Variable length coding method and variable length decoding method

1. A coding method for performing variable-length coding on two-dimensional coefficients of a block which are obtained by performing frequency transformation on picture data of the block that has a predetermined size of pixels, the method comprising:

scanning the two-dimensional coefficients of the block in a predetermined order starting from a low frequency component toward a high frequency component so as to obtain one-dimensional coefficients;

transforming the one-dimensional coefficients into Run values and Level values, wherein each of the Level values is a value of a coefficient having a non-zero value in the one-dimensional coefficients and each of the Run values is the number of successive zero-value coefficients in the one-dimensional coefficients; and

performing variable-length coding on the Run values and the Level values independently from a high frequency component toward a low frequency component using a plurality of variable length code tables for Run and for Level;

wherein each of the plurality of variable length code tables for Level includes a plurality of variable length codes which correspond to the Level values of the coefficients,

wherein said performing variable-length coding on the Level values comprises:

coding a Level value of a first coefficient using a first variable length code table of the plurality of variable length code tables for Level;

determining whether or not an absolute Level value of the first coefficient exceeds a first threshold value by comparing the absolute Level value of the first coefficient to the first threshold value;

switching to a second variable length code table of the plurality of variable length code tables for Level only when it is determined that the absolute Level value of the first coefficient exceeds the first threshold value;

coding a Level value of a second coefficient using the second variable length code table of the plurality of variable length code tables for Level;

determining whether or not an absolute Level value of the second coefficient exceeds a second threshold value by comparing the absolute Level value of the second coefficient to the second threshold value, the second threshold value being greater than the first threshold value;

switching to a third variable length code table of the plurality of variable length code tables for Level only when it is determined that the absolute Level value of the second coefficient exceeds the second threshold value;

coding a Level value of a third coefficient using the third variable length code table of the plurality of variable length code tables for Level;

determining whether or not an absolute Level value of the third coefficient exceeds a third threshold value by comparing the absolute Level value of the third coefficient to the third threshold value, the third threshold value being greater than the second threshold value;

switching to a fourth variable length code table of the plurality of variable length code tables for Level only when it is determined that the absolute Level value of the third coefficient exceeds the third threshold value; and

coding a Level value of a fourth coefficient using the fourth variable length code table of the plurality of variable length code tables for Level,

wherein each of the first threshold value, the second threshold value, and the third threshold value is determined based on a rate of change in code length of the variable length codes included in each of the variable length code tables for Level.

2. A coding apparatus that performs variable-length coding on two-dimensional coefficients of a block which are obtained by performing frequency transformation on picture data of the block that has a predetermined size of pixels, the apparatus comprising:

a scanning unit operable to scan the two-dimensional coefficients of the block in a predetermined order starting from a low frequency component toward a high frequency component so as to obtain one-dimensional coefficients;

a transforming unit operable to transform the one-dimensional coefficients into Run values and Level values, wherein each of the Level values is a value of a coefficient having a non-zero value in the one-dimensional coefficients and each of the Run values is the number of successive zero-value coefficients in the one-dimensional coefficients; and

a variable-length coding unit operable to perform variable-length coding on the Run values and the Level values independently from a high frequency component toward a low frequency component using a plurality of variable length code tables for Run and for Level;

wherein each of the plurality of variable length code tables for Level includes a plurality of variable length codes which correspond to the Level values of the coefficients,

wherein said variable-length coding unit is operable to:

code a Level value of a first coefficient using a first variable length code table of the plurality of variable length code tables for Level; and

determine whether or not an absolute Level value of the first coefficient exceeds a first threshold value by comparing the absolute Level value of the first coefficient to the first threshold value;

switch to a second variable length code table of the plurality of variable length code tables for Level only when it is determined that the absolute Level value of the first coefficient exceeds the first threshold value;

code a Level value of a second coefficient using the second variable length code table of the plurality of variable length code tables for Level;

determine whether or not an absolute Level value of the second coefficient exceeds a second threshold value by comparing the absolute Level value of the second coefficient to the second threshold value, the second threshold value being greater than the first threshold value;

switch to a third variable length code table of the plurality of variable length code tables for Level only when it is determined that the absolute Level value of the second coefficient exceeds the second threshold value;

code a Level value of a third coefficient using the third variable length code table of the plurality of variable length code tables for Level;

determine whether or not an absolute Level value of the third coefficient exceeds a third threshold value by comparing the absolute Level value of the third coefficient to the third threshold value, the third threshold value being greater than the second threshold value;

switch to a fourth variable length code table of the plurality of variable length code tables for Level only when it is determined that the absolute Level value of the third coefficient exceeds the third threshold value; and

code a Level value of a fourth coefficient using the fourth variable length code table of the plurality of variable length code tables for Level,

wherein each of the first threshold value, the second threshold value, and the third threshold value is determined based on a rate of change in code length of the variable length codes included in each of the variable length code tables for Level.