A method comprising: receiving an image block unit of a frame, the image block unit comprising samples in color channels comprising at least one chrominance channel and one luminance channel; reconstructing samples of said luminance channels of the image block unit; determining a reference area for predicting target samples of at least one color channel of the image block unit, wherein said reference area comprises one or more of reference samples in a neighboring block in current color channel/frame, in the neighboring of a co-located block in reference color channel/frame; and/or inside the co-located block in reference color channel/frame; determining filter coefficients of a filter for said predicting based the reference samples and a shape of the filter; reconstructing the target samples of at least one color channel of the image block unit using said prediction based on the samples of said reference area and the filter coefficients; computing an autocorrelation matrix for the reconstructed target samples of the image block unit; computing autocorrelation matrices for a plurality of neighboring image blocks units of said image block unit; mixing the autocorrelation matrices of each of the plurality of neighboring image blocks units separately with the autocorrelation matrix of said image block unit; and storing the mixed autocorrelation matrices for said image block unit.