Image coding method, image decoding method, image coding apparatus, image decoding apparatus, and image coding-decoding apparatus
An image coding method includes: generating a predicted block; calculating a residual block; calculating quantized coefficients by performing transform and quantization on the residual block; calculating a coded residual block by performing inverse quantization and inverse transform on the quantized coefficients; generating a temporary coded block; determining whether or not an offset process is required, to generate first flag information indicating a result of the determination; executing the offset process on the temporary coded block when it is determined that the offset process is required; and performing variable-length coding on the quantized coefficients and the first flag information.
1. An image coding method of coding an input block included in an image, the image coding method comprising:
generating a predicted block by predicting the input block;
calculating a residual block by subtracting the predicted block from the input block;
calculating quantized coefficients by performing transform and quantization on the residual block;
calculating a coded residual block by performing inverse quantization and inverse transform on the quantized coefficients;
generating a temporary coded block by adding the coded residual block to the predicted block;
determining whether or not an offset process for correcting an error included in the temporary coded block is required, to generate first flag information indicating a result of the determination, the error being caused by the quantization in the calculating of quantized coefficients;
executing the offset process on the temporary coded block when it is determined in the determining that the offset process is required; and
performing variable-length coding on the quantized coefficients and the first flag information.
2. The image coding method according to claim 1 ,
wherein the offset process is executed to add an offset value to a value of a pixel included in the temporary coded block,
in the determining, whether an offset value for a previously-coded block adjacent to the input block or the offset value newly calculated for the temporary coded block is used in the offset process to be executed on the temporary coded block is further determined to generate second flag information indicating a result of the determination,
in the executing, the offset process is executed on the temporary coded block using the offset value indicated by the second flag information, and
in the performing, variable-length coding is further performed on the second flag information.
3. The image coding method according to claim 1 ,
wherein, in the executing, the offset process is executed selectively on a pixel (i) that is one of pixels included in the temporary coded block and (ii) that corresponds to a pixel included in the input block and having a value included in a predetermined range where subjective color distortion is apparent.
4. The image coding method according to claim 3 ,
wherein, in the determining, when each of values of all pixels included in the input block is outside the predetermined range, it is determined that the offset process is not required to be executed on the temporary coded block that corresponds to the input block.
5. The image coding method according to claim 1 ,
wherein each of the values of the pixels included in the input block is expressed in a YUV format.
6. The image coding method according to claim 1 ,
wherein the image coding method (i) switches between a coding process based on a first standard and a coding process based on a second standard, (ii) performs the determining, the executing, and the performing, as the coding process based on the first standard, and (iii) codes an identifier indicating a standard of a coding process.
7. An image coding apparatus that codes an input block included in an image, the image coding apparatus comprising:
a prediction unit configured to generate a predicted block by predicting the input block;
a calculation unit configured to calculate a residual block by subtracting the predicted block from the input block;
a transform-quantization unit configured to calculate quantized coefficients by performing transform and quantization on the residual block;
an inverse quantization-transform unit configured to calculate a coded residual block by performing inverse quantization and inverse transform on the quantized coefficients;
a generation unit configured to generate a temporary coded block by adding the coded residual block to the predicted block;
a determination unit configured to determine whether or not an offset process for correcting an error included in the temporary coded block is required, to generate first flag information indicating a result of the determination, the error being caused by the quantization performed by the transform-quantization unit;
an offset processing unit configured to execute the offset process on the temporary coded block when it is determined by the determination unit that the offset process is required; and
a variable-length coding unit configured to perform variable-length coding on the quantized coefficients and the first flag information.