The present invention relates to image compression and in particular to variable length coding of quantized transform coefficients.
Variable length coding (VLC) is commonly used in digital video compression. Known transform based compression schemes, for example, divide an image into small blocks, e.g., 8×8 block, transform the blocks (interframe and/or intraframe) using a transform, e.g., the discrete cosine transform (DCT), quantize the resulting coefficients, order the quantized coefficients along a predefined path in the block to form a series of coefficients—sometimes called serializing the coefficients of the block, then statistically encode the series of coefficients using VLC to code the interframe and/or intraframe transform coefficients of each block.
Two-dimensional variable length coding (2D-VLC) is a commonly used VLC technique. In 2D-VLC, each symbol—called an “event”—that is encoded by VLC is not a single coefficient, but combines a run of preceding zero-amplitude coefficients with a non-zero coefficient. That is, each event is a doublet that includes 1) the run length of zero-amplitude coefficients preceding any non-zero coefficient, and 2) the amplitude of that non-zero coefficient. The events are statistically encoded using a variable length code such that the most frequently occurring event is encoded with the shortest codeword, and the least frequently encoded events are encoded with the longest codeword.
2D-VLC is used, for example, in common coding schemes such as ISO/IEC JPEG, MPEG1, MPEG2, and MPEG4 and ITU H.261, H.262, and H.263. 2D-VLC is currently being considered for use in MPEG4-part10 and H.264.
With the advance of integrated circuit technology, computational power and memory are becoming more available. It is therefore becoming feasible to implement variable length coding schemes that provide for more compression than conventional two-dimensional VLC.
Because of the widespread use of image coding, many patents have been issued on different forms of VLC. U.S. Pat. No. 4,698,672 issued Oct. 6, 1987 to Wen-hsiung Chen, one of the inventors of the present invention, for example described one form of a two-dimensional variable length coding method.
Implementing 2D-VLC typically includes modifying the method, for example to reduce the complexity of a hardware implementation. According to one modification, only a subset of the events is encoded using a VLC. Each of a set of less frequently occurring events is encoded with a relatively long, fixed-length codeword, and the other, relatively more frequent events are each encoded with a variable length codeword. An escape code at the beginning of the codeword is used to indicate the fixed-length codewords of the relatively infrequent events.
FIG. 1 shows how a table lookup may be used to implement a 2D-VLC scheme. Prior to the table look up, the runlength of zero amplitudes preceding any non-zero amplitude and the non-zero amplitude are determined. The table look up uses a 2D table for those likely events encoded using variable length encoding. The escape code fixed length codes are used for a set of relatively less likely-to-occur combinations.
In typical 2D-VLC implementations, a short end of block (EOB) code indicates the last non-zero coefficient in the block has been encountered, i.e., that the remaining coefficients in the block are all zero.
Extensions and variations to the common 2D-VLC method are known. For example, the ITU H.263 compression standard defines one such variation sometimes called three-dimensional VLC (3D-VLC). See PCT patent publication WO 9318616 published Sep. 16, 1993 titled PICTURE DATA ENCODING METHOD and also the ITU-T H.263 standard. In 3D-VLC, each symbol (“event”) is a triplet (LAST, RUN, LEVEL) that includes: LAST, a binary flag that indicates whether or not the current non-zero amplitude-value is the last non-zero coefficient in the block, RUN, the run-length of zero-value coefficients that precede the current non-zero amplitude, i.e., the number of zeroes since the last non-zero coefficient amplitude, and LEVEL, the current non-zero coefficient amplitude value. Thus, there is no need for a separate EOB codeword; whether or not the non-zero coefficient is the last one is incorporated into the event.
FIG. 2 shows how a table lookup may be used to implement 3D-VLC.
The existence of patents on 2D-VLC and extensions thereof, and the resulting patent disputes has made a search for alternate VLC methods important.
Thus there is a need for an efficient VLC scheme that can be used as a replacement of known 2D-VLC. There further is a need for an efficient VLC scheme that can provide better compression performance than known 2D-VLC techniques.