TECHNICAL FIELD

The present disclosure relates to an image coding technique and, more particularly, to a method and an apparatus for coding an image based on transform in an image coding system.

RELATED ART

Nowadays, the demand for high-resolution and high-quality images/videos such as 4K, 8K or more ultra high definition (UHD) images/videos has been increasing in various fields. As the image/video data becomes higher resolution and higher quality, the transmitted information amount or bit amount increases as compared to the conventional image data. Therefore, when image data is transmitted using a medium such as a conventional wired/wireless broadband line or image/video data is stored using an existing storage medium, the transmission cost and the storage cost thereof are increased.

Further, nowadays, the interest and demand for immersive media such as virtual reality (VR), artificial reality (AR) content or hologram, or the like is increasing, and broadcasting for images/videos having image features different from those of real images, such as a game image is increasing.

Accordingly, there is a need for a highly efficient image/video compression technique for effectively compressing and transmitting or storing, and reproducing information of high resolution and high quality images/videos having various features as described above.

SUMMARY

A technical aspect of the present disclosure is to provide a method and an apparatus for increasing image coding efficiency.

Another technical aspect of the present disclosure is to provide a method and an apparatus for increasing efficiency in transform index coding.

Still another technical aspect of the present disclosure is to provide an image coding method and apparatus using LFNST.

Still another technical aspect of the present disclosure is to provide an image coding method and apparatus for zero-out performed when LFNST is applied.

According to an embodiment of the present disclosure, there is provided an image decoding method performed by a decoding apparatus. The method may include: obtaining residual information from a bitstream; deriving transform coefficients for a current block based on the residual information; determining whether a significant coefficient exists in a second region other than a first region at the top-left of the current block; parsing an LFNST index from the bitstream when the significant coefficient does not exist in the second region; deriving modified transform coefficients by applying an LFNST matrix derived based on the LFNST index to the transform coefficients in the first region; deriving residual samples for the current block based on an inverse primary transform for the modified transform coefficients; and generating a reconstructed picture based on the residual samples for the current block.

Here, the first region is derived based on the size of the current block, when the size of the current block is 4×4 or 8×8, the first region is from the top-left of the current block to the 8th sample position in a scan direction, and when the size of the current block is not 4×4 or 8×8, the first region is a 4×4 area at the top-left of the current block.

The scan direction is a diagonal scan direction.

A predetermined number of the modified transform coefficients are derived based on the size of the current block, when the height and width of the current block are greater than or equal to 8, 48 modified transform coefficients are derived, and when the width and height of the current block are 4 or more and the width or height of the current block is less than 8, 16 modified transform coefficients are derived.

The 48 modified transform coefficients are arranged in the top-left, top-right, and bottom-left 4×4 areas within the top-left 8×8 area of the current block.

The 16 modified transform coefficients are arranged in the top-left 4×4 area of the current block.

According to another embodiment of the present disclosure, there is provided an image encoding method performed by an encoding apparatus. The method may include: deriving prediction samples for a current block; deriving residual samples for the current block based on the prediction sample; deriving transform coefficients for the current block based on a primary transform for the residual samples; deriving modified transform coefficients for the current block based on the transform coefficients of a first region at the top-left of the current block and a predetermined LFNST matrix; zeroing out a second region of the current block in which the modified transform coefficients do not exist; and encoding residual information derived through quantization of the modified transform coefficients and an LFNST index indicating the LFNST matrix.

According to still another embodiment of the present disclosure, there may be provided a digital storage medium that stores image data including encoded image information and a bitstream generated according to an image encoding method performed by an encoding apparatus.

According to yet another embodiment of the present disclosure, there may be provided a digital storage medium that stores image data including encoded image information and a bitstream to cause a decoding apparatus to perform the image decoding method.

According to the present disclosure, it is possible to increase overall image/video compression efficiency.

According to the present disclosure, it is possible to increase efficiency in transform index coding.

A technical aspect of the present disclosure may provide an image coding method and apparatus using LFNST.

A technical aspect of the present disclosure may provide an image coding method and apparatus for zero-out performed when LFNST is applied.

The effects that can be obtained through specific examples of the present disclosure are not limited to the effects listed above. For example, there may be various technical effects that a person having ordinary skill in the related art can understand or derive from the present disclosure. Accordingly, specific effects of the present disclosure are not limited to those explicitly described in the present disclosure and may include various effects that can be understood or derived from the technical features of the present disclosure.

