Quantization parameter prediction using luminance information

In an image and/or video decoder, a method for luma-based quantization parameter (QP) prediction using a processor, the method comprising:

• receiving a coded bitstream comprising syntax elements related to QP values encoded by an encoder;
• examining a first flag in the bitstream denoting whether luma-based QP prediction is enabled or not, and upon determining that luma-based QP prediction is enabled:
• examining a second flag in the bitstream denoting whether a custom QP mapping look-up-table is included or not in the bitstream, wherein a QP mapping look-up-table maps a function of block-luminance values into delta QP value, and upon detecting that a custom QP mapping look-up-table is included, reading the custom QP mapping look-up-table using a set of QP mapping look-up-table syntax elements including a set of QP mapping look-up-table syntax parameters in the bitstream;
• based on the value of the second flag, selecting a decoding delta QP mapping look-up-table as either a default QP mapping look-up-table or the custom QP mapping look-up-table;
• generating a delta QP value based on the decoding delta QP mapping look-up-table, wherein generating the delta QP value further comprises:
• determining a method to reconstruct the delta QP value according to a value of a third flag in the bitstream denoting one or more types of QP prediction based on the custom QP mapping look-up-table or the default QP mapping look-up-table and luminance values in coded blocks in the bitstream, wherein the one or more types of QP prediction comprise:
• for a first value of the third flag, a default type, wherein the delta QP value is extracted from the bitstream;
• for a second value of the third flag, a first prediction type, wherein the delta QP value for a current block in the bitstream is generated based on the decoding delta QP mapping look-up-table and a function of luminance values in a prediction block used to decode the current block;
• and
• for a third value of the third flag, a second prediction type, wherein the delta QP value for the current block in the bitstream is generated based on the decoding delta QP mapping look-up-table, a function of luminance values in the prediction block used to decode the current block, and functions of luminance values in one or more neighbor blocks to the current block;
• and
• based on the reconstructed delta QP value, generating a QP value to decode a coded block in the bitstream by adding the delta QP value to a base QP value, wherein the base QP value denotes a value known by both, encoder and decoder.

In an image and/or video encoder, a method for generating a coded bitstream using a processor, the bitstream supporting luma-based quantization parameter (QP) prediction, the method comprising:

• setting a first flag to a value denoting that luma-based QP prediction is enabled in the bitstream;
• setting a second flag to a value denoting whether luma-based QP prediction utilizes default QP mapping look-up-tables or a custom QP mapping look-up-table, wherein a QP mapping-look-up table maps a function of block-luminance values into delta QP values;
• and
• setting values to a set of QP mapping look-up-table syntax parameters included in a set of QP mapping look-up-table syntax elements so that a decoder can read the custom QP mapping look-up-table when the custom QP mapping look-up-table is included in the bitstream,
• setting a third flag to a value denoting one or more types of QP prediction based on the custom QP mapping look-up-table or a default QP mapping look-up-table and luminance values in coded blocks in the bitstream for reconstructing a delta QP value to generate a QP value by adding the reconstructed delta QP value to a base QP value, wherein the base QP value denotes a value known by both, encoder and decoder, and wherein the one or more types of QP prediction comprise:
• a default type, wherein the delta QP value is embedded in the bitstream;
• a first prediction type, wherein the delta QP value for a current block in the bitstream is generated based on a decoding delta QP mapping look-up-table and a function of luminance values in a prediction block used to decode the current block;
• and
• a second prediction type wherein the delta QP value for the current block in the bitstream is generated based on the decoding delta QP mapping look-up-table, a function of luminance values in the prediction block used to decode the current block, and functions of luminance values in one or more neighbor blocks to the current block.

An apparatus for image and/or video processing comprising a bitstream stored on a non-transitory machine-readable media, wherein the bitstream is characterized by image data in a compressed format and syntax elements for determining quantization parameter (QP) values, wherein the syntax elements for determining QP values comprise:

• a first flag denoting whether luma-based prediction of QP values is enabled or not;
• a second flag denoting whether a custom QP mapping look-up-table is included in the bitstream or not, wherein a QP mapping-look-up table maps a function of block-luminance values into delta QP values;
• and
• a set of QP mapping look-up-table syntax elements including a set of QP mapping look-up-table syntax parameters allowing a decoder to read the custom QP mapping look-up-table when the second flag denotes that the custom QP mapping look-up-table is included in the bitstream,
• wherein the syntax elements for determining QP values further comprise a third flag denoting one or more types of QP prediction based on the custom QP mapping look-up-table or a default QP mapping look-up-table and luminance values of coded blocks in the bitstream for reconstructing a delta QP value to generate a QP value by adding the reconstructed delta QP value to a base QP value, wherein the base QP value denotes a value known by both, encoder and decoder, and wherein the one or more types of QP prediction comprise:
• a default type, wherein the delta QP value is embedded in the bitstream;
• a first prediction type, wherein the delta QP value for a current block in the bitstream is generated based on a decoding delta QP mapping look-up-table and a function of luminance values in a prediction block used to decode the current block;
• and
• a second prediction type wherein the delta QP value for the current block in the bitstream is generated based on the decoding delta QP mapping look-up-table, a function of luminance values in the prediction block used to decode the current block, and functions of luminance values in one or more neighbor blocks to the current block.