Provided is an image encoding method. The method includes: determining a first reference sample value of an encoding block according to a pixel sample value in the encoding block; determining an intra prediction mode of the encoding block according to the first reference sample value, and calculating an intra prediction value of the encoding block; obtaining a prediction difference parameter according to original values of the encoding block and the intra prediction value; and encoding the first reference sample value, the intra prediction mode, and the prediction difference parameter, and writing encoded bits into a bitstream. Further provided are an image decoding method, an encoder, a decoder, and a computer storage medium.