Abstract:
The invention provides spatial scalable compression of video, comprising base layer encoding for providing a bitstream with a relatively low resolution, and enhancement layer encoding for encoding a residual signal for providing a second bitstream, wherein a modification is provided prior to the enhancement layer encoding for transforming the residual signal into a signal with a level range of a normal input video signal.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a video coding.  
         BACKGROUND OF THE INVENTION  
         [0002]    Because of the massive amounts of data inherent in digital video, the transmission of full-motion, high-definition digital video signals is a significant problem in the development of high-definition television. More particularly, each digital image frame is a still image formed from an array of pixels according to the display resolution of a particular system. As a result, the amounts of raw digital information included in high resolution video sequences are massive. In order to reduce the amount of data that must be sent, compression schemes are used to compress the data. Various video compression standards or processes have been established, including, MPEG-2, MPEG-4, and H.263.  
           [0003]    Many applications are enabled where video is available at various resolutions and/or qualities in one stream. Methods to accomplish this are loosely referred to as scalability techniques. There are three axes on which one can deploy scalability. The first is scalability on the time axis, often referred to as temporal scalability. Secondly, there is scalability on the quality axis, often referred to as signal-to-noise scalability or fine-grain scalability. The third axis is the resolution axis (number of pixels in image) often referred to as spatial scalability or layered coding. In layered coding, the bitstream is divided into two or more bitstreams, or layers. Each layer can be combined to form a single high quality signal. For example, the base layer may provide a lower quality video signal, while the enhancement layer provides additional information that can enhance the base layer image.  
           [0004]    In particular, spatial scalability can provide compatibility between different video standards or decoder capabilities. With spatial scalability, the base layer video may have a lower resolution than the input video sequence, in which case the enhancement layer carries information which can restore the resolution of the base layer to the input sequence level.  
           [0005]    [0005]FIG. 1 illustrates a known spatial scalable video encoder  100 . The depicted encoding system  100  accomplishes layer compression, whereby a portion of the channel is used for providing a low resolution base layer and the remaining portion is used for transmitting edge enhancement information, whereby the two signals may be recombined to bring the system up to high-resolution. The high resolution video input Hi-Res is split by splitter  102  whereby the data is sent to a low pass filter  104  and a subtraction circuit  106 . The low pass filter  104  reduces the resolution of the video data, which is then fed to a base encoder  108 . In general, low pass filters and encoders are well known in the art and are not described in detail herein for purposes of simplicity. The encoder  108  produces a lower resolution base stream which can be broadcast, received and via a decoder, displayed as is, although the base stream does not provide a resolution which would be considered as high-definition.  
           [0006]    The output of the encoder  108  is also fed to a decoder  112  within the system  100 . From there, the decoded signal is fed into an interpolate and upsample circuit  114 . In general, the interpolate and upsample circuit  114  reconstructs the filtered out resolution from the decoded video stream and provides a video data stream having the same resolution as the high-resolution input. However, because of the filtering and the losses resulting from the encoding and decoding, loss of information is present in the reconstructed stream. The loss is determined in the subtraction circuit  106  by subtracting the reconstructed high-resolution stream from the original, unmodified high-resolution stream. The output of the subtraction circuit  106  is fed to an enhancement encoder  116  which outputs a reasonable quality enhancement stream.  
         SUMMARY OF THE INVENTION  
         [0007]    It is an object of the invention to provide advantageous coding of the enhancement stream. To this end, the invention provides an apparatus, a layered encoder, a method, a decoder and a method for decoding as defined in the independent claims. Advantageous embodiments are defined in the dependent claims.  
           [0008]    It is an insight of the inventors that a prior art enhancement encoder needs non-standard components to process the residual signal which has pixel values concentrated around zero. Accordingly, there is a need for a method and apparatus which changes the concentration of pixel values of the residual signal before the residual signal is supplied to the enhancement encoder.  
           [0009]    By providing a modification unit prior to the enhancement unit for transforming the residual signal into a signal with a signal level range of a normal input video signal, standard components can be used in the enhancement coder. Preferably, the modification unit consists of an adder for adding a DC offset to the residual signal, preferably such that the pixel values of the residual signal are shifted to the middle of a predetermined input range of the enhancement coder.  
           [0010]    According to one embodiment of the invention, a method and apparatus for providing spatial scalable compression using adaptive content filtering of a video stream is disclosed. The video stream is downsampled to reduce the resolution of the video stream. The downsampled video stream is encoded to produce a base stream. The bsae stream is decoded and upconverted to produce a reconstructed video stream. The reconstructed video stream is subtracted from the video stream to produce a residual stream. The residual signal is transformed into a signal with a signal level range of a normal input video signal used for video compression. The resulting residual stream is encoded and outputted as an enhancement stream.  
           [0011]    According to another embodiment of the invention, a method and apparatus for decoding compressed video information received in a base stream and an enhancement stream is disclosed. The base stream is decoded and then upconverted to increase the resolution of the decoded base stream. The enhancement stream is decoded. A signal range of the decoded enhancement stream is transformed into a signal range of an original residual signal. The upconverted decoded base stream is combined with the transformed enhancement stream to produce a video output.  
           [0012]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereafter. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The invention will now be described, by way of example, with reference to the accompanying drawings, wherein:  
         [0014]    [0014]FIG. 1 is a block schematic representation of a known encoder with spatial scalability;  
         [0015]    [0015]FIG. 2 is a block schematic representation of an encoder with spatial scalability according to one embodiment of the invention;  
         [0016]    [0016]FIG. 3 is a block schematic representation of a decoder according to one embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    [0017]FIG. 2 illustrates a spatial scalable video encoder  200  according to one embodiment of the invention. The depicted encoding system  200  accomplishes layer compression, whereby a portion of the channel is used for providing a low resolution base layer and the remaining portion is used for transmitting edge enhancement information, whereby the two signals may be recombined to bring the system up to high-resolution. The high resolution video input is split by splitter  202  whereby the data is sent to a low pass filter  204  and a subtraction circuit  206 . The low pass filter  204  reduces the resolution of the video data, which is then fed to a base encoder  208 . In general, low pass filters and encoders are well known in the art and are not described in detail herein for purposes of simplicity. The encoder  208  produces a lower resolution base stream BS which can be broadcast, received and via a decoder, displayed as is, although the base stream does not provide a resolution which would be considered as high-definition.  
         [0018]    The output of the encoder  208  is also fed to a decoder  212  within the system  200 . From there, the decoded signal is fed into an interpolate and upsample circuit  214 . In general, the interpolate and upsample circuit  214  reconstructs the filtered out resolution from the decoded video stream and provides a video data stream having the same resolution as the high-resolution input. However, because of the filtering and the losses resulting from the encoding and decoding, loss of information is present in the reconstructed stream. The loss is determined in the subtraction circuit  206  by subtracting the reconstructed high-resolution stream from the original, unmodified high-resolution stream. The output of the subtraction circuit  206  is fed into a modification unit  207 . The modification unit  207  transforms the residual signal into a signal within the same signal level range as a normal input video signal as used for video compression. The modification unit  207  adds a DC-offset value  209  to the residual signal. The modification unit  207  also comprises a clip function which prevents the output of the modification unit from going below a predetermined value and above another predetermined value. This DC-offset and clipping operation allows the use of existing standards, e.g., MPEG, for the enhancement encoder where the pixel values are in a predetermined range, e.g., 0 . . . 255. The residual signal is normally concentrated around zero. By adding a DC-offset value  209 , the concentration of samples can be shifted to the middle of the range, e.g., 128 for 8 bit video samples. The transformed residual signal from the modification unit  207  is fed to an enhancement encoder  216  which outputs a reasonable quality enhancement stream ES. The advantage of this addition is that the standard components of the encoder for the enhancement layer can be used and result in a cost efficient (re-use of IP blocks) solution.  
         [0019]    [0019]FIG. 3 illustrates a decoder  300  according to one embodiment of the invention. FIG. 3 illustrates a decoder for decoding the encoded signals processed by the layered encoder  200 . The base stream is decoded in a base decoder  302 . The decoded output from the decoder  302  is upconverted by an upconverter  306  and then supplied to an addition unit  310 . The enhancement stream is decoded in a decoder  304 . The output of the decoder  304  is supplied to a modification unit  308 . The modification unit  308  performs the inverse operation of the modification  207  in the encoder  200 . The modification unit  308  converts the decoded enhancement stream from a normal video signal range to the signal range of the original residual signal. The output of the modification unit  308  is supplied to the addition unit  310 , where it is combined with the output of the upconverter  306  to form the output of the decoder  300 .  
         [0020]    It will be understood that the different embodiments of the invention are not limited to the exact order of the above-described steps as the timing of some steps can be interchanged without affecting the overall operation of the invention. Furthermore, the term “comprising” does not exclude other elements or steps, the terms “a” and “an” do not exclude a plurality and a single processor or other unit may fulfill the functions of several of the units or circuits recited in the claims.