A method, comprising: receiving reconstructed video data associated with a block boundary between two coding blocks in a video coding system, the block boundary having N lines of samples crossing the block boundary from a P side of the boundary to a Q side of the boundary, the two coding blocks each corresponding to a coding unit, a prediction unit, or a transform unit; determining whether to apply a first filter set to reduce block artifacts at the block boundary, the determining whether to apply the first filter set to reduce block artifacts at the block boundary including: determining whether an intra-side difference of a first line of the N lines of samples is smaller than a first intra-side difference threshold, the intra-side difference of the first line indicating variations of sample values at the P side and Q side of the first line of the N lines of samples, and determining whether to apply the first filter set to two consecutive lines of samples among the N lines of samples according to whether the two consecutive lines of samples each satisfy a same set of conditions, the same set of conditions comprising at least one of an inter-side difference of the respective one of the two consecutive lines of samples is greater than an inter-side difference threshold and an intra-side difference of the respective one of the two consecutive lines of samples is smaller than a second intra-side difference threshold; determining a filter length of a filter in the first filter set based on a first side length (len p) of samples in each line of samples at the P side, and a second side length (len q) of samples in each line of samples at the Q side, the first side length (len p) being a width or height of one of the two coding blocks at the P side, the second side length (len q) being a width or height of one of the two coding blocks at the Q side; and applying at least one filter in the first filter set with the determined filter length on the block boundary. receiving reconstructed video data associated with a block boundary between two coding blocks in a video coding system, the block boundary having N lines of samples crossing the block boundary from a P side of the boundary to a Q side of the boundary, the two coding blocks each corresponding to a coding unit, a prediction unit, or a transform unit; determining whether to apply a first filter set to reduce block artifacts at the block boundary, the determining whether to apply the first filter set to reduce block artifacts at the block boundary including: determining whether an intra-side difference of a first line of the N lines of samples is smaller than a first intra-side difference threshold, the intra-side difference of the first line indicating variations of sample values at the P side and Q side of the first line of the N lines of samples, and determining whether to apply the first filter set to two consecutive lines of samples among the N lines of samples according to whether the two consecutive lines of samples each satisfy a same set of conditions, the same set of conditions comprising at least one of an inter-side difference of the respective one of the two consecutive lines of samples is greater than an inter-side difference threshold and an intra-side difference of the respective one of the two consecutive lines of samples is smaller than a second intra-side difference threshold; determining whether an intra-side difference of a first line of the N lines of samples is smaller than a first intra-side difference threshold, the intra-side difference of the first line indicating variations of sample values at the P side and Q side of the first line of the N lines of samples, and determining whether to apply the first filter set to two consecutive lines of samples among the N lines of samples according to whether the two consecutive lines of samples each satisfy a same set of conditions, the same set of conditions comprising at least one of an inter-side difference of the respective one of the two consecutive lines of samples is greater than an inter-side difference threshold and an intra-side difference of the respective one of the two consecutive lines of samples is smaller than a second intra-side difference threshold; determining a filter length of a filter in the first filter set based on a first side length (len p) of samples in each line of samples at the P side, and a second side length (len q) of samples in each line of samples at the Q side, the first side length (len p) being a width or height of one of the two coding blocks at the P side, the second side length (len q) being a width or height of one of the two coding blocks at the Q side; and applying at least one filter in the first filter set with the determined filter length on the block boundary.