Methods for determining a prediction value, an encoder, and a decoder are provided. Reconstructed values of neighboring samples of a current block are acquired, and then filtered to obtain a reference value set of the current block. When a size of the current block is smaller than a preset threshold value, a first constant value is calculated according to a bit depth value of a luma component of a sample in the current block. A difference between the first constant value and a first reference value in the reference value set is determined as a first prediction input value in a prediction input value set. Other prediction input values in the prediction input value set other than the first prediction input value are determined according to the reference value set. Prediction values of samples at specific positions in the current block is calculated and then filtered.