Abstract:
A system and method to convert a CIF compressed video to a QCIF video bitstream provides compatibility between the various digital video sources and application uses. A coding mode and a motion vector for a macroblock in a QCIF video sequence are selected from those of a corresponding CIF video sequence without motion estimation.

Description:
FIELD OF THE INVENTION 
     This invention relates to video systems and more particularly to digital video. 
     BACKGROUND OF THE INVENTION 
     Presently, there is extensive interest in advanced video services using technologies of digital signal processing, VLSI, and packet networkings. Examples of these services include video on the Internet, videoconferencing, interactive network video, video editing/publishing, and virtual video library/store. For many digital video applications, compressed video bitstreams are usually transmitted over networks and/or stored in tapes or databases in Common Intermediate Format (CIF) or Quarter Common Intermediate Format (QCIF). 
     Various digital video applications utilize CIF and QCIF digital formats. For example, in multi-point videoconferencing over networks, multi-point control unit (MCU) receives QCIF compressed video bitstreams from several users, combines them into one CIF video, down converts it into QCIF video, encodes it, and sends the QCIF video bitstream to all the users. There is a need to down convert a CIF compressed video to a QCIF video bitstream to provide compatibility between the various digital video sources and application uses. 
     SUMMARY OF THE INVENTION 
     The present invention is a system and method to convert a CIF compressed video to a QCIF video bitstream. A coding mode and a motion vector for a macroblock in a QCIF video sequence are selected from those of a corresponding CIF video sequence without motion estimation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     A more complete understanding of the present invention may be obtained from consideration of the following description in conjunction with the drawing, in which: 
     FIG. 1 shows a functional block diagram of a pixel-domain CIF to QCIF down-conversion scheme; 
     FIG. 2 shows a functional block diagram of a CIF to QCIF down-conversion scheme in the DCT domain; 
     FIG. 3 shows a diagrammatic representation of GOBs mapping from a CIF format to a QCIF format using the H.261 standard; and, 
     FIG. 4 shows a functional flow chart for selecting the coding mode and the motion vector. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Digital video is utilized in a great variety of applications including video on the Internet, videoconferencing, interactive network video, video editing/publishing, and video libraries. Many advanced video applications require converting a compressed video bitstream from CIF (Common Intermediate Format) to QCIF (Quarter Common Intermediate Format). The conversion task can be performed in the pixel domain or in the discrete cosine transform (DCT) domain. The present invention utilizes the DCT-domain processing for H.261 video bitstream down-conversion, which requires lower complexity than that in the pixel domain. The present invention utilizes a scheme to select a coding mode and a motion vector for a macroblock (MB) in a QCIF video sequence from those of the corresponding CIF video without motion estimation (ME). 
     There are two possible domains to perform down-conversion for H.261 bitstream: discrete cosine transform (DCT) domain and pixel domain. Referring to FIG. 1 there is illustrated a pixel-domain CIF-to-QCIF down-conversion scheme. The decoding loop  102  performs variable length decoding (VLD)  104 , inverse quantization (IQ)  106 , Inverse discrete cosine transform (IDCT)  108 , and motion compensation (MC)  110  which is added to the output of the IDCT  108  by adder  112 . The output of adder  112  is coupled to frame memory (FM)  114 , which is coupled to MC  110 . Output of the adder  112  is then coupled to pixel domain down converter  116 . The encoding loop  118  is composed of DCT  120 , quantization (Q)  122 , IQ  124 , IDCT  126 , FM  128 , ME/MC  130  and variable length coding (VLC)  132 . The output of the ME/MC  130  is coupled to adder  132  and subtracter  134 . The output of pixel domain down converter  116  is coupled to the subtracter  134  and the ME/MC  130 . Output of the adder  132  is coupled to FM  128 , which is coupled to ME/MC  130 . Output of the subtracter  134  is coupled to DCT  120 , which is coupled to Q  122 . The output of Q  122  is coupled to VLC  132  and IQ  124 . 
     Referring to FIG. 2 there is shown a diagrammatic representation of the present invention DCT-domain down-conversion system. It consists of decoding loop  202 , DCT-domain down sampling  204  and encoding loop  206 . In the DCT-domain decoding loop  202 , besides VLD  208  and IQ  210 , which are the same as in the pixel-domain decoding loop  102 , MC is performed in the DCT domain (DCT-MC)  212 . In the DCT-domain down-conversion loop  204 , the down-conversion is performed on an MB by MB basis. More specifically, four 8×8 luminance (Y) blocks are scaled down to one 8×8 Y block while two chrominance blocks (Cr and Cb for 4:1:1 format) are kept unchanged. Once all four adjacent macroblocks are available, the four 8×8 Cr or Cb blocks are down converted to one 8×8 Cr or Cb block, respectively. The DCT-domain encoding loop  206  is also different from that of the pixel-domain approach. It performs DCT-MC  216  instead of doing DCT and IDCT pair, and ME in the pixel domain. 
     Referring to FIG. 1 in conjunction with FIG. 2, it can be seen that the DCT-domain down-conversion approach has the advantage of lower complexity than that in the pixel domain. This is because DCT, IDCT, and ME, which are computationally expensive for the pixel-domain approach, are saved for the DCT-domain approach. Accurate quantitative comparison of complexity between DCT- and pixel-domain approaches is very difficult since the complexity for the DCT-domain approach is highly dependent on the characteristics of a particular video sequence. According to reported results, the complexity of DCT-MC and/or DCT-domain down sampling is comparable to that of DCT (and/or IDCT) used in the pixel-domain approach when exploiting the sparseness of quantized DCT coefficients and zero motion vector distribution in DCT-MC and DCT-domain down sampling. Notice that for an 8×8 DCT block most high frequency coefficients are quantized to zero, and a large percentage of motion vectors are zero for the head-shoulder video sequences. These result in significant computational savings. When counting the computation from ME of the pixel-domain method, which usually requires lots of computation, the overall computational saving of DCT-domain approach is very large. 
     Referring to FIG. 3 there is illustrated a scheme to select a coding mode and a motion vector for an MB. The proposed DCT-domain CIF-to-QCIF down converter is based on the H.261 video compression standard. In the H.261 standard, a CIF frame consists of 12 Group of Blocks (GOBs)  302  and a QCIF frame is composed of 3 GOBs  304 . A GOB consists of 33 Mbs. In order to perform CIF-to-QCIF down-conversion, four GOBs in CIF format have to be mapped to one GOB in QCIF format. For the pixel-domain down-conversion approach, this can be done by scaling every 8×8 block in CIF format down to a 4×4 block in QCIF format. For the DCT-domain approach, the down-conversion has to be performed from four 8×8 blocks in CIF format to an 8×8 block in QCIF format, as stated above. 
     Once the DCT coefficients of each block for the QCIF video are available after down-conversion, compose the four blocks to generate a new MB. To encode the MB, a motion vector and a coding mode should be determined. Although this can be done by ME in the DCT domain, it is computationally much more efficient to obtain the motion vector from those of the CIF video. The new motion vector can be inferred from the four motion vectors of the CIF video by taking there mean or median. However, we found that for many MBs, the inferred motion vectors even result in worse prediction performance than the zero motion vector. 
     To solve this problem, we propose a new scheme to select the MB coding mode and the motion vector. Let NMV H , NMV V , and NMtype represent the horizontal and vertical components of a motion vector and the coding mode of the MB of the QCIF video, respectively. Denote Mtype [i], i=1,2,3,4, as coding modes for the four MBs of the CIF video. Let MV H [i], and MV V [i], i=1,2,3,4, denote the horizontal and vertical components of a motion vector, respectively. Note that a typical coding mode in the H.261 standard is 0 or 2 or 5, which respectively means that the corresponding MB is encoded by using intra-frame coding, inter-frame coding without MC, and motion compensated inter-frame coding. Then choose the coding mode and the motion vector for the new MB as follows (refer to FIG.  4 ): 
     Step 1: In step  402 , count NumIntra the number of MBs with Mtype[i]=0, i=1, 2, 3, 4. 
     Step 2: In step  404 , check if NumIntra&gt;2. If NumIntra&gt;2, go to step  406  and set Nmtype to 0, then go to END (step  408 ). Otherwise, continue. 
     Step 3: In step  410 , infer the new motion vector as                  (       NMV   H     ,     NMV   V       )     =     (         1   4                       ∑     i   =   1     4                       MV   H          [   i   ]           ,       1   4                       ∑     i   =   1     4                       MV   V          [   i   ]             )       ,           (   1   )                                
     or 
     
       
         ( NMV   H   ,NMV   V )=(median{ MV   H   [i], i =1,2,3,4}, median{ MV   V   [i], i =1,2,3,4}),  (2) 
       
     
     where median represents the median of the elements. Test in step  412  if NMV H =0 and NMV V =0, then in step  414 , set NMtype to 2 and go to END  408 . Otherwise, continue. 
     Step 4: Get six DCT prediction error blocks without MC and count the number of bits B 2  required to encode them in step  416 . 
     Step 5: Do DCT-MC using the motion vector obtained using Eq. (1), and then count the number of bits, B 5 , required to encode the MC prediction error blocks in step  418 . 
     Step 6: Test in step  420  if B 2 &gt;B 5 , then in step  422  set NMtype to 5. Otherwise in step  424 , set NMtype to 2, and NMV H =NMV V =0. Go to END  408 . 
     END: If MBA (MB address) is less than 33, go to the next MB. Otherwise, go to the next GOB. 
     One advantage of this scheme is that we perform DCT-MC only on MBs with NMV H ≠0 or NMV V ≠0 in Step 3. The computational saving can be clearly seen. Another advantage is using brute-force mode determination to achieve the best result with a little additional computation. This is because the DCT coefficients are already obtained and thus there is no need to perform DCT. The brute-force method presented here requires even less complexity than calculating the variance of the error block. 
     After the above process, the resulting DCT coefficients are quantized and variable length coded to generate a QCIF bitstream. 
     Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. The proposed approach can be applied to QCIF to QCIF video bridging for videoconferencing. In addition, it can also be applied to H.263 baseline code with some modifications. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. Details of the structure may be varied substantially without departing from the spirit of the invention and the exclusive use of all modifications, which will come within the scope of the appended claims, is reserved.