A method for decoding a picture, performed by a decoding apparatus, the method comprises: deriving neighboring chroma reference samples for a chroma block; deriving neighboring luma reference samples of a luma block related to the chroma block and luma samples in the luma block; deriving down-sampled neighboring luma reference samples and down-sampled luma samples by down-sampling the neighboring luma reference samples and the luma samples; deriving linear model parameters based on the down-sampled neighboring luma reference samples and the neighboring chroma reference samples; generating prediction samples for the chroma block based on the linear model parameters and the down-sampled luma samples of the luma block; and reconstructing the chroma block based on the prediction samples for the chroma block, wherein the neighboring luma reference samples includes top neighboring luma reference samples located at the upper side of a top boundary of the luma block, and left neighboring luma reference samples located at the left side of a left boundary of the luma block, and wherein the down-sampled neighboring luma reference samples includes a down-sampled top neighboring luma reference sample and a down-sampled left neighboring luma reference sample, characterized in that, when the luma block is located in a current coding tree unit, CTU, that is different from a CTU in which the top neighboring luma reference samples are located, three top neighboring luma reference samples among the top neighboring luma reference samples in one horizontal sample line adjacent to the top boundary of the luma block and six left neighboring luma samples among the left neighboring luma reference samples in two horizontal sample lines neighboring the left boundary of the luma block are used for deriving the down-sampled neighboring luma reference samples. deriving neighboring chroma reference samples for a chroma block; deriving neighboring luma reference samples of a luma block related to the chroma block and luma samples in the luma block; deriving down-sampled neighboring luma reference samples and down-sampled luma samples by down-sampling the neighboring luma reference samples and the luma samples; deriving linear model parameters based on the down-sampled neighboring luma reference samples and the neighboring chroma reference samples; generating prediction samples for the chroma block based on the linear model parameters and the down-sampled luma samples of the luma block; and reconstructing the chroma block based on the prediction samples for the chroma block, wherein the neighboring luma reference samples includes top neighboring luma reference samples located at the upper side of a top boundary of the luma block, and left neighboring luma reference samples located at the left side of a left boundary of the luma block, and wherein the down-sampled neighboring luma reference samples includes a down-sampled top neighboring luma reference sample and a down-sampled left neighboring luma reference sample, characterized in that, when the luma block is located in a current coding tree unit, CTU, that is different from a CTU in which the top neighboring luma reference samples are located, three top neighboring luma reference samples among the top neighboring luma reference samples in one horizontal sample line adjacent to the top boundary of the luma block and six left neighboring luma samples among the left neighboring luma reference samples in two horizontal sample lines neighboring the left boundary of the luma block are used for deriving the down-sampled neighboring luma reference samples.