Adaptive transfer function for video encoding and decoding

Inventors

Tourapis, Alexandros • Singer, David

Assignees

Apple Inc

Abstract

A video encoding and decoding system that implements an adaptive transfer function method internally within the codec for signal representation. A focus dynamic range representing an effective dynamic range of the human visual system may be dynamically determined for each scene, sequence, frame, or region of input video. The video data may be cropped and quantized into the bit depth of the codec according to a transfer function for encoding within the codec. The transfer function may be the same as the transfer function of the input video data or may be a transfer function internal to the codec. The encoded video data may be decoded and expanded into the dynamic range of display(s). The adaptive transfer function method enables the codec to use fewer bits for the internal representation of the signal while still representing the entire dynamic range of the signal in output.

Core Innovation

The described invention provides a video encoder that accesses a plurality of frames of source video data and a plurality of transfer functions used to map a full dynamic range of the source video data to a focus range of luminance values of the source video data. For each frame, the encoder selects a particular transfer function and determines particular luminance values of the frame using the focus range of luminance values and the particular transfer function. Different transfer functions are used corresponding to the plurality of frames, and the resulting particular luminance values are encoded into encoded video data.

The invention further generates format metadata providing information on the focus range luminance values and the plurality of transfer functions, and then generates a bit stream comprising the encoded video data and the format metadata. The format metadata is used to provide information needed for interpreting which focus range luminance values and which transfer functions are associated with the encoded data. The approach is framed around mapping from a full dynamic range to a focus range of luminance values.

In additional refinements described in the document, the transfer functions include operations such that source video data is mapped to a focus range and corresponding encoded video data representation uses a different number of bits. A decoder is described that decodes the encoded video data from the bit stream using a third number of bits different from the source and encoded representations. The document also describes using different transfer functions across frame groupings or for adjacent frames, and having the format metadata provide information identifying one or more portions of a transfer function used for encoding.

Claims Coverage

The independent claims identified are clm-00001, clm-00009, and clm-00016. Across these independent claims, the coverage centers on frame-by-frame selection of a transfer function to map full dynamic range source video data to focus range luminance values, encoding those luminance values, and generating a bit stream that includes encoded video data and format metadata describing the focus range and the transfer functions; further independent-claim-adjacent refinements add bit-count relations for source/encoded/decoder representations and constrain transfer-function selection across frames, including adjacent frames, with metadata identifying transfer-function portions.

Overall claim coverage is directed to selecting transfer functions for frames to determine focus-range luminance values mapped from a full dynamic range, encoding those luminance values, and signaling the focus range and transfer-function information via format metadata within a bit stream. Additional disclosed refinements include maintaining different bit-count relationships among source, encoded, and decoder representations, constraining transfer-function usage across frame subsets or adjacent frames, and using metadata to identify which portions of a transfer function were used.

Stated Advantages

Documented Applications