1. A method of Intra coding for a depth block in a three-dimensional coding system, the method comprising: receiving input data associated with a current depth block, wherein the current depth block is partitioned into one or more segments; receiving reconstructed neighboring depth samples of the current depth block; deriving a prediction value for each segment based on the reconstructed neighboring depth samples directly without deriving depth prediction samples or depth prediction subsamples of each segment, wherein the prediction value is derived for each segment based on a function of at least two reconstructed neighboring depth samples adjacent to a depth sample of the current depth block, and wherein the prediction value for each segment is derived based on an average of a first reconstructed neighboring depth sample and a second reconstructed neighboring depth sample; and encoding or decoding each segment of the current depth block using the prediction value, wherein the current depth block is coded using simplified depth coding (SDC) Planar mode having one segment and the prediction value, P for said one segment is derived according to P=(X+Y)>>1, wherein X corresponds to a first reconstructed neighboring depth sample diagonally across from a lower-left corner depth sample of the current depth block and Y corresponds to a second reconstructed neighboring depth sample diagonally across from an upper-right corner depth sample of the current depth block.