The invention claimed is:1. A method for video coding, the method comprising:receiving reconstructed chroma samples and related reconstructed luma samples in a colour picture with a target chroma format;applying first ALF (Adaptive Loop Filter) processing to the reconstructed chroma samples for a target reconstructed chroma sample to generate a first filtered chroma sample;applying second ALF processing to the related reconstructed luma samples to generate a second filtered chroma sample for the target reconstructed chroma sample, wherein positions of the related reconstructed luma samples selected for the second ALF processing are determined based on the target chroma format; andgenerating a final filtered chroma sample by combining the first filtered chroma sample and the second filtered chroma sample, wherein the final filtered chroma sample is used as a video decoding output or used for further video encoding or decoding process,wherein the positions of the related reconstructed luma samples selected for the second ALF processing are determined according to a calculation with one or more variables derived based on the target chroma format.2. The method of claim 1, wherein if the target chroma format corresponds to YUV444/YCrCb444, a target position of the related reconstructed luma samples selected for the second ALF processing is at a same position as the target reconstructed chroma sample.3. The method of claim 1, wherein if the target chroma format corresponds to YUV422/YCrCb422 or YUV420/YCrCb420, a target position of the related reconstructed luma samples selected for the second ALF processing is at a pre-defined spatial position among multiple reconstructed luma samples associated with the target reconstructed chroma sample.4. The method of claim 1, wherein the calculation comprises a first term corresponding to x-position of the target reconstructed chroma sample scaled by an x-subsampling factor and a second term corresponding to y-position of the target reconstructed chroma sample scaled by a y-subsampling factor.5. The method of claim 4, wherein the x-subsampling factor is equal to 1 for monochrome and YUV444/YCrCb444, and the x-subsampling factor is equal to 2 for YUV420/YCrCb420 and YUV422/YCrCb422; and wherein the y-subsampling factor is equal to 1 for the monochrome, the YUV422/YCrCb422 and the YUV444/YCrCb444, and the y-subsampling factor is equal to 2 for the YUV420/YCrCb420.6. The method of claim 1, wherein one target related reconstructed luma sample selected for the second ALF processing corresponds to a weighted sum of all corresponding related reconstructed luma samples related to one target reconstructed chroma sample.7. An apparatus for video coding, the apparatus comprising one or more electronic circuits or processors arranged to:receive reconstructed chroma samples and related reconstructed luma samples in a colour picture with a target chroma format;apply first ALF (Adaptive Loop Filter) processing to the reconstructed chroma samples for a target reconstructed chroma sample to generate a first filtered chroma sample;apply second ALF processing to the related reconstructed luma samples to generate a second filtered chroma sample for the target reconstructed chroma sample, wherein positions of the related reconstructed luma samples selected for the second ALF processing are determined according to the target chroma format; andgenerate a final filtered chroma sample by combining the first filtered chroma sample and the second filtered chroma sample, wherein the final filtered chroma sample is used as a video decoding output or used for further video encoding or decoding process,wherein the positions of the related reconstructed luma samples selected for the second ALF processing are determined according to a calculation with one or more variables derived based on the target chroma format.
