1. A method of evaluating an effect of chroma downsampling in a compression process of an input image, comprising: computing, using a CDS evaluation module, an error for a target CDS block based on characteristics of DCT coefficients in U and/or V planes of a respective CDS candidates group in the input image, and further based on a diversity of the DCT coefficients in the U and/or V planes of the respective CDS candidates group in the input image; and computing, using said CDS evaluation module, an estimated perceptual effect of CDS over the input image based on a plurality of target CDS blocks error values, wherein said characteristics include an average of DCT coefficients in the U and/or V planes of the respective CDS candidates group.