  In pixel-by-pixel spatial prediction methods for lossless intra coding, the
prediction is obtained by a weighted sum of neighbouring pixels. The proposed
prediction approach in this paper uses a weighted sum of three neighbor pixels
according to a two-dimensional correlation model. The weights are obtained
after a three step optimization procedure. The first two stages are offline
procedures where the computed prediction weights are obtained offline from
training sequences. The third stage is an online optimization procedure where
the offline obtained prediction weights are further fine-tuned and adapted to
each encoded block during encoding using a rate-distortion optimized method and
the modification in this third stage is transmitted to the decoder as side
information. The results of the simulations show average bit rate reductions of
12.02% and 3.28% over the default lossless intra coding in HEVC and the
well-known Sample-based Angular Prediction (SAP) method, respectively.
