1. A method for luma-based chroma intra-prediction in a video encoder or a video decoder, the method comprising: down sampling a first reconstructed luma block of a largest coding unit (LCU); computing parameters α and β of a linear model using immediate top neighboring reconstructed luma samples and left neighboring reconstructed luma samples of the first reconstructed luma block and reconstructed neighboring chroma samples of a chroma block corresponding to the first reconstructed luma block, wherein the linear model is Pred C [x,y]=α·Rec L ′[x,y]+β, wherein x and y are sample coordinates, Pred C is predicted chroma samples, and Rec L ′ is samples of the down sampled first reconstructed luma block, and wherein the immediate top neighboring reconstructed luma samples are the only top neighboring reconstructed luma samples used; and computing samples of a first predicted chroma block from corresponding samples of the down sampled first reconstructed luma block using the linear model and the parameters.