GB2356507A

GB2356507A - Method of encoding a video signal to enable logo insertion

Info

Publication number: GB2356507A
Application number: GB9926311A
Authority: GB
Inventors: Michael James Knee
Original assignee: Snell and Wilcox Ltd
Current assignee: Snell Advanced Media Ltd
Priority date: 1999-11-05
Filing date: 1999-11-05
Publication date: 2001-05-23
Also published as: WO2001033861A1; GB9926311D0; AU1162701A

Abstract

When MPEG encoding a video signal, a set of macroblocks is defined to form the site for downstream insertion of a logo. The encoder forces the coding of the set of macroblocks into an intra-coding mode. It also inhibits the use, in coding of any other macroblock, of a motion vector directed to any of the set of macroblocks. The aggregate bit rate over the set of macroblocks is controlled so as to simplify logo insertion or to maintain picture quality in the logo region.

Description

2356507 COMPRESSION ENCODING This invention relates to compression

encoding of video and images and in the most important example relates to MPEG-2 encoding of digital television.

There is a common requirement to insert a logo or other fixed element into a video signal. This is a relatively straightforward procedure with broad band video but is considerably more complex if the video signal is received in compressed form. One approach to the problem is to decode the incoming MPEG-2 signal, insert the logo conventionally into the uncompressed video signal and then re-encode. If the decoding and encoding processes are independent, there is the risk of introducing additional coding artifacts into the signal. This risk can be removed if the decoder is arranged to pass to the encoder, an appropriate set of coding decisions. Re-use by an encoder of the decisions taken by a decoder can result in effectively lossless decoding/encoding. This technology, as shown for example in EP 0 765 576 and EP 0 913 058. Whilst this technology is effective, there are instances which a technically less complex solution is required.

An alternative approach is to decode only that spatial region of the video signal which is to receive the logo or other element. Because of the use of motion vectors in MPEG-2, it is necessary to decode and re- encode not only the macroblocks comprising the spatial region in which the logo is to be inserted, but also all those other macroblocks which are linked by motion vectors. This remains a relatively complex processing operation. Moreover, the complexity of the processing which is required will vary with the nature of the picture material and particularly with the number of macroblocks which require decoding and re-encoding.

It is frequently the case that a centrally produced video feed is distributed to a number of locations, at each of which a distinctive logo is inserted before local distribution of the video signal. The problem of inserting a logo without introducing coding artefacts is therefore faced at many different locations, on the same video signal.

It is an object of this invention to provide a system solution to the problem of inserting a logo without introducing coding artefacts.

Accordingly, the present invention consists in one aspect in a method of motion compensated compression encoding operating on macroblocks of a picture, comprising the step of identifying a set of macroblocks defining a site for downstream insertion of a logo or like element, forcing the coding of said set of macroblocks into an intra-coding mode and inhibiting the use, in coding of any other macroblock, of a motion vector directed to any one of said set of macroblocks.

The invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a block diagram of a system according to the present invention; and Figure 2 is a block diagram of an MPEG-2 encoder according to the present invention.

Referring to Figure 2, an MPEG-2 encoder 102 receives a video signal input together with a control signal from a "select logo region" block 104. This enables a region of the picture to be defined (typically a corner of the picture) into which a logo will optionally be inserted. This region will comprise a number of macroblocks and preferably consists of a number of whole slices in the MPEG-2 bitstream.

The encoder 102 is so arranged that the macroblocks in the selected area are encoded in intra-mode and that no predictions are taken from this area in coding the remainder of the picture.

The compressed MPEG-2 bitstream, prepared in accordance with this invention, is received by one or a number of logo inserters as shown at 106.

The logo inserter comprises a splitter 108 which identifies the macroblocks in the region which has been defined for subsequent logo insertion. These are passed to a local decoder 110. Decoded video is combined in video mixer 112 with the output of logo generator 114. The resulting video signal is encoded at local encoder 116.

The macroblocks outside the logo region are passed directly from the splifter 108 to a combiner 118 in which they are re-combined with the output from local encoder 116.

In this way, a logo is inserted into the MPEG bitstream with a minimum and constant amount of processing. Outside the prepared area, the processing is transparent.

If the logo is the same size as the prepared area and is not transparent, it will be possible to proceed in a simpler manner. In this case, the macroblocks in the prepared area are simply replaced by a pre-encoded logo bitstream.

It will be necessary to pay careful attention to the relative number of bits in the prepared area and in the inserted logo. In the case where a pre encoded logo directly replaces the prepared macroblocks, the encoder 102 will be arranged to ensure through quantiser control and the addition, if necessary, of stuffing, that the aggregate number of bits of the prepared macroblocks equals that of the pre-encoded logo. Alternatively, the logo generator can be arranged to insert stuffing to make up a shortfall of bits.

In the more general case, the encoder 102 Gan be arranged through quantiser control or the use of stuffing, to ensure that there are sufficient bits employed in the prepared area so that the output of the local encoder 116, producing the same number of bits, offers sufficient visual quality. It will generally be regarded as important that the logo or other fixed element is re produced on a display with a resolution that does not vary with the nature of the picture material. It will be understood that, without this feature of the present invention, the number of bits available for encoding of a logo would depend upon the number of bits employed in the upstream encoding of the corresponding region of the picture and thus, in turn upon the relative difficulty in encoding the picture and specific regions without the picture.

Turning to Figure 2, there is shown an example of MPEG-2 encoder 102. This encoder employs the technology disclosed in the above published patent applications to which reference is again directed.

The incoming video signal is received by a coding decision block 202.

This analyses the video signal and makes the necessary MPEG-2 coding decisions. These include coding modes, allocation of motion vectors and quantiser levels. The coding decision block 202 has two outputs: a video output 204 and an information bus output 206. A so-called "dim" coder 208 operates on the coding decisions carried without the information bus signal to encode the video signal 204 and output an MPEG-2 bitstream. In accordance with the present invention, the information bus passes through an information bus processor 210 which modifies the coding decisions, as appropriate, to ensure that the macroblocks within a selected area are intra-coded, that no use is made of these macroblocks in prediction coding of the remaining macroblocks of the picture and, preferably, that the defined area is encoded with the appropriate number of bits. For this purpose, the information bus processor 210 will receive control inputs specifying the location of the logo and the relative quality with which the logo is to be displayed.

Claims

1. A method of motion compensated compression encoding operating on macroblocks of a picture, comprising the step of identifying a set of macroblocks defining a site for downstream insertion of a logo or like element, forcing the coding of said set of macroblocks into an intracoding mode and inhibiting the use, in coding of any other macroblock, of a motion vector directed to any one of said set of macroblocks.