17 replacing the colour or stencil data previously stored therein.

Concurrently with the creation of image and stencil data in the two stores 16, 17 decompressed image data 1D from the first image store 12 and created image data Ic from the second image store 16 are combined on a pixel-by-pixel basis by the combiner 14 depending on the stencil data K in the stencil store 17 (where 0 < K < 1) to create output image data loud.

In this regard it will be appreciated that the image data held in the first image store 16 need not be decompressed data derived from compressed data, but could instead be data representing an image at broadcast resolution derived from data representing the image at a higher resolution, for example movie film resolution. In this case the source 2 could be a movie film scanner and the buffer 3 would be adapted to reduce the incoming data from the scanner to a resolution suitable for storage in the first image store 12. Creation of image data representing the image in the second store 16 and/or the stencil data representing the stencil in the stencil store 17 would proceed as described previously herein. Once a modified image acceptable to the user had been achieved the aforementioned up-converting technique could be applied to the stencil data and/or image data stored in the MO 22 for subsequent use in a movie film resolution processing apparatus.

The output image data from the combiner 14 can be expressed by the equation Iotrr = IC+ID(l-K). (3) It will be noted that in the above equation (3) of Io= the data from the image store Ic is not weighted by the stencil data K. As is explained in detail in our British Patent Application published as GB-A-2256557 and corresponding US Patent Application No. 07/880,236, the teachings of which are incorporated herein by reference, it is inappropriate to apply the stencil data K to the image data Ic because the image data Ic is already weighted by keying data P (see equation (1)). The keying data P has values which are between 1 and 0 inclusive. Keying data corresponding to the foreground insert F will have a value of 1 and keying data corresponding to the background B will have a value of 0. A zone between the foreground and the background has normalized values which decline from 1 near to the foreground insert towards zero near the background to produce a soft edge between the foreground insert and the background.

Considering the equations (1) and (2) for Ic, and K namely Ic = FP+B(1-P) and K = TP+S(1-P), the backgrounds B and S are both black with a value equal to zero and T has a value equal to unity. Therefore, by substituting these values into the equations (1) and (2) for Ic and K it can be shown that Ic = FP and P = K, and by substituting these equations into the equation (3) for 1our it can be shown that Iow = FP+ID(1-P)- From the foregoing it will be appreciated by those possessed of the necessary skills that the resulting data Iotrr output from the combiner 14 includes foreground data and background data which has only been weighted once by keying data and therefore does not suffer from the aforementioned problem of unwanted artifacts.

The resulting data Iowt from the combiner is input to the monitor 15 for display of the image represented thereby. This enables the user to view the effect of his manipulation of the stylus/touch tablet 19 substantially instantaneously.

During the drawing process the user may select another colour (for example from a menu of colours displayed on the monitor). When another colour is selected the data representative thereof is placed in the colour register 20, replacing the data previously there. Further manipulation of the stylus and touch tablet 19 will cause data representing the new colour to be written to the second store 16 by the drawing processor 18. Creation of the corresponding stencil data continues unaffected.

The system 1 is provided with a magneto-optical (MO) disc store 22 for storing image data from the second image store 16 and the stencil store 17. Once the user is satisfied with the painting or retouching of the image as displayed on the monitor 15, the created image data and the stencil data in the two stores 16, 17 can be stored in the MO disc store 22.

Since the MO disc is a transportable storage medium, storing the created image data and the stencil data thereon enables the data to be transferred to another editing system, for example an on-line editing system, so that the retouching or painting defined by the created image data and stencil data can be applied to respective video frames defined by data to broadcast quality. Keying circuitry suitable for combining the created image data with broadcast quality video data using the created stencil data is described in our aforementioned British Patent Application published as GB-A-2256557.

Whilst in the foregoing the system 1 has been described with reference to data processing simulating the painting or drawing of an image into the second store 16 it will be appreciated that the system 1 can be configured to accommodate other types of image modification. For example in operations such as "wash", where the value of the chrominance in luminance/chrominance data is reduced, and "shade", where the value of the luminance data is reduced, the data for an entire image frame is processed and stored so that the change is applied to the entire image.

This is usually done so that the original image can be modified by substituting or combining portions of original image data with corresponding portions of processed image data, which portions are selected by manipulation of a stylus/touch tablet device. Such operations may for example be displayed as a menu of options on the monitor.

The system 1 can be configured to enable decompressed data for an entire image from the first image store 12 to be processed and the processed decompressed data to be stored in the second image store 16. In this configuration subsequent manipulation of the stylus/touch tablet device 19 will cause stencil data to be created in the stencil store 17 only. Original image data from the first store 12 and processed image data from the second store 16 are combined by the combiner 14 under the control of the stencil data, and the combined data is output to the monitor 15 for display of the image represented thereby. Once the user is satisfied with the modified image as displayed on the monitor 15, the stencil data is output to the MO disc store 22 together with processing data defining the processing that was applied to the decompressed data in order to produce the processed data in the second store 16. There is no need to store the processed data from the second store 16 in the MO store 22 because the processed data is derived from decompressed data and therefore is not of broadcast quality. The processing data that is stored with the stencil data enables the same processing to be applied subsequently to uncompressed data in an on-line system to produce processed data for combination with the video data depending on the stencil data.

Furthermore, whilst the system 1 has been described with reference to images at broadcast resolution it will be appreciated that the system can also be used in the production of higher resolution images. Under these circumstances the image data held in the first image store 16 would not be decompressed data derived from compressed data, but would instead be data representing an image at say ordinary or high definition broadcast resolution derived from data representing the image at a higher resolution, for example movie film resolution.

Figure 2 of the accompanying drawings shows a second system 23 embodying the invention. It will be noted that in many respects the system 23 is similar to the system 1 shown in Figure 1 and that parts which are the same in both systems are identified by the same legends and reference numbers. The following description is directed to those features of the system 23 that are different to or not present in the system 1. Turning now to Figure 2, image data is supplied to the system 23 from a source 2 which in this case could be a movie film scanner for example.

The source 2 provides high resolution data which is input to a down converter 24 adapted to reduce the incoming data from the source to a resolution suitable for storage in the first image store 12. Many techniques are known for reducing the resolution of image data and are suitable for use by the down converter 24.

Creation of image data representing the image in the second store 16 and/or the stencil data representing the stencil in the stencil store 17 in the system 23 is the same as described previously herein in respect of the system 1 and enables modifications to be effected to an image displayed on the monitor by creating image data in the second store 16 and combining the same with the data in the first store 12 under the control of stencil data as it is created in the stencil store 17. Once the user is satisfied with the modifications made to the displayed image, the stencil data from the stencil store 17 is stored on the MO 22 together with either image data created in the second image store 16 or processing data identifying the manner in which processed data in the second image store 16 was derived from the data in the first image store 12.

In our British Patent Application published as GB-A-2235856 and corresponding US Patent 5142616, the teachings of which are incorporated herein by reference, we describe the modification of a high resolution image using a stencil created at a low resolution and then up-converted to the high resolution. This up-converting technique can be applied to the stencil data and the created image data stored on the MO 22 when the data is required subsequently for effecting corresponding modifications to the original high resolution data.

Having thus described the present invention by reference to preferred embodiments it is to be well understood that the embodiments in question are exemplary only and that modifications and variations such as will occur to those possessed of appropriate knowledge and skills may be made without departure from the spirit and scope of the invention as set forth in the appended claims and equivalents thereof.

Claims (32)

CLAIMS:

1. An image processing apparatus comprising: a first image store for storing digital image data representing a first image; a second image store for storing generated digital image data representing a generated image; a user operable input device; an image processor responsive to user manipulation of the input device for producing digital stencil data representing a stencil identifying data defining a portion or portions of said generated image to be combined with said first image data; a stencil store for storing said stencil data; a combiner for continuously combining the image data from the first image store and the generated image data from the second image store depending on the stencil data from the stencil store to produce combined data representing a combined image; a monitor for displaying the combined image represented by the combined data to enable a user to view the effect of the manipulation of the user operable input device on the combined image without altering the data in the first image store; and a transportable storage medium for storing the stencil data from the stencil store together with data relating to the generated image data, once the user is satisfied with the combined image displayed on the monitor, for subsequent use in effecting processing of high quality image data providing a high quality representation of said first image in order to obtain image data defining a high quality representation of an image equivalent to the combined image displayed on the monitor.

2. An image processing apparatus as claimed in claim 1, further comprising: a source of high quality image data providing said high quality representation of said first image; and deriving means for deriving from said high quality image data low quality image data defining a low quality representation of said first image for use as said image data.

3. An apparatus as claimed in claim 2, wherein the deriving means comprising a compressor for compressing said high quality data for storage as compressed data in a compressed data store and a decompress or for decompressing said compressed data to produce decompressed data for use as said low quality image data.

4. An apparatus as claimed in claim 3, further comprising a frame random access store for storing compressed data representing at least one image.

5. An apparatus as claimed in claim 2, wherein the deriving means comprises a down converter for reducing the resolution of said high quality data to produce low resolution data for use as said low quality image data.

6. An apparatus as claimed in any preceding claim, wherein the stencil store initially contains data representing an opaque stencil and the processor is operable in response to said manipulation of the input device to read a patch of data from the stencil store, to modify the read data with data representing a transparent stencil thereby creating a patch of modified stencil data, and to write the patch of modified stencil data to the stencil store replacing the data previously stored therein.

7. An apparatus as claimed in claim 6, wherein the stencil data is created according to the algorithm K=TP+S(1-P), as defined hereinabove.

8. An apparatus as claimed in any preceding claim, wherein the generated image data is produced by the image processor in response to said user manipulation of the input device and represents an image having a correspondence with the image represented by the stencil data.

9. An apparatus as claimed in claim 8, wherein the data relating to the generated image stored in the transportable storage medium comprises the generated image data.

10. An apparatus as claimed in claim 8 or 9, further comprising a colour register for storing data representing a user selected colour, and wherein the second image store initially contains data representing a black image and the processor is operable in response to said manipulation of the input device to read a patch of image data from the second image store, to modify the read data with the selected colour data thereby creating a patch of modified image data, and to write the patch of modified image data to the second image store replacing the data previously stored therein.

11. An apparatus as claimed in claim 10, wherein the image data in the second store is created according to the algorithm IC=FP+B(l-P), as defined hereinabove.

12. An apparatus as claimed in any of claims 1 to 7, wherein the generated image data is produced by the image processor from the image data in the first image store and represents a processed version of substantially the entire first image.

13. An apparatus as claimed in claim 12, wherein the data relating to the generated image stored in the transportable storage medium comprises processing data defining the processing applied by the image processor to produce the generated image data.

14. An apparatus as claimed in any preceding claim, wherein the first image store, the second image store and the stencil store comprise separate distinct storage areas in a large random access storage device.

15. An apparatus as claimed in any preceding claim, wherein the user operable input device comprises a stylus and touch tablet device.

16. An apparatus as claimed in any preceding claim, wherein the transportable storage medium comprises a magneto-optical disc.

17. An apparatus as claimed in any preceding claim, wherein the combined data is produced by the combiner according to the algorithm Io==IC+ID(l-K)r as defined hereinabove.

18. A method of image processing comprising: storing digital image data representing a first image; storing generated digital image data representing a generated image; manipulating a user operable input device; producing digital stencil data representing a stencil identifying data defining a portion or portions of said generated image to be combined with said first image data and storing said stencil data in a stencil store; continuously combining the image data from the first image store and the generated image data from the second image store depending on the stencil data from the stencil store to produce combined data representing a combined image; continuously outputting the combined data to a monitor for display of the combined image, without altering the data in the first image store; and storing the stencil data from the stencil store together with data relating to the generated image data, once a satisfactory combined image is displayed on the monitor, for subsequent use in effecting processing of high quality image data providing a high quality representation of said first image in order to obtain image data defining a high quality representation of an image equivalent to the combined image displayed on the monitor.

19. A method as claimed in claim 18, further comprising: providing high quality image data comprising said high quality representation of said first image; and deriving from said high quality image data low quality image data defining a low quality representation of said first image for use as said image data.

20. An apparatus as claimed in claim 19, wherein the deriving comprises compressing said high quality data for storage as compressed data in a compressed data store and decompressing said compressed data to produce decompressed data for use as said low quality image data.

21. A method as claimed in claim 20, further comprising storing the compressed data in a frame random access store.

22. A method as claimed in claim 19, wherein the deriving comprises reducing the resolution of said high quality data to produce low resolution data for use as said low quality image data.

23. A method as claimed in any of claims 18 to 22, further comprising initially storing in the stencil store data representing an opaque stencil, and wherein in response to said manipulation of the input device a patch of data is read from the stencil store and is modified with data representing a transparent stencil thereby creating a patch of modified stencil data which is written to the stencil store replacing the data previously stored therein.

24. A method as claimed in claim 23, wherein the stencil data is created according to the algorithm K=TP+S(1-P), as defined hereinabove.

25. A method as claimed in any of claims 18 to 24, wherein the generated image data is produced in response to said manipulation of the input device and represents an image having a correspondence with the image represented by the stencil data.

26. A method as claimed in claim 25, wherein the data relating to the generated image stored in the transportable storage medium comprises the generated image data.

27. A method as claimed in claim 25 or 26, further comprising storing data representing a user selected colour and initially storing in the second image store data representing a black image, and wherein in response to said manipulation of the input device a patch of image data is read from the second image store and modified with the selected colour data thereby creating a patch of modified image data which is written to the second image store replacing the data previously stored therein.

28. A method as claimed in claim 27, wherein the image data in the second store is created according to the algorithm IC=FP+B(l-P), as defined hereinabove.

29. A method as claimed in any of claims 18 to 24, further comprising producing the generated image data from the image data in the first image store so that the generated image data represents a processed version of substantially the entire first image.

30. A method as claimed in claim 29, wherein the data relating to the generated image stored in the transportable storage medium comprises processing data defining the processing applied to produce the generated image data.

31. A method as claimed in any of claims 18 to 30, wherein the combined data is produced according to the algorithm Iotn=IC+ID(lK), as defined hereinabove.

32. A method or apparatus substantially as described herein with reference to the accompanying drawings.