This application claims the benefit, under 35 U.S.C. § 365 of International Application PCT/EP2018/067847, filed Jul. 2, 2018, which was published in accordance with PCT Article 21(2) on Jan. 10, 2019, in English, and which claims the benefit of European Patent Application No. 17305884.3, filed Jul. 6, 2017.

1. TECHNICAL FIELD

The present principles generally relate to a method and a device for picture encoding, and more particularly, to a method and a device for encoding a block of a picture of high dynamic range and wide color gamut.

2. BACKGROUND ART

New generation video formats include wider color gamut, higher frame rate, and higher dynamic range. New standards have been created to support this type of content from capture, through encoding, distribution and up to end user rendering. A transfer function (TF) is used to convert an optical representation of a signal (linear light value) at capture into an integer format compatible with image processing such as encoding. An electro-optical TF (EOTF) converts non-linear values into linear-light values, and the opto-electronic TF (OETF) converts linear-light values into non-linear values for display. For example, ITU-R Recommendation BT.2100 defines two sets of transfer functions namely Perceptual Quantizer (PQ) and Hybrid-Log Gamma (HLG). HLG is a relative transfer function (the OETF applies to normalized linear-light values, and the normalization depends on the actual peak luminance considered for the mastering display or camera). Rec. ITU-R BT.2100 also specifies color primaries for HDR, which actually correspond to the BT.2020 color primaries. To summarize, BT.2100 specifies the format of HDR video signals, that are represented in BT.2020 color primaries, with two possible transfer functions, PQ and HLG.

Besides, it is known that many current HDR content are represented in a PQ Y′CbCr 4:2:0 10-bits format. Therefore, many efforts are being made to optimally encode such signal, for instance using HEVC encoders. Technical Report ISO/IEC 23008-14 (2017), “Conversion and Coding Practices for HDR/WCG Y′CbCr 4:2:0 Video with PQ Transfer Characteristics” discloses solutions for optimal encoding based dedicated codec parameters responsive to luma-dependent local QP adaptation, to signal remapping prior to encoding, and to residual signal remapping inside the encoding process. Typically, the codec parameters are implemented as indexable tables in the encoder or decoder.

However, no specific solution to encode current HDR content represented with an HLG transfer function has been developed. One noticeable difficulty with HLG is that, contrarily to PQ, it is a relative transfer function wherein statistic distribution of different HLG signals may differ noticeably depending if they are natively scene-referred or display-referred, or depending on the peak luminance of the native linear-light signal. It is therefore required to be able to handle these different cases in the HLG coding/decoding process. Therefore, as many sets of codec parameters as different cases HLG are expected for optimally encoding current HDR content represented with an HLG transfer function. It is desirable to optimize the implementation of codec parameters for conversion and coding for HDR/WCG Y′CbCr 4:2:0 Video with HLG Transfer function or any other HDR/WCG representation format. More specifically, an implementation reducing the complexity and memory needs, especially at decoder side, and improving the coding efficiency is desirable.

3. BRIEF SUMMARY

A method for encoding a block of a picture represented in a current format (for instance defined for a PQ or an HLG transfer function, a color gamut and a peak luminance) is disclosed that comprises for at least one sample of the block and for one current component determining a quantization parameter QP responsive to the current format for encoding the block of picture with respect to a default dQP table and a QP offset, the determining using an indication representative of a default dQP table (index_dQP_table) and an indication representative of a QP offset (slice_qp_delta); and encoding the at least one sample of the block into a bitstream using the determined quantization parameter QP.

Advantageously, the disclosed method drastically reduces the set of codec parameters or tables (implemented as QP tables, scaling tables or mapping tables), whatever the PQ or HLG signal characteristics, which advantageously results in a reduction of the required tuning for the encoder and memory needs at encoder and decoder.

A device for encoding a block of a picture represented in a current format comprising means for determining a quantization parameter QP responsive to a current format for encoding the block of picture with respect to a default dQP table and a QP offset, wherein the quantization parameter QP is determined using an indication representative of a default dQP table (index_dQP_table) and an indication representative of a QP offset (slice_qp_delta); and means for encoding the at least one sample of the block of into a bitstream using the determined quantization parameter QP.

In variant, an encoding device is disclosed that comprises a communication interface configured to access a block of a picture represented in a current format and at least one processor configured to:determine a quantization parameter QP responsive to the current format for encoding the block of picture with respect to a default dQP table and a QP offset, wherein the quantization parameter QP is determined using an indication representative of a default dQP table (index_dQP_table) and an indication representative of a QP offset (slice_qp_delta);encoding the at least one sample of the accessed block into a bitstream using the determined quantization parameter QP. That is a transform coefficient for the at least one sample of the accessed block is quantized using the determined quantization parameter QP.

A bitstream representative of a block of a picture represented in a current format is disclosed that comprises:coded data representative of the block of a picture wherein the block is coded into a bitstream using a determined quantization parameter QP responsive to current format and;coded data representative of an indication of a default dQP table (index_dQP_table) and an indication of a QP offset (slice_qp_delta) responsive to the current format.

In a variant, a non-transitory processor readable medium having stored thereon a bitstream representative of a block of a picture represented in a current format is disclosed, wherein the bitstream comprises:coded data representative of the block of picture wherein the block is coded into a bitstream using a determined quantization parameter QP responsive to current format and;coded data representative of an indication of a default dQP table (index_dQP_table) and an indication of a QP offset (slice_qp_delta) responsive to the current format.

A transmitting method is disclosed that comprises:transmitting coded data representative of a block of a picture represented in a current format wherein the block is coded into a bitstream using a determined quantization parameter QP responsive to current format and;transmitting coded data representative of an indication of a default dQP table (index_dQP_table) and an indication of a QP offset (slice_qp_delta).

A transmitting device is disclosed that comprises:means for transmitting coded data representative of a block of a picture represented in a current format wherein the block is coded into a bitstream using a determined quantization parameter QP responsive to current format and;means for transmitting coded data representative of an indication of a default dQP table (index_dQP_table) and an indication of a QP offset (slice_qp_delta).

In a variant, a transmitting device is disclosed that comprises a communication interface configured to access a block of a picture and at least one processor configured to:transmit coded data representative of the accessed block of a picture represented in a current format wherein the accessed block is coded into a bitstream using a determined quantization parameter QP responsive to current format and;transmit coded data representative of an indication of a default dQP table (index_dQP_table) and an indication of a QP offset (slice_qp_delta).

Advantageously, a signaling for indicating which table (or function) among the default ones that are pre-defined, is actually used for the current format of the picture to encode is advantageously disclosed. The signaling may be implicit (the indication is deduced from information on the current format) or explicit (the indication is explicitly coded in the bitstream).

The following embodiments apply to the encoding method, encoding devices, bitstream, processor readable medium, transmitting method and transmitting devices disclosed above.

In a first specific and non-limiting embodiment, the indication representative of a default dQP table (index_dQP_table) and the indication representative of a QP offset (slice_qp_delta) are explicitly encoded into the bitstream.

In a second specific and non-limiting embodiment, the indication representative of a default dQP table (index_dQP_table) is implicitly encoded into the bitstream with VUI information.

In a third specific and non-limiting embodiment, the quantization parameter QP is determined for luma component only.

In a fourth specific and non-limiting embodiment, wherein a format of the video signal data is defined by a color gamut, transfer function and a peak luminance.

A method for decoding a block of a picture represented in a current format is disclosed that comprises:receiving a bitstream representative of a coded block of picture, an indication representative of a default dQP table (index_dQP_table), an indication representative of a QP offset (slice_qp_delta);determining a quantization parameter responsive to the current format based on the default dQP table and the QP offset;decoding the coded block of a picture using the determined QP.

A device for decoding a block of a picture represented in a current format is disclosed that comprises:means for receiving a bitstream representative of a coded block of picture, an indication representative of a default dQP table (index_dQP_table), an indication representative of a QP offset (slice_qp_delta);means for determining a quantization parameter OP responsive to the current format based on the indicated default dQP table and the indicated QP offset;means for decoding the coded block of picture using the determined quantization parameter QP.

In a variant, a decoding device is disclosed that comprises a communication interface configured to access a bitstream and at least one processor configured to:receive a bitstream representative of a coded block of picture, an indication representative of a default dQP table (index_dQP_table), an indication representative of a QP offset (slice_qp_delta);determine a quantization parameter QP responsive to the current format based on the indicated default dQP table and the indicated QP offset;decode the coded block of picture using the determined quantization parameter QP.

Advantageously, a normative implementation in the decoder of the reduced number of dQP tables, covering both PQ and HLG signal representations, results in a reduction of the memory needs for the decoder, and enables to improve coding efficiency to code PQ and HLG content.

According to another aspect, a method for encoding a block of a picture represented in a current format is disclosed that comprises, for at least one sample of the block and for luma and chroma components:determining a scaling function responsive to the current format for encoding the block of picture, the scaling function being applied for deriving the mapping of the luma component and for scaling the chroma components;encoding the block of picture using the derived mapping function applied to the luma component and the determined scaling function applied to the chroma components.

A method for decoding a coded block of a picture represented in a current format is disclosed that comprise:receiving a bitstream representative of a coded block of picture;determining an inverse scaling function responsive to the current format for decoding the coded block of picture, the inverse scaling function being applied for deriving an inverse mapping of the luma component and for scaling the chroma components;decoding the coded block of picture using the derived inverse mapping function applied to the luma component and the determined inverse scaling function applied to the chroma components.

