A METHOD AND APPARATUS FOR MANIPULATING VIDEO IMAGE SIGNALS

Abstract: A system for manipulating video image signals, in which digitized composite video signals (i.e. D2 digitally encoded PAL or NTSC) are converted to component video signals (i.e. luminance and colour difference signals) allowing selected pixels to be modified. In addition, the unconverted composite video signals are also stored and a key signal is generated which indicates which of the converted (i.e. component) pixels have been modified. An output signal is then produced by combining the manipulated signals and the unmanipulated signals in proportions determined by said key signal.

Description: A METHOD AND APPARATUS FOR MANIPULATING VIDEO IMAGE SIGNALS BACKGROUND OF THE INVENTION The present invention relates to manipulating video signals in the form of a composite encoded signals, such as digitized PAL or NTSC etc encoded according to the D2 standard. A video graphics system is disclosed in United States Patent No 4514818, assigned to the present Assignee and included herein as part of the present disclosure, in which a video image is viewed on a monitor by generating an analogue video signal from digitally stored image data. The image data may be modified by the operation of a stylus upon a touch tablet, in which movement of the stylus simulates the operation of a graphic implement such as a paintbrush, chalk or an airbrush. A machine embodying the techniques disclosed in the aforesaid patent is manufactured by the present Assignee and sold under the trade mark "PAINTBOX". Video graphics systems are often used during the post production stages of video making, in which a clip of video is manipulated by the system (frame by frame) so that elements may be added or removed from a live action shot, or animated sequences generated. The original recordings are often in the form of composite signals, such as PAL or NTSC, which are termed composites because they may be transmitted over a single channel, the chrominance signal (hue and saturation) being frequency multiplexed above the luminance signal. However, to allow manipulation by a video graphics system, the frames must be decoded into separate components, with a baseband signal for each of the three primary colours of red, green and blue, or for each of a luminance and two colour difference signals. If so required, the component signals may be re-coded back into composite form after manipulation. A problem which results from decoding between composite video (say PAL) and component video is that clips which undergo this process do not match perfectly with unprocessed clips. A possible solution would be to perform the decoding and re-encoding process on all of the video clips as part of the editing process, however, this is not only time consuming but also adds unecessary degradation to the whole of the production. SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved method and apparatus for manipulating video image signals. In particular it is an object of the invention to allow a decoded composite signal to be manipulated, re-coded and then edited with non-decoded clips while minimising differences between the two clips. According to the present invention there is provided a method of manipulating video image signals, comprising the steps of decoding input composite video signals to give input component signals, storing digitized pixels of said decoded component signals, modifying selected pixels of said stored signals and coding said stored component pixels to produce manipulated composite video signals; characterised by storing said input composite video signals, storing an indication of said modified pixels of said stored component signals, and producing output composite video signals by selecting from said manipulated and re-coded composite video signals or from said stored input composite video signals substantially in response to said stored indication of the modified pixels. DESCRIPTION OF THE PREFERRED EMBODIMENT A system for manipulating video image signals is shown in the accompanying Figure, in which individual frames are played from a first digital video tape recorder DVTRl and recorded on a second similar recorder DVTR2. The recorders sample a conventional composite video signal without demodulating it; in NTSC machines the sampling rate is 14.4 MHz conforming to a recording standard identified as D2. The recorders may form part of a conventional edit suite, in which they would be connected together; a solid line representing such a connection is not shown in the Figure. The system is provided with video graphics apparatus, similar to that manufactured by the present Assignee and sold under the trademark "PAINTBOX". Individual frames are supplied to a component framestore 15, arranged to store a luminance value (Y) and two colour difference signals (U and V) via a decoder 16, which decodes the NTSC or PAL composite signals. The YUV signal conforms to the standard identified as 4:2:2 and is read at video rate under the control of a processor 17, allowing the stored frame to be viewed on a monitor 18. The video graphics apparatus also includes a touch tablet 19 and a stylus 20 so that, in response to operations made by a video artist with the stylus 20 upon the tablet 19, selected pixels stored in the framestore 15 are modified. After each frame has been manipulated, the component signals may be re-encoded by a coder 21 to composite form and recorded on DVTR2. However, the coder 21 cannot perform an exact reciprocal process to that performed by the decoder 16, therefore, frames which have been decoded, stored in store 15 and re-coded to composite form will appear slightly different to those which have been transferred directly to DVTR2 under normal editing operations. The system shown in the Figure, embodying the present invention, overcomes this problem by storing each frame of the image in two forms. Thus, in addition to the decoded version of a frame in component form, the original coded version of that same frame is also stored in composite form. After manipulation, areas of the image frame, stored in component form, will have been modified and these areas are selected and coded to provide an output. However, in areas which have not been modified, signals are obtained from the composite framestore ensuring that these areas match with similar areas in frames which have not been manipulated by the system. The system is provided with a D2 framestore 22, an 8-bit key store 23 and a video selector 24. Each input frame which is converted to YUV for manipulation is also stored in its original coded form in the D2 framestore 22. The video selector is then arranged to select from either the manipulated signals, identified as V m or from the original video input signal, V^, in response to a key signal k. The key signal is a multi-bit signal and, as such, may combine proportions of both the input signal and the manipulated signal. Thus, the output video signal V Q is obtained by the equation shown in the Figure, i.e. the output is given by the product of k and the manipulated video signal V m plus the product of one minus k and the original input video signal v i- The values of k are stored on a pixel by pixel basis and generated in response to operations of the stylus 20. Thus, as YUV signals in store 15 are modified in response to signals from the stylus, key signals are simultaneously generated in the store 23. The generation of a key signal in this way is similar to the generation of a stencil signal, as described in United States Patent No. 4602286 (equivalent to British Patent No. 2113950B) therefore, the key signal is eight bits deep and has soft edges. In some applications, the edges of the key may be too hard and the system may be provided with filtering means for softening edges defined by key signals. Alternatively, key signals may be produced from brush profiles simulating the operation of an air brush, even when other types of brushes are being used on the image. The Figure illustrates an application of the system, in which the original input image consists of a square divided into three triangles. The D2 video input signals for the image frame are written directly to the D2 composite store 22 while, composite store in parallel, they are also written to the component store 15 via decoder 16. An image is now displayed on the monitor 18, by reading the stored component data at video rate and an artist "paints" the image, which, in this example, involves changing the colour of the lower triangle. In response to the operations of the stylus 20 on the touch tablet 19, an eight bit "shadow" of the modified triangle is also created in the key store 23 and, on completing the manipulation, the new image is recorded on DVTR2. For pixels within the modified triangle, the image data read from store 15, and coded by coder 21, is supplied to DVTR2; these pixels having been modified by the artist. However, for pixels outside the shaded triangle, the coded output from coder 21 is ignored and the data is taken directly from store 22, so that these un-manipulated areas cannot be distinguished from images supplied directly from tape to tape. Along the edge of the modified triangle, the key signals have intermediate values, so that the signals supplied to DVTR2 consist of a blend of outputs from coder 21 and framestore 22, ensuring that no visible lines are seen at the transition between the sources. A second monitor 25 is also shown in the figure, arranged to receive the composite output from combiner 24. This monitor shows the actual frame which will be recorded by DVTR2, therefore better results may be obtained if the artist uses this monitor instead of monitor 18.

Claims: CLAIMS 1. A method of manipulating video image signals, comprising the steps of decoding input composite video signals to give input component signals, storing digitized pixels of said decoded component signals, modifying selected pixels of said stored signals and coding said stored component pixels to produce manipulated composite video signals; characterised by storing said input composite video signals, storing an indication of said modified pixels of said stored component signals, and producing output composite video signals by selecting from said manipulated and re-coded composite video signals or from said stored input composite video signals substantially in response to said stored indication of the modified pixels. 2. A method according to claim 1, characterised in that the selected pixels are modified by the operation of a video graphics system, including a monitor, a stylus and a touch tablet. 3. A method according to claim 2, characterised in that said indication of the particular pixels manipulated is obtained in response to the position of said stylus when in contact with said touch tablet. 4. A method according to claim 3, characterised in that the said indication comprises a multi-bit signal having a range of values between two extreme values, wherein a first extreme value indicates that pixels having this value have i not been modified and said output signals are to be selected only from said stored input composite signals, a second extreme value indicates extensive modification to decoded signals and said output signals are to be selected only from the manipulated and re-coded signals and other values within said range indicate that the output signal is to be produced from a mixture of both stored input composite signals and manipulated and re-coded composite signals. 5. Video image manipulation apparatus, comprising a decoder (16) for decoding input composite video signals (PAL, NTSC etc) , storage means (15) for storing digitized pixels of said decoded input signals, modifying means (17, 18, 19, 20) for modifying selected pixels of said stored signals and a coder (21) for re-coding said stored pixels to produce manipulated composite video signals; characterised by a second framestore (22) for storing said input composite video signals, a key store (23) for storing key signals indicating the particular pixels modified by said modifying means, and a video selector (24) for producing output composite video signals, by selecting from said coder (21) or from said second framestore (22) in response to said key signals. 6. Video image manipulation apparatus according to claim 5, characterised in that said means for modifying selected pixels of said decoded and stored signals is a video graphics system including a processor (17) , a monitor (18) , a touch tablet (19) and a stylus (20) . 7. Video image manipulation apparatus according to claim 6, characterised in that said processor (17) includes means for generating said key signals in response to the position of the stylus (20) when in contact with the touch tablet and means for writing the key signals to the key store (23). 8. Video image manipulation apparatus according to claim 5, characterised in that the video selector produces an output from either the modified pixels from the coder (21) , un- odified pixels from the second framestore (22), or a combination of said modified and un-modified signals in response to a multi-bit key signal from said key store (23) . 9. A video editing system comprising means (DVTRl) for replaying digitally encoded composite video signals; means (DVTR2) for recording digitally encoded composite video signals; means (16, 15, 21) for decoding said composite signals to digital luminance and colour difference signals, means for storing a frame of said decoded signals and re-encoding said stored frame to digital composite video signals; means for modifying selected pixels of said stored decoded video signals including a processor (17), a monitor (18) , a touch tablet (19) and a stylus (20) ; characterised by means for storing a frame of said composite video signals corresponding to said stored frame of decoded and manipulated video signals; means for storing key signals indicating modified pixels of said manipulated frame, and a video selector arranged to produce an output signal by combining the manipulated signals and the unmanipulated signals in proportions determined by said key signals. 10. A video editing system according to claim 9, characterised in that a key signal may have, a first value indicating that the corresponding output pixel is to be derived from only the manipulated signals, a second value indicating that the corresponding output pixel is to be derived from the un-manipulated input signals or one of a plurality of intermediate values, wherein the manipulated and unmanipulated pixels are combined in proportions determined by the value of said key signal.

Inventors: CAWLEY ALEXANDER ROBIN, KELLAR PAUL RODERICK NOEL, CAWLEY, ALEXANDER, ROBIN, KELLAR, PAUL, RODERICK, NOEL
Applicants: QUANTEL LTD, QUANTEL LIMITED
Application year: 1989
Main classification: H04N9
IPC codes: G06T11, H04N7, H04N9, H04N11