Intra prediction mode derivation for chrominance values

A video decoding method where luminance and chrominance samples of a 4:2:2 format video signal are predicted from other respective reference samples according to a prediction direction associated with a current sample to be predicted, the chrominance samples having a lower horizontal and/or vertical sampling rate than the luminance samples so that a ratio of luminance horizontal resolution to chrominance horizontal resolution is different to a ratio of luminance vertical resolution to chrominance vertical resolution so that a block of luminance samples has a different aspect ratio to a corresponding block of chrominance samples, the method comprising:

• detecting, by circuitry, a first prediction direction defined in relation to a first grid of a first aspect ratio in respect of a set of current samples to be predicted;
• and
• applying a direction mapping to the prediction direction so as to generate a second prediction direction defined in relation to a second grid of a different aspect ratio, wherein
• the first grid, used to detect the first prediction direction, is defined in respect of sample positions of luminance samples, and
• the second grid, used to generate the second prediction direction, is defined in respect of sample positions of chrominance samples.

A video coding method where luminance and chrominance samples are predicted from other respective reference samples according to a prediction direction associated with a current sample to be predicted, the chrominance samples having a lower horizontal and/or vertical sampling rate than the luminance samples so that a ratio of luminance horizontal resolution to chrominance horizontal resolution is different to a ratio of luminance vertical resolution to chrominance vertical resolution so that a block of luminance samples has a different aspect ratio to a corresponding block of chrominance samples, the method comprising:

• providing the luminance samples and the chrominance samples as a 4:2:2 format video signal;
• detecting, by circuitry, a first prediction direction defined in relation to a first grid of a first aspect ratio in respect of a set of current samples to be predicted;
• and
• applying a direction mapping to the prediction direction so as to generate a second prediction direction defined in relation to a second grid of a different aspect ratio, wherein
• the first grid, used to detect the first prediction direction, is defined in respect of sample positions of luminance samples, and
• the second grid, used to generate the second prediction direction, is defined in respect of sample positions of chrominance samples.

A video decoding apparatus comprising:

• circuitry configured to:
• process luminance and chrominance samples of a 4:2:2 format video signal which are predicted from other respective reference samples according to a prediction direction associated with a current sample to be predicted, the chrominance samples having a lower horizontal and/or vertical sampling rate than the luminance samples so that a ratio of luminance horizontal resolution to chrominance horizontal resolution is different to a ratio of luminance vertical resolution to chrominance vertical resolution so that a block of luminance samples has a different aspect ratio to a corresponding block of chrominance samples;
• detect a first prediction direction defined in relation to a first grid of a first aspect ratio in respect of a set of current samples to be predicted;
• and
• apply a direction mapping to the prediction direction so as to generate a second prediction direction defined in relation to a second grid of a different aspect ratio, wherein
• the first grid, used to detect the first prediction direction, is defined in respect of sample positions of luminance samples, and
• the second grid, used to generate the second prediction direction, is defined in respect of sample positions of chrominance samples.

A video coding apparatus comprising:

• circuitry configured to:
• process luminance and chrominance samples which are predicted from other respective reference samples according to a prediction direction associated with a current sample to be predicted, the chrominance samples having a lower horizontal and/or vertical sampling rate than the luminance samples so that a ratio of luminance horizontal resolution to chrominance horizontal resolution is different to a ratio of luminance vertical resolution to chrominance vertical resolution so that a block of luminance samples has a different aspect ratio to a corresponding block of chrominance samples;
• receive the luminance samples and the chrominance samples as a 4:2:2 format video signal;
• detect a first prediction direction defined in relation to a first grid of a first aspect ratio in respect of a set of current samples to be predicted;
• and
• apply a direction mapping to the prediction direction so as to generate a second prediction direction defined in relation to a second grid of a different aspect ratio, wherein
• the first grid, used to detect the first prediction direction, is defined in respect of sample positions of luminance samples, and
• the second grid, used to generate the second prediction direction, is defined in respect of sample positions of chrominance samples.

A video decoding apparatus comprising:

• circuitry configured to:
• process luminance and chrominance samples of a 4:2:2 format video signal which are predicted from other respective reference samples according to a prediction direction associated with a current sample to be predicted, the chrominance samples having a lower horizontal and/or vertical sampling rate than the luminance samples so that a ratio of luminance horizontal resolution to chrominance horizontal resolution is different to a ratio of luminance vertical resolution to chrominance vertical resolution so that a block of luminance samples has a first aspect ratio which is different to a second aspect ratio for a corresponding block of chrominance samples;
• detect a first prediction direction for a current chrominance sample defined in relation the first grid of a first aspect ratio in respect of a set of current chrominance samples to be predicted;
• and
• apply a direction mapping to the prediction direction so as to generate a second prediction direction defined in relation to a second grid of the second aspect ratio, wherein
• the first grid, used to detect the first prediction direction, is defined in respect of sample positions of luminance samples, and
• the second grid, used to generate the second prediction direction, is defined in respect of sample positions of chrominance samples.