Local illumination compensation for video encoding and decoding using stored parameters

#This application claims the benefit, under 35 U.S.C. § 371 of Patent Cooperation Treaty Application No. PCT/US2019/050510, filed Sep. 11, 2019, which was published in accordance with PCT Article 21(2) on Mar. 26, 2020, in English, and which claims the benefit of European Patent Application No. 18306215.7, filed Sep. 19, 2018 and European Patent Application No. 18306261.1, filed Sep. 26, 2018.

#At least one of the present embodiments generally relates to the field of video compression. At least one embodiment particularly aims at Local Illumination Compensation and more particularly to using a stored set of parameters for the Local Illumination Compensation.

#To achieve high compression efficiency, image and video coding schemes usually employ prediction and transform to leverage spatial and temporal redundancy in the video content. Generally, intra or inter prediction is used to exploit the intra or inter frame correlation, then the differences between the original block and the predicted block, often denoted as prediction errors or prediction residuals, are transformed, quantized, and entropy coded. To reconstruct the video, the compressed data are decoded by inverse processes corresponding to the entropy coding, quantization, transform, and prediction.

#FIG. 1 illustrates a block diagram of an example of video encoder 100, such as a High Efficiency Video Coding (HEVC) encoder.

#FIG. 2 illustrates a block diagram of an example of video decoder 200, such as an HEVC decoder.

#FIG. 3 illustrates a block diagram of an example of a system in which various aspects and embodiments are implemented.

#Various embodiments relate to a LIC system that uses memory for storing illumination compensation (IC) parameters and does not require access to reconstructed pixels of neighboring blocks. In at least one of the embodiments, a set of LIC parameters is stored in a dedicated buffer, which is of limited size, and which is decoupled from the coding unit level storage of information. The buffer contains a set of LIC parameters, which may be, for example, computed (or determined in some other manner) on the fly by the decoder (and the encoder as well) or determined beforehand (for example obtained from the video signal or from a device such as an encoder).

#With regards to the coding syntax of a coding unit (CU), a flag indicates the use of illumination compensation function for Coding Units where LIC can be applied. In addition, according to at least one embodiment, a LIC parameter index is signaled to indicate which LIC parameters, from the considered buffer or LIC parameter set, are used for a current CU. In the case of on-the-fly computed LIC parameters, the LIC parameter set of at least one embodiment consists of the LIC parameters associated with the right edge part of one or more previously coded CUs. Such parameters are computed or determined in some other manner according to a vertical right-hand sub-part of one or more coded/decoded CUs. The set of parameters gathers LIC parameters over a limited spatial height, typically equal to the height of a coding tree unit (CTU). In another embodiment, an additional buffer may also store the LIC parameters associated with a horizontal lower part of a coded/decoded CU inside the current CTU. Advantageously, this embodiment, combined with the previous one, may increase the coding efficiency at the cost of a limited memory load increase. Another form of on-the-fly LIC parameters computation may consist in computing the illumination change in a current CU being coded/decoded and storing the resulting LIC parameters in a First-In-First-Out (FIFO) buffer. This buffer serves as a LIC parameter set for one or more subsequent CUs in the considered slice. In at least one embodiment using this form of on-the-fly LIC parameter computer for LIC in a CU, an index indicating the LIC parameters used in the FIFO is signaled in the bit-stream. Additionally, this FIFO concept typically comes with a policy for updating/maintaining the LIC parameters contained in the FIFO, which is detailed later in this document. Another embodiment uses a set of LIC parameters determined beforehand. LIC parameters determined beforehand can be defined prior to the use of the application and may not change. LIC parameters determined beforehand can also be adjustable so that they can be changed and thus may be obtained before their use and updated during their use. In at least one embodiment an index signals the LIC parameters determined beforehand to be used during the temporal prediction, for at least one CU where the use of LIC is signaled as true.

#Moreover, the present aspects, although describing principles related to particular drafts of VVC (Versatile Video Coding) or to HEVC (High Efficiency Video Coding) specifications, are not limited to VVC or HEVC, and can be applied, for example, to other standards and recommendations, whether pre-existing or future-developed, and extensions of any such standards and recommendations (including VVC and HEVC). Unless indicated otherwise, or technically precluded, the aspects described in this application can be used individually or in combination.