Image coding and decoding system using variable length coding and variable length decoding
An image coding apparatus provides a run-length encoding unit RLE 1 that subjects quantized coefficients obtained by quantizing frequency components of an image signal to a variable length coding process by using a run value Run that indicates the number of successive zero coefficients and a level value Lev that indicates a value of a non-zero coefficient following the zero coefficients. The run-length encoding unit RLE 1 includes a reordering unit Lreodr for reordering level values Lev; a variable length coder LVLC for coding reordered level values ROLev by using a code table selected according to the value of a quantization parameter QP; a reordering unit Rreodr for reordering run values Run from high frequency component of the quantized coefficients to low frequency component; and a variable length coder RVLC for coding reordered run values RORun by using a code table selected according to the number of already-processed run values.
1. An image coding and decoding system which includes an image coding apparatus which codes an image signal of a picture and an image decoding apparatus which decodes a coded image signal of a picture,
wherein said image coding apparatus comprises:
a frequency transformation unit operable to frequency-transform a block image into a two-dimensional array of transform coefficients of a frequency component;
a run-level transforming unit operable to transform the two-dimensional array of transform coefficients of a frequency component into a run value and a level value;
a level value coding unit operable to code the level value; and
a run value coding unit operable to code the run value,
wherein the run value coding unit includes:
a number calculation unit operable to calculate a number of zero-value transform coefficients with a frequency component which is not higher than a frequency component corresponding to a zero-value transform coefficient related to the run value to be coded;
a table selecting unit operable to select a code table in accordance with the number of zero-value transform coefficients; and
a variable length coding unit operable to variable length code the run value by using the selected code table,
wherein said image decoding apparatus comprises:
a receiving unit operable to receive coded data including a coded run value and a coded level value, wherein the coded run value is obtained by coding a run value which indicates the number of continuous transform coefficients, each having a zero value, and the coded level value is obtained by coding a level value which indicates a value of a transform coefficient having a non-zero value;
a run value decoding unit operable to decode the coded run value to obtain a decoded run value;
a level value decoding unit operable to decode the coded level value to obtain a decoded level value; and
an inverse scanning unit operable to obtain a two-dimensional array of transform coefficients of the block image from the decoded level value and the decoded run value; and
an inverse frequency transformation unit operable to inverse frequency transform the two-dimensional array of transform coefficients of the block image into a decoded block image,
wherein the run value decoding unit includes:
a number calculation unit operable to calculate a number of zero-value transform coefficients which have not been decoded;
a table selecting unit operable to select a code table in accordance with the number of zero-value transform coefficients which have not been decoded; and
a variable length decoding unit operable to variable length decode the coded run value from a high frequency component toward a low frequency component by using the selected code table.
2. The image coding and decoding system according to claim 1 ,
wherein the coding of an image signal of a picture is performed on a block by block basis and the decoding of a coded image signal of a picture is performed on a block by block basis.