A method in a decoding process for determining full-resolution chroma pixel information (Cx) corresponding to a spatial fraction of a still-image or a video-frame represented by full-resolution luma pixel information (Y) and decimated chroma pixel information (Cxd) decimated by a decimation process, including: receiving the full-resolution luma pixel information at video or image processing apparatus; decimating, at the video or image processing apparatus, the full-resolution luma pixel information (Y) by said decimation process resulting in a decimated spatial luma fraction (Yd); determining, with the video or image processing apparatus, if the decimated chroma pixel information (Cxd) at least approximately can be expressed by {(Yd+shift1)*scale−shift2}; storing, in an electronic memory of the video or image processing apparatus, values of scale, shift1, and shift2 that result in a minimum deviation between {(Yd+shift1)*scale−shift2} and Cxd; and calculating, with the video or image processing apparatus, {(Y+shift1)*scale−shift2} as a first candidate (Cx1) for the full-resolution chroma pixel information (Cx).