TECHNICAL FIELD

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

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 transform efficiency.

Still another technical aspect of the present disclosure is to provide an image coding method and an image coding apparatus which are based on a reduced secondary transform (RST).

Yet another technical aspect of the present disclosure is to provide a method and an apparatus for increasing the efficiency of a secondary transform by changing the array of transform coefficients according to an intra prediction mode.

Still another technical aspect of the present disclosure is to provide an image coding method and an image coding apparatus for increasing the efficiency of a secondary transform by optimizing the transformation kernel matrix applied to the secondary transform.

Still another technical aspect of the present disclosure is to provide an image coding method and an image coding apparatus which are based on a transform set for increasing coding efficiency.

According to an embodiment of the present disclosure, there is provided an image decoding method performed by a decoding apparatus. The method may include: deriving transform coefficients through dequantization based on quantized transform coefficients for a target block; deriving modified transform coefficients based on an inverse reduced secondary transform (RST) using a preset transform kernel matrix for the transform coefficients; deriving residual samples for the target block based on an inverse primary transform for the modified transform coefficients; and generating a reconstructed picture based on the residual samples for the target block, wherein the deriving the modified transform coefficients derive the modified transform coefficients of the top-left 4×4 region, the top-right 4×4 region and the bottom-left 4×4 region of the 8×8 region by applying the transform kernel matrix to the transform coefficients of the top-left 4×4 region of the 8×8 region of the target block.

When performing a matrix operation between the transform coefficients of the top-left 4×4 region of the 8×8 region and the transform kernel matrix, the transform coefficients of the top-left 4×4 region of the 8×8 region are one-dimensionally arranged according to a forward diagonal scanning order.

The transform coefficients arranged one-dimensional are two-dimensionally arranged the top-left 4×4 region, the top-right 4×4 region and the bottom-left 4×4 region of the 8×8 region according to a row-first direction or a column-first direction corresponding to the intra prediction mode applied to the target block after the matrix operation with the transform kernel matrix.

According to another embodiment of the present disclosure, there is provided a decoding apparatus for performing image decoding. The decoding apparatus may include: an entropy decoder to derive quantized transform coefficients for a target block and information on prediction from a bitstream; a predictor to generate a prediction sample for the target block based on the information on prediction; a dequantizer to derive transform coefficients through dequantization based on the quantized transform coefficients for the target block; an inverse transformer to include an inverse reduced secondary transformer (RST) that derives modified transform coefficients based on inverse RST of the transform coefficients and an inverse primary transformer that derives residual samples for the target block based on first inverse transform of the modified transform coefficients; and an adder to generate reconstructed samples based on the residual samples and the prediction samples, wherein the inverse reduced secondary transformer derive the modified transform coefficients in the top-left 4×4 region of the 8×8 region by applying the transform kernel matrix to the transform coefficients of the top-left 4×4 region, the top-right 4×4 region and the bottom-left 4×4 region of the 8×8 region of the target block.

According to still 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 based on an intra prediction mode applied to a target block; deriving residual samples for the target block based on the prediction samples; deriving transform coefficients for the target block based on a primary transform for the residual samples; deriving modified transform coefficients based on a reduced secondary transform (RST) for the transform coefficients; and deriving quantized transform coefficients by performing quantization based on the modified transform coefficients, wherein the deriving of the modified transform coefficients one-dimensionally may include arranging the transform coefficients according to one of a row-first direction or a column-first direction according to the intra prediction mode applied to the target block and applying the RST to the transform coefficients.

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 generated according to an image encoding method performed by an encoding apparatus.

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 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 the efficiency of a secondary transform by changing the array of transform coefficients according to an intra prediction mode.

According to the present disclosure, it is possible to increase image coding efficiency by performing image coding based on a transform set.

According to the present disclosure, it is possible to increase the efficiency of a secondary transform by optimizing the transformation kernel matrix applied to the secondary transform.

