Coding of intra modes

A method of decoding an encoded video image comprising:

• for a block of the encoded video image,
• (a) calculating first gradient values in a causal neighborhood of the block;
• (b) for each of intra prediction directions, calculating second gradient values in the block by extrapolating the first gradient values along the each of intra prediction directions (S120);
• (c) for the each of the intra prediction directions, acquiring an energy value from the second values in the block;
• (d) acquiring an energy value for at least one non-directional intra prediction mode by using the acquired energy values for the intra prediction directions;
• and
• (e) determining, for the block, an intra prediction mode based on said acquired energy values corresponding to directional modes for the intra prediction directions and the at least one non-directional intra prediction mode.

An apparatus for encoding a video image or decoding a decoding an encoded video image, comprising a processor configured to for each one of blocks of the video image,

• (a) calculate first gradient values in a causal neighborhood of the block;
• (b) for each of intra prediction directions, calculate second gradient values in the block by predicting the first gradient values along the each of intra prediction directions (S120);
• (c) for the each of the intra prediction directions, acquire an energy value from the second gradient values in the block;
• (d) acquire an energy value for at least one non-directional intra prediction mode by using the acquired energy values for the prediction directions;
• and
• (e) determine, for the block, an intra prediction mode among directional modes for the intra prediction directions and the at least one non-directional intra prediction mode based on the acquired energy values.

An apparatus for decoding an encoded video image, comprising a processor configured, for a block of the encoded video image, to:

• (a) calculate first gradient values in a causal neighborhood of the block;
• (b) for each of intra prediction directions, calculate second gradient values in the block by extrapolating the first gradient values along the each of intra prediction directions;
• (c) for the each of the intra prediction directions, acquire an energy value from the second values in the block;
• (d) acquire an energy value for at least one non-directional intra prediction mode by using the acquired energy values for the intra prediction directions;
• and
• (e) determine, for the block, an intra prediction mode based on said acquired energy values corresponding to directional modes for the intra prediction directions and the at least one non-directional intra prediction mode.