obtaining reconstruction samples by summing prediction samples and residual samples;

decoding filter information;

classifying the reconstruction samples in a coding tree block into classes on a per block classification unit basis;

assigning block class indices to the coding tree block having the reconstruction samples classified into the classes on a per block classification unit basis; and

applying a filtering on the coding tree block having the reconstruction samples classified into the classes on a per block classification unit basis by using the filter information and the block class indices,

wherein the block class indices are determined according to directionality information and activity information,

wherein at least one of the directionality information and the activity information is determined on the basis of a gradient value with respect to at least one of a vertical direction, a horizontal direction, a first directional direction, and a second directional direction,

wherein the gradient value is obtained using a one-dimensional Laplacian operation for the block classification unit,

wherein the one-dimensional Laplacian operation is performed only for a specific sample position included in the block classification unit, and

wherein a horizontal position and a vertical position of the specific sample are both even or both odd based on each block classification unit.