Patent Publication Number: US-2005135478-A1

Title: Reduction of layer-decoding complexity by reordering the transmission of enhancement layer frames

Description:
BACKGROUND OF THE INVENTION  
      The present invention generally relates to video coding, and more particularly to rearranging the transmission order of enhancement layer frames.  
      In MPEG-4 base-layer decoders as well as MPEG-2 decoders for that matter, the transmission order of the various frames differs from the display order. An example of this is shown in  FIG. 1 . As can be seen, the transmission order of both the base layer frames and corresponding enhancement layer frames differs from the display order.  
      The reason for the rearrangement of the frames of  FIG. 1  is that the bi-directional motion compensation (MC) employed for the B-frames requires the anchor frames (I and P-frames) on which the prediction is made to be already available in the memory at the encoder/decoder side, when the B-frames are encoded/decoded. This requires that the I- and P-frames to be transmitted to the decoder prior to the B-frames. However, since the B-frames is typically displayed between the I- and P-frames, the transmission and display order of the frames are different due to the MC-prediction.  
      A block diagram of one example of a scalable (layered) decoder is shown in  FIG. 2 . During operation, the decoder  2  receives the encoded base and enhancement layer frames in the transmission order shown in  FIG. 1 . Further, the decoder  2  will decode and reorder these frames into the display order shown in  FIG. 2 .  
      As can be seen, the decoder  2  includes two separate paths for decoding the base layer and enhancement layer bit steams. Since these two paths are separate the decoding process of each of the two streams does not need to be synchronized.  
      The path for the base layer stream includes a variable length decoder  4 , an inverse quantization block  6  and an inverse discrete cosine transform block (IDCT)  8  to convert the base layer bit-steam into picture frames. A motion compensation block  12  is also included for performing motion compensation on picture frames previously stored in a frame memory  14  based on the received motion vectors. Further, an adder  10  is also included to combine the outputs of the IDCT block  8  and the motion compensation block  12 .  
      The path for the enhancement layer stream includes a variable length decoder (VLD  15 , a bit plane decoding block  17  and another IDCT block  18  to convert the enhancement layer bit-steam into picture frames. During operation, the bit-plane decoding block  17  will decode the output of the variable length decoder  12  into individual bit planes using any suitable fine granular scalable decoding technique.  
      As can be further seen, a bit plane memory  16  is also included to store the individual bit planes until all of the bit planes for a current frame are decoded. Further, after the IDCT block  18  a frame memory  22  is included. The frame memory  22  is used to compensate for the encoded frames being received in a transmission order different from the display order, as shown in  FIG. 1 .  
      For example, if the enhancement layer frames are transmitted at the same time instance as the corresponding base-layer frames, the frame-memory  22  is required to store the enhancement-layer frames until its display time, which coincides with the base-layer display time. Referring back to the transmission order of  FIG. 1 , the enhancement picture E 3  after being decoded is stored in the frame memory  22  until after the enhancement frame E 2  is decoded and displayed. Thereafter, the enhancement frame E 3  is retrieved from the frame memory and than displayed. Therefore, in this manner, the transmission order of the frames is converted into the display order, as shown in  FIG. 1 .  
      The decoder  2  also includes another adder  20  to combine the picture frames from each of the paths in order to produce enhanced video  24 . The enhanced video  24  can be either displayed immediately in real time or stored in an output frame memory for display at a later time.  
     SUMMARY OF THE INVENTION  
      The present invention is directed to a method for encoding video data. The method includes coding a portion of the video data to produce base layer frames. Also, coding another portion of the video data to produce enhancement layer frames. Further, rearranging the enhancement layer frames into a display order.  
      The present invention is also directed to a method for decoding a video signal including a base layer and an enhancement layer, where the enhancement layer includes enhancement frames arranged in a display order. The method includes decoding the base layer to produce decoded base layer frames. Also, decoding the enhancement layer to produce decoded enhancement layer frames and rearranging the decoded base layer frames into the display order. Further, combining the decoded base layer frames with the decoded enhancement layer frames without storing any of the decoded enhancement layer frames to form video frames. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Referring now to the drawings were like reference numbers represent corresponding parts throughout:  
       FIG. 1  is a diagram showing the transmission and display order for a conventional encoding system;  
       FIG. 2  is a block diagram showing one example of a decoder;  
       FIG. 3  is a diagram showing one example of the transmission and display order according to the present invention;  
       FIG. 4  is a block diagram showing one example of a decoder according to the present invention;  
       FIG. 5  is a diagram showing one example of the transmission timing of the frames according to the present invention;  
       FIG. 6  is a block diagram showing one example of a encoder according to the present invention;  
       FIG. 7  is a diagram showing another example of the transmission and display order according to the present invention; and  
       FIG. 8  is a block diagram showing one example of a system according to the present invention. 
    
    
     DETAILED DESCRIPTION  
      The present invention is directed to rearranging the transmission order of coded enhancement-layer frames. By making the display and transmission order of the enhancement layer frames identical, a frame memory is no longer necessary at the decoder-side to hold the enhancement-layer frames until being displayed since the display can take place immediately after the decoding. Reducing the amount of memory is desirable for mobile applications or other low-power consumption devices.  
      In the conventional encoding system, where the enhancement layer transmission order is the same as for the base-layer, more than two frames stores are necessary for decoding. Referring to  FIG. 1 , one frame memory is used to store the E 1  frame, one frame memory is used to store the E 3  frame (which has been decoded, but cannot be displayed until E 2  is received, decoded and displayed) and one frame memory is used for the decoding and storing of E 2 . However, according to the present invention, the memory to store the compressed E 3  data is no longer necessary.  
      One example of the transmission and display order according to the present invention is shown in  FIG. 3 . For purposes of explanation,  FIG. 3  only shows five base layer frames and corresponding enhancement layer frames. However, it should be noted that in an actual system the present invention would be applied to a variety of different groups of picture (GOP) structures.  
      As can be seen from  FIG. 3 , the transmission order of the base layer frames is same as in the conventional system shown in  FIG. 1 . However, according to the present invention, the transmission order of the enhancement frames has been rearranged to be the same as the display order of the enhancement frames on the decoder side, as shown in  FIG. 3 .  
      By rearranging the transmission order of the enhancement frames to be the same as the display order no local memory is necessary for the enhancement frames since the FGS frames are displayed immediately after the decoding. Of course, the display takes place after the FGS residual has been added to the base-layer frame.  
      One example of a decoder according to the present invention is shown in  FIG. 4 . As can be seen, the decoder  26  of this figure is the same as the conventional decoder of  FIG. 2  except that a frame memory  22  at the output of the IDCT block  18  is no longer required. As described above, this frame memory is no longer required since the transmission order of the enhancement frames has been rearranged to be the same as the actual display order of the frames. Therefore, the enhancement layer frames can be displayed in the ordered received after being combined with the base layer frames.  
      During operation, the decoder  26  will receive the base and enhancement layer frames in the transmission order shown in  FIG. 3 . However, in  FIG. 3 , the transmission order of the base layer frames is different than the enhancement layer frames. In order to compensate for this, the order of the base layer frames is changed and the timing of the enhancement layer frames is changed, as described below.  
      One example of the transmission timing of the enhancement layer frames according to the present invention is shown in  FIG. 5 . As can be seen, the transmission timing of the enhancement layer frames is delayed with respect to the corresponding base layer frames. In the first time period, the base layer frame I 1  is transmitted. Since the transmission of the corresponding enhancement layer frame E 1  has been delayed to the next period, the decoder  26  of  FIG. 4  will decode the base layer frame I 1  and just store it in the frame memory  14  until the base layer frame P 3  and the enhancement frame E 1  is received.  
      In the second time period of  FIG. 5 , the enhancement layer frame E 1  and the base layer frame P 3  is transmitted. At this time, the decoder  26  of  FIG. 4  will decode the base layer frame P 3  and again just store it in the frame memory  14  until the delayed enhancement. frame E 3  is received and decoded. Further, the decoder  26  of  FIG. 4  will decode the enhancement layer frame E 1  and combine it with the corresponding base layer frame I 1  previously stored in the frame memory  14  to form a frame of enhanced video.  
      In the third time period of  FIG. 5 , the base layer frame B 2  and the corresponding enhancement layer frame E 2  is transmitted at the same time. Thus, the decoder  26  of  FIG. 4  will decode the base layer frame B 2  and the corresponding enhancement layer frame E 2  at the same time and then combine the decoded frames to form another frame of enhanced video.  
      In the fourth time period of  FIG. 5 , the enhancement layer frame E 3  and the base layer frame P 5  is transmitted. At this time, the decoder  26  of  FIG. 4  will decode the base layer frame P 5  and again just store it in the frame memory  14  until the delayed enhancement frame E 5  is received and decoded. Further, the decoder  26  of  FIG. 4  will decode the enhancement layer frame E 3  and combine it with the corresponding base layer frame P 3  previously stored in the frame memory  14  to form another frame of enhanced video. As can be seen from  FIG. 5 , the above-described process will continue until all of the enhancement and corresponding base layer frames transmitted in the subsequent time periods are decoded and combined to produce an enhanced video sequence.  
      One example of an encoder according to the present invention is shown in  FIG. 6 . According to the present invention, the encoder will produce a stream of base layer frames and a stream of enhancement layer frames according to the transmission order shown in  FIG. 3 .  
      As can be seen from  FIG. 7 , the encoder  28  includes a base layer encoder  30  and enhancement layer encoder  54 . The base layer encoder  30  includes a discrete cosine transform (DCT) block  34 , a quantization block  36  and an entropy encoder  38  to encode the original video into I frames and the motion compensated residuals into P and B frames.  
      The layer base encoder  30  also includes an inverse quantization block  42 , an IDCT block  44 , an adder  46  and a compensation block  48  connected to the other input of the adder  46 . During operation, these elements  42 , 44 , 46 , 48  provide a decoded version of the current frame being coded, which is stored in a frame memory  50 .  
      A motion estimation block  52  is also included which produces the motion vectors from the current frame and a decoded version of the previous frame stored in the frame memory  50 . The use of the decoded version of the previous frame enables the motion compensation performed on the decoder side to be more accurate since it is the same as received on the decoder side.  
      As can be further seen, the output of the motion compensation block  48  is also connected to one side of the subtracter  32 . This enables motion compensated residuals based on predictions from previously transmitted coded frames to be subtracted from the current frame being coded. A multiplexer  40  is also included to combine the outputs of the entropy encoder  38  and the motion estimation block  52  to form the base layer stream.  
      The enhancement layer encoder  54  includes another subtracter  62 . The subtracter  62  is utilized to subtract the output of the inverse quantization block  42  from the output of the DCT block  34  in order to form residual images. A fine granular scalable (FGS) encoder  58  is also included to encode the residual images produced by the subtracter  62 . The residual images are encoded by performing bit-plane DCT scanning and entropy encoding. A frame memory  56  is connected to the FGS encoder  58 , which is utilized to store each of the bit-planes after being decoded. After all of the bit-planes of the current frame are decoded, the frame memory  56  will output that frame.  
      As can be further seen, another frame memory  60  is connected to the output of the FGS encoder  60 . According to the present invention, the frame memory  60  rearranges the enhancement layer frames into the transmission order shown in  FIG. 3 . In order to perform the rearrangement of the enhancement layer frames, the encoded enhancement layer frames are stored in the frame memory  60  and then transmitted according to the timing shown in  FIG. 5 .  
      As previously described, the transmission order of the enhancement layer frames is the same order as the frames are displayed on the decoder side. This is significant since it eliminates the need for one of the frame memories on the decoder-side, which is desirable for mobile and other low power applications.  
      According to the present invention, in addition to the applicability of the present invention to enhancement-layers with no inter-enhancement prediction, the present invention is also applicable to the case where single direction prediction (i.e. no bi-directional MC prediction) is used with the enhancement layer. An example of this scenario is shown in  FIG. 8 .  
      The present invention is also applicable in the case where multiple enhancement layers are used on the top of the base layer. In this case, each of the enhancement layers can either have no intra-enhancement-layer prediction or has a single-direction prediction (from that enhancement layer or any other layer with the overall layered-coding structure).  
      One example of a system in which the present invention may be implemented is shown in  FIG. 9 . By way of examples, the system may represent a television, a set-top box, a desktop, laptop or palmtop computer, a personal digital assistant (PDA), a video/image storage device such as a video cassette recorder (VCR), a digital video recorder (DVR), a TiVO device, etc., as well as portions or combinations of these and other devices. The system includes one or more video sources  64 , one or more input/output devices  74 , a processor  66  and a memory  68 .  
      The video/image source(s)  64  may represent, e.g., a television receiver, a VCR or other video/image storage device. The source(s)  74  may alternatively represent one or more network connections for receiving video from a server or servers over, e.g., a global computer communications network such as the Internet, a wide area network, a metropolitan area network, a local area network, a terrestrial broadcast system, a cable network, a satellite network, a wireless network, or a telephone network, as well as portions or combinations of these and other types of networks.  
      The input/output devices  74 , processor  66  and memory  68  communicate over a communication medium  72 . The communication medium  72  may represent, e.g.,. a bus, a communication network, one or more internal connections of a circuit, circuit card or other device, as well as portions and combinations of these and other communication media. Input video data from the source(s)  64  is processed in accordance with one or more software programs stored in memory  66  and executed by processor  66  in order to generate output video/images supplied to a display device  70 .  
      In a preferred embodiment, the coding, decoding and rearranging of the enhancement layer frames described in conjunction with  FIGS. 3-8  is implemented by computer readable code executed by the system. The code may be stored in the memory  68  or read/downloaded from a memory medium such as a CD-ROM or floppy disk. In other embodiments, hardware circuitry may be used in place of, or in combination with, software instructions to implement the invention. For example, the elements shown in  FIGS. 4 and 7  also can be implemented as discrete hardware elements.  
      While the present invention has been described above in terms of specific examples, it is to be understood that the invention is not intended to be confined or limited to the examples disclosed herein. It should be noted that the application of the framework described herein goes beyond the examples shown in the figures. The present invention is applicable to all schemes employing motion compensation (MC)at the base-layer and having an enhancement-layer without MC (i.e. Intra coded). Therefore, this mechanism can be applied to all scalable schemes where no Bi-directional prediction is done within the enhancement-layer (i.e., with no intra-enhancement-layer prediction) or single direction prediction MC.  
      Further, the present invention is adaptable to any coding algorithm used for the enhancement-layer residual—progressive coding or normal quantization, wavelet or DCT etc. Examples of such enhancement-layer coding schemes are the MPEG-4 Fine-Granular-Scalability (FGS) method and the SNR scalability of MPEG-2, where no prediction in the enhancement layer is used.