IMPROVEMENTS RELATING TO VIDEO EDITING SYSTEMS

Abstract: A system for combining two or more video signal inputs using a chroma key like system. The keying is not done to one colours, like saturated blue, but to any colour within a range. One of the video signal inputs 1 is frozen and appears as a still picture on a display 6. Graphic means (16) are used to define an area of the picture and the maximum and minimum values of the colour components of the signals in that area are detected. Whenever a signal with components falling within the detected range is present in a switch (14) it operates to select a channel for output. this works to build up a composite picture.

Description: This invention relates to video editing systems, and especially to such systems which utilise so called chroma keying for combining video signals representing different pictures to produce a composite picture. These systems are frequently used for adding background scenery to a picture of actors, or for introducing illustrations behind a newsreader or lecturer. Such systems comprise at least two input channels for video signals representing two different pictures, an output channel and switch means for selectively switching the video signals from one or the other of the input channels to the output channel. It is usually arranged that the signals in one channel represent the subjects which are to appear in the foreground in the composite picture set against a uniform background say of saturated blue at a uniform luminance. The switch means is responsive to these signals in such a way that when the signals representing foreground subjects are present the switch connects the respective input channel to the output channel, but when signals representing the saturated blue are present, the switch connects the input channel carrying the background illustration with the output channel. The signals representing the saturated blue of uniform luminance are called chroma key signals. Indeed GB-A-2,086,180 discloses a video editing system in which at least two input video signals representing respective video images are selectively combined to produce an output signal representing a composite video image, the system comprising a video signal switching means for switching between the said video signals to produce said output signal in dependence on a predetermined characteristic of one of the input video signals. One of the problems with this type of system is that the chroma key signal, though intended to be of uniform chrominance and luminance, is liable to exhibit unwanted variation and so the keying of the scene may become erratic. The variations in the chroma key signal can be caused by a combination of factors such as uneven lighting within the studio and also because of noise. Current systems try to overcome this problem by viewing the video sequence and manually varying the threshold levels at which the switching circuit operates until the best result is obtained. This method is obviously time consuming and does not always give satisfactory results. The aim of the present invention is to produce a chroma keying or other similar editing system in which the switching is less susceptible to error due to unwanted variation in the keying signals. According to one aspect of the present invention there is provided a video editing system in which at least two input video signals representing respective video images are selectively combined to produce an output signal representing a composite video image, the system comprising a video signal switching means for switching between the said video signals to produce said output signal in dependence on predetermined colour values of a first one of the input video signals, characterised in that the system further comprises operator controlled means for designating a plurality of pixels within an area of the image represented by said first video signal detecting means for detecting the colour values of said designated pixels and for deriving a keying range of colour values, the switching means being arranged to switch between said video signals when a pixel in said first video signal has colour values in said keying range. The system may further comprise frame freezing means for freezing a frame of said first one of the video signals for display; and said operator controlled means may comprise graphics signal means for creating a graphics signal for the designating of said plurality of pixels; and address generating means for generating address signals for use in applying said designated plurality of pixels to said detecting means and may further include a touch tablet and stylus means. The colour may be represented by three colour components the signals detected by representing say a range of luminance values and a range of two colour difference values. The three dimensions may alternatively be values of R, G, B or other colour components. The switching means may be operative to perform the selective switching in response to the presence of the signals representing all three colour components within the respective ranges of the colour components. According to another aspect of the invention there is provided a method of video editing comprising the steps of: providing at least two input video signals representing respective video images; and selectively combining the video signals to produce a composite video image by switching between the video signals in dependence on a predetermined characteristic of one of the input video signals; the method being characterised by the steps of: designating a plurality of pixels within an area of the image represented by said first video signal; detecting the colour values of said designated pixels and deriving a keying range of colour values; and switching between the video signals when a pixel in said first video signal has colour values in said keying range. 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 an exemplary embodiment of the invention given with reference to the accompanying drawings. In the drawings: Figures 1a to 1c shows one example of the input and output video signals for which this invention can be used; and Figure 2 shows a block diagram of an exemplary system embodying the invention. Referring now to the drawings, the scene shown in Fig. 1a and 1b represent an example of the type of situation for which the invention is useful. Fig. 1a shows a subject, which is to appear in the foreground of the composite picture, behind which is the chroma key forming a background of uniform colour, say saturated blue. Fig. 1b shows the background illustration which in the composite picture Fig. 1c can be seen as replacing the areas of chroma key. As will become clearer from the description that follows, in the system shown in Figure 2, a frame of video signals from one input channel, in this case the ones representing the scene in Fig. 1a, is frozen and displayed. This enables the operator to view the picture and decide which area within the chroma key is to be used to define the range of the predetermined characteristic to be used in a switching means. Once this area has been analysed and the range defined the switching means operates to combine the two input channels wherever signals representing components falling within the detected range are present. An exemplary system embodying the invention is shown in Figure 2 of the accompanying drawings. In Figure 2, references 1 and 2 represent two sources of colour video signals in television format and it will be assumed that the source 1 provides signals representing the subjects to be seen in the foreground of the composite picture, while source 2 provides signals representing the background for said subjects. Thus source 1 may be assumed to provide signals representing a picture such as shown in Fig. 1a while source 2 likewise provides signals representing a picture such as shown in Fig. 1b. The signals from each source are arranged to be in digital form, each pixel comprising say eight binary bits, and each signal comprising three components at base band. The components are denoted as Y, U, V namely luminance and two colour difference signals; the colour difference signals are for example R-Y and B-Y or some other combination of the primary colour signals. The signals from the two sources 1 and 2 are fed via two channels 21 and 22 to a switch 14, each channel in this case comprising three parallel paths each eight bits wide for carrying respectively the three components Y, U, V. All the video signal channels represented in the drawings by a single line are of this form. Channel 21 branches to a frame store 5 which forms part of a frame freezing arrangement, another part of this frame freezing arrangement being a frame selector 4. In order to produce a still picture the frame selector 4 is operated and this causes a frame of video signals from channel 21, and thus from input 1, to be stored in the frame store 5. The contents of this frame store 5 are then fed repeatedly to a display device 6 so that a still picture from the sequence of video signals input at 1 can be seen thereon. The still picture will be of the form of Fig. 1a. The two inputs 1 and 2 and the frame freezing arrangement may be part of a video editing system such as that disclosed in UK Patent Application 8506652 published 9 October 1985 as GB-A-2,156,627 and claiming priority from UK Application 8,408,113. Once a still picture has been obtained in this way an operator controlled means 16 can be used to mark an outline identifying an area, for example the area referenced 24 in figure 1a, on the picture as displayed on the display device 6. The area enclosed within the outline is the area which is to be used in a detector 13 to detect the range of the chroma key characteristic. The operator controlled means 16 consists of a touch tablet and stylus means 7, an address generator 8, a frame store 9 and a graphics signal source 10. The graphic signal generator 10 produces a signal which initially is stored in the frame store 9 in response to the address generator 8 and then is combined with the signals from frame store 5 in a combiner 11, the resultant signal being continuously displayed on the display device 6. The combiner 11 may be in the form of an inhibitor which inhibits signals from the frame store 5 at the addresses where there are video graphics signals in the frame store 9, thus producing a "black" point for each point identified by the touch tablet and stylus combination 7 as the stylus is moved to define the area to be used to detect the video key characteristic. In this way the graphics signal generator 10 produces a signal which represents the outline of the identified area. The addresses in frame store 9 at which the graphic signals are to be stored are generated by the address generator 8 in response to signals from the touch tablet and stylus means 7. The touch tablet 7 generates signals representative of the co-ordinates of a point where the stylus has touched the tablet and these signals are converted by the address generator 8 to address signals for the frame store 9. The area defining arrangement 16 could be part of a system such as described in UK Patent Application 8136539 published as GB-A-2,089,625. When the area has been defined a signal selector 12 selects video signals from within this area from the frame store 5. The signal selector 12 selects the signals in response to the addresses generated in address generator 8 by blocking the signals from outside the defined area. The selected signals are then input into the detector 13 which detects the range of Y, U, V present in order to define a volume in colour space which is to be used as the chroma key. This information is stored in a store 25 to be used by the switch 14. The Y, U, V values of the video signals in channel 21 are analysed in the switch 14 and whenever all three values fall within the defined range the switch 14 operates to connect to channel 22. At other times the switch connects to channel 21. The analyser may be a simple window circuit to detect whether the incoming Y, U, V values are greater than the minimum Y, U, V values detected and less than the maximum detected values. Other criteria may however be used for operating the switch 14. The switch 14 outputs the signals from the selected input to display device 6.

Claims: A video editing system in which at least two input video signals (1, 2) representing respective video images are selectively combined to produce an output signal representing a composite video image, the system comprising a video signal switching means (14) for switching between the said video signals (1, 2) to produce said output signal in dependence on predetermined colour values of a first one of the input video signals, characterised in that the system further comprises operator controlled means (16) for designating a plurality of pixels within an area of the image represented by said first video signal,detecting means (13) for detecting the colour values of said designated pixels and for deriving a keying range of colour values, the switching means (14) being arranged to switch between said video signals when a pixel in said first video signal has colour values in said keying range. A system as claimed in Claim 1, characterised by frame freezing means (4, 5) for freezing a frame of said first one of the video signals for display; and in that said operator controlled means (16) comprises graphics signal means (10) for creating a graphics signal for the designating of said plurality of pixels; and address generating means (8) for generating address signals for use in applying said designated plurality of pixels to said detecting means (13). A system as claimed in Claim 1 or 2, characterised in that said operator controlled means (16) comprises a touch tablet and stylus means (7). A system as claimed in any preceding claim, wherein said colour is represented by three colour components (Y, U, V), characterised in that said switching means (14) is operative when signals representing the three colour components are within respective ranges for each component of the detected range of colour. A system as claimed in any preceding claim, characterised in that the system further comprises a display means (6) for displaying video images. A method of video editing comprising the steps of: providing at least two input video signals representing respective video images; and selectively combining the video signals to produce a composite video image by switching between the video signals in dependence on a predetermined characteristic of one of the input video signals; the method being characterised by the steps of: designating a plurality of pixels within an area of the image represented by said first video signal; detecting the colour values of said designated pixels and deriving a keying range of colour values; and switching between the video signals when a pixel in said first video signal has colour values in said keying range. A method as claimed in claim 6, characterised by the steps of freezing a frame of said one of the video signals for display; and identifying pixels in the displayed image as the designated pixels.

Inventors: CAWLEY, ROBIN ALEXANDER, PRATT, RODERICK JERMYN
Applicants: QUANTEL LIMITED
Application year: 1985
Main classification: H04N9
IPC codes: H04N9, H04N5