STORAGE OF DIGITAL IMAGE COMPOSITION CONTROL SIGNAL

Abstract: A digital video signal and a control plane are stored together by replacing (20) a data word in the video signal (Y, U, V) with a predetermined combination (24) of bits in response to a bit or bits (22) in a corresponding pixel of the control plane (Fig. 5). An image composition system receives the thus combined video signal and control plane and regenerates (2, 6) the control plane data from the predetermined combination of bits (Fig. 2). The control plane can then be used to combine the video image with a second video image.

Description: Storage of digital image composition control signal This invention relates to digital video image composition systems and in particular to those which use separate "control" planes to control the composition of two or more video images. One digital video image ccaiposition system is described in British patent no. 2113950. In this a multi-bit per pixel control plane is used to control the composition of two images. The value in the control plane for a pixel determines the proportions of the corresponding pixels in the two images which will be used for the final σαπposed image. Thus the proportions of the two images can be varied by varying the values in the control plane. This variation enables the boundary between two images to be soft-edged or anti-aliased by varying the control plane values across a number of pixels crossing the boundary. In video production systems the control plane values will be used to control video mixers (or eyers). The main problem with the use of these control planes is the fact that they require additional storage space. If, for example, an 8-bit per pixel control plane is used a considerable increase in memory is required. Also, when the video signals are supplied with their control planes for real-time image composition, it is necessary to transmit at a higher bit-rate. One object of the present invention is to eliminate or at least reduce the need for additional storage space and higher bit-rate transmissions in compositing images. The invention is defined in the appended claims to which reference should now be made. The invention will now be described in more detail by way of example with reference to the drawings in which: Figure 1 shows a prior art storage arrangement for a YUV video image and a 3 bit control plane; Figure 2 shows a circuit for decoding a video signal supplied in accordance with a first embodiment of the invention; Figure 3 shows the storage structure of a YUV video image and a control plane in a second ernbodi ent of invention; Figure 4 shows the transmission sequence of a YUV signal and a control plane in 4:2:2 format in the second embodiment of the invention; and Figure 5 shows a circuit for encoding a video signal in accordance with the first embodiment at the invention. A YUV digital video signal is conventially stored with n bits for each of the luminence (Y) and chrominance (U and V) components. Such a signal is shown in Figure 1 with 8-bits being used for the storage of each of the Y, U, and V components. This data is used to represent the video signal for one pixel. In addition to the Y, U, and V components for each pixel the video signal considered here has a 3 bit control plane associated with it. This is shown in Figure la as an addtional 3 bits of data storage per pixel. Each of the Y, U, and V corrponents of the signal has a binary level in the range 00000000 to 11111111 equivalent to decimal 0 to 255 giving a total of 256 levels. A first embodiment of the invention includes the 3 control plane bits within the 24 bit YUV data word by assigning one of the levels of the Y, U, and V components to represent a control plane bit. For example, the all I's state of each 8 bit component could be used to represent a control plane bit. Three control plane bits, one in each component, will give a total of 8 control plane values. However, the presence of each control bit in a word comprising Y, U, and V values for a pixel will lead to at least one and possibly all three video corrponents for that pixel being lost. Thus, when the video signal is keyed into a background video signal in accordance with the control plane values, the video components in some of the pixels will have to be interpolated from neighbouring pixels. In order to π___n__mise the loss of resolution caused by the loss of the video cαπponents in pixels which have control plane values, the control plane values are only transmitted at the boundaries of an object to be keyed into the background. The area within the boundary can then be filled in without any loss of resolution wit_hin the boundary. The boundary transition from background to foreground can take place over a single pixel or a number of pixels. A mixer receiving the keying signals can be arranged to fill in the area inside the toundary with full opacity for the keyed in foreground or with transparency. A circuit for extracting control plane data from an incxar ng video signal is shown in Figure 2. This circuit receives the Y, U, and V components of the incx-ming video signal at an input means 1 and three comparators 2 determine whether any of the three corrponents have the all I's state denoting a control plane bit. If there are no control plane bits the the Y, U, and V cαrrponents pass directly to respective outputs via three adders 4. If any one of the three input components is in the all I's state then a signal will be generated at a second output of the respective corrparator 2. The three second outputs of the comparators are coupled to three inputs of a key generation circuit 6. This outputs a 3 bit control plane word in accordance with the outputs of the comparators and this is used to control the mixer. A further output of the key generation circuit 6 is coupled to enable input of an interpolation circuit 8 which also receives the jncαming Y, U, and V words. When a control plane bit is present the key generation circuit 6 enables the interpolation circuit 8 to identify which components need to be interpolated from neighbouring pixels and derives these in a well known manner using an arrangement of delays, adders, and multipliers. When none of the three corrponents are in the all I's state then the control plane value will be 000 and there will be no keying in of the input video signal into the background. The control plane bits can be inserted into the video signal by simply using the three control plane bits to switch each of the Y, U, and V components between the source of video signals and an all I's signal in accordance with the value of the respective control bit. A circuit for inserting the control plane bits into the video data is shown in Figure 5. Each of the Y, U, and V data words is connected to an input of a respective switch 20. Each switch is controlled by a control plane bit supplied by a source of control plane bits 22 to switch its output between the j ccming corrponent of the video signal and a source of an all 1's signal 24. The source 24 could typically be a shift register or a series of switches connected to logic 1. When the control plane bit controlling a switch is a logic O then the respective corrponent of the video signal will be switched through to the output. When the control plane bit is a logic 1 the all I's signal will be switched through to the output of the switch 20. Thus each, of the components of a video signal can be replaced by an all I's state representing a control bit. The resultant output video signal can then be stored for future use if desired or supplied direct to a decoding circuit of the type shown in Figure 2. When the video signal is supplied in 4:2:2 format in accordance with CCIR recommendation 601, the U and V corrponents are shared by adjacent pixels. For this reason, the MSB of the control plane preferably replaces the Y corrponent for a pixel. Since the U and V values for adjacent pixels are shared, the two LSB's of the control plane word are also shared. When respective control plane bits are the same in adjacent pixels then there is no problem in this. For example if both the bits represented by the U component are 0 then the U corrponent is left unchanged and when they are both 1 the U corrponent is switched to the all I's state. Where the bits are different, a random selection takes place and either a logical 0 or 1 is selected and the respective component switched to either the incoming video component or the all I's state. Figure 3 shows a second embodiment of how a 24 bit YUV video signal and a 3 bit control plane signal for a pixel can be stored in a total of 24 bits of memory. This is done by using only 7 bits for each of the Y,U and V values and using the 3 spare bits thus created for the storage of the control plane. Firstly the luminance word is considered and if the least significance bit (which represents the darkest level of luminance) is used the second less significance bit is incremented. Thus this is equivalent to a rounding up operation. The most significant bit of the control plane is then inserted into the spare bit created by the rounding up operation. This could similarly be achieved by roiinding down or simply discarding the LSB. The same operation is performed on the two chrcxminance words and the second LSB and the LSB are inserted into the two spare bits thus created. The operation described above can be implemented in software or alternatively a dedicated hardware circuit could be used. This circuit would include inputs for the video signal and the control plane and circuitry to discard the appropriate LSB's and replace them with control plane bits. An output to a frame buffer would enable the resultant signal to be stored. The loss of the LSB in each of the Y,U and V corrponents does not lead to any perceptable degregation of the image in normal viewing circumstances. It also enables existing video data storage units e.g. frame stores to be used for the storage of the data without the need for a separate store for the control plane. The standard transmission format specified in CCIR recαrrrnendation 601 for YUV video data is the 4:2:2 structure. This involves transmitting alternate luminence and ch__taτ_inance values. The order is shown in Figure 4. In this it will be seen that the order of transmission is; Yl, Ul, Y2, V2, Y3, U3 When this transmission sequence is received adjacent pixels have to share the drα inance (U and V) values. For example, in a line of video data pixels 1 and 2 would share the same U value which would be derived from an average between the original U values for those two pixels, pixels 2 and 3 would share the same V values which would be similarly derived, pixels 3 and 4 would share the same U values etc. A three bit control plane could be incorporated in such a signal either after the averaging process, or before the averaging process. In the latter case the averaging would then only take place on seven bits of data. Clearly it is necessary also to average the control plane bits stored with the U and V corrponents. In the case where both bits care the same in adjacent pixels there is no problem in this. Where they are different then a random selection of either logical 1 or logical 0 could be made for that bit. The MSB is of course stored with the Y component and thus is different for each pixel. The storage of the control plane with the sequence of Y, U, Y, V, Y, U transmission is also shown in Figure 4. The control bits are designated Cl,l etc where the first numeral represents the pixel and the second numeral the bit, one representing the MSB and three the LSB. It can thus be seen that the U and V bits used for each pixel will also contain the respective control plane bits. A video signal received in the sequence of Figure 4 is reconstructed into YUV components and control plane for each pixel and if required can be converted to 8 bit words for each corrponent by randomly assigning a bit to the LSB. The signal can then be keyed into a second video signal preferably also of 8 bit format using the control plane value stored in the three control bits to control the proportions in which the two signals are combined. Any video signal embodying the invention can be separated into video data and control plane data either in software or in dedicated hardware. The hardware solution would require means to receive the video signal and means to separate the control plane data from it before supplying the video data to a video mixer to be combined with a second video signal under the control of the control plane data. Such a system is described above with reference to Figure 2. In other respects the composition system can be based on that described in the aforementioned British patent. The invention has been described here with reference to a YUV or YIQ signal but is clearly not limited to this and could equally well be used with an RGB signal.

Claims: CLAIMS 1. A method of storing a digital video signal and a control plane, the method comprising the steps of receiving a digital video signal having at least one data word for each pixel of the video signal and also receiving a corresponding control plane signal having at least one data bit for a pixel, replacing a data word in the video signal with a predetermined combination of bits when a respective control plane data bit is present, and storing the resultant data word for each pixel in a memory means. 2. A method according to claim 1 in which the video signal has three data words for each pixel, the control plane has three data bits for each pixel and the presence of a control plane bit causes replacement of a respective data word with the predetermined combination of bits . 3. A system for storing a digital video signal and a control plane coπprising means for receiving a digital video signal having at least one data word per pixel, means for receiving a control plane signal having at least one data bit per pixel, means for replacing a data word in the video signal with a predetermined combination of bits in response to the presence of a respective control bit, and means for storing the resultant data word for each pixel. 4. A system according to claim 3 in which the video signal comprises three data words for each pixel, the control plane comprises three data bits for each pixel and a data word is replaced with the predetermined combination of bits in response to a respective control plane data bit. 5. A digital video image composition system comprising means for receiving a first digital video signal having at least one data word in which at least one cαπibination of bits in the data word indicates the presence of a control plane data bit, means for generating a control plane data bit in response to said cxjrribination of bits in the data word, means for interpolating a replacement data word for that pixel from corresponding data words in adjacent pixels, means for receiving a second digital video siganl, and means for cαrbining the first and second video signals for each pixel in proportions dependent upon the value of the control plane data bit or bits correspondi g to that pixel. 6. A digital video image composition system according to claim 5 in which the first video signal comprises three data words per pixel each of which has at least one combination of bits indicating the presence of a control plane data bit. 7. A method of storing a digital video signal and a control plane, the method comprising the steps of receiving a digital video signal having at least one data word for each pixel of the video signal and also receiving a corresponding control plane signal having at least one data bit for each pixel, discarding at least the least significant bit of each data word, replacing each discarded bit with a control plane data bit, and storing the resultant data word for each pixel in a memory means. 8. A method according to claim 7 in which the video signal has three data words for each pixel, the control plane has three data bits for each pixel and each of the three data bits replaces a discarded least significant bit from a respective data word. 9. A system for storing a digital video signal and a control plane, comprising means for receiving a digital video signal having at least one data word per pixel, means for receiving a control plane signal having at least one data bit per pixel, means for discarding at least the least significant bit of the data word and replacing it with a control plane data bit, and means for storing the resultant data word for each pixel. 10. A system according to claim 9 in which the video signal comprises three data words for each pixel, the control plane comprises three data bits for each pixel and the means for discarding and replacing discards the least significant bit of each of the three data words and replaces it with a respective one of the three control plane data bits. 11. A digital video image composition system comprising means for receiving a first digital video signal having N data bits per pixel and in which at least one of the said data bits comprises control plane data, means for receiving a second digital video signal having N data bits per pixel, means for separating the control plane data bit or bits from the first video signal, and means for cαmbining the first and second video signals for each pixel in p_τ_portions dependent upon the value of the control plane data bit or bits coresponding to that pixel. 12. A digital video image composition system according to claim 11 in which the first video signal comprises three data words per pixel and the least significant bit of each data word comprises a portion of a control plane data word. 13. A digital video image composition system according to claim 7 in which the first video signal conprises a YUV signal received in 4:2:2 format.

Inventors: STANSFIELD PETER WILLIAM, STANSFIELD, PETER, WILLIAM
Applicants: RANK CINTEL LTD, RANK CINTEL LIMITED
Application year: 1990
Main classification: H04N9
IPC codes: H04N5, H04N7, H04N9