A variable length coding method is comprised of: a coefficient value scanning step in which an RL sequence generation unit, a reordering unit, and a binarization unit scan coefficient values within a block in a predetermined scanning order starting at a higher-frequency component toward a lower-frequency component; and an arithmetic coding step in which an arithmetic coding unit and a table storage unit perform arithmetic coding on the absolute values of the coefficient values according to the scanning order used in the coefficient value scanning step, by switching between probability tables for use, wherein, in the arithmetic coding step, a probability table to be used is switched to another probability table in one direction, when the arithmetic-coded absolute values of the coefficient values include an absolute value exceeding a predetermined threshold value.

The present disclosure generally relates to a method and device for inverse-tone mapping a picture. The method comprising: —obtaining (20) a first component (Y) comprising: —obtaining a luminance component (L) from said color picture; —obtaining a resulting component by applying (20), a non-linear function on said luminance component (L) in order that the dynamic of the resulting component is increased compared to the dynamic of the luminance component (L)—obtaining (50) a modulation value (Ba) from the luminance of said color picture; —obtaining the first component (Y) by multiplying said resulting component by said modulation value (Ba); —obtaining two chrominance components (C1, C2) from said color picture; —obtaining (40) a first factor (r(L(i))) that depends on the value (L(i)) of a pixel (i) of said luminance component (L); —obtaining (30) at least one color component (Ec) from said first component (Y), said two chrominance components (C1, C2) and said first factor (r(L(i))); and—forming the inverse-tone mapped color picture by combining together said at least one color component (Ec).

The present disclosure generally relates to a method and device for inverse-tone mapping a picture. The method comprising: —obtaining (20) a first component (Y) comprising: —obtaining a luminance component (L) from said color picture; —obtaining a resulting component by applying (20), a non-linear function on said luminance component (L) in order that the dynamic of the resulting component is increased compared to the dynamic of the luminance component (L)—obtaining (50) a modulation value (Ba) from the luminance of said color picture; —obtaining the first component (Y) by multiplying said resulting component by said modulation value (Ba); —obtaining two chrominance components (C1, C2) from said color picture; —obtaining (40) a first factor (r(L(i))) that depends on the value (L(i)) of a pixel (i) of said luminance component (L); —obtaining (30) at least one color component (Ec) from said first component (Y), said two chrominance components (C1, C2) and said first factor (r(L(i))); and—forming the inverse-tone mapped color picture by combining together said at least one color component (Ec).

For obtaining an good yet easy to use luminance dynamic range conversion, we describe an image color processing apparatus (200) arranged to transform an input color (R,G,B) of a pixel of an input image (Im_in) having a first luminance dynamic range into an output color (Rs, Gs, Bs) of a pixel of an output image (Im_res) having a second luminance dynamic range, which first and second dynamic ranges differ in extent by at least a multiplicative factor 2, comprising: a maximum determining unit (101) arranged to calculate a maximum (M) of color components of the input color, the color components at least comprising a red, green and blue component; —a uniformization unit (201) arranged to apply a function (FP) to the maximum (M) as input, which function has a logarithmic shape and was predetermined to be of a fixed shape enabling to transform a linear input to a more perceptually uniform output variable (u); a function application unit (203) arranged to receive a functional shape of a function, which was specified previously by a human color grader, and apply the function to the uniform output variable (u), yielding a transformed uniform value (TU); a linearization unit (204) arranged to transform the transformed uniform value (TU) to a linear domain value (LU); a multiplication factor determination unit (205) arranged to determine a multiplication factor (a) being equal to the linear domain value (LU) divided by the maximum (M); and a multiplier (104) arranged to multiply at least three linear color components (R,G,B) by the multiplication factor (a), yielding the output color.

For obtaining an good yet easy to use luminance dynamic range conversion, we describe an image color processing apparatus (200) arranged to transform an input color (R,G,B) of a pixel of an input image (Im_in) having a first luminance dynamic range into an output color (Rs, Gs, Bs) of a pixel of an output image (Im_res) having a second luminance dynamic range, which first and second dynamic ranges differ in extent by at least a multiplicative factor 2, comprising: a maximum determining unit (101) arranged to calculate a maximum (M) of color components of the input color, the color components at least comprising a red, green and blue component; —a uniformization unit (201) arranged to apply a function (FP) to the maximum (M) as input, which function has a logarithmic shape and was predetermined to be of a fixed shape enabling to transform a linear input to a more perceptually uniform output variable (u); a function application unit (203) arranged to receive a functional shape of a function, which was specified previously by a human color grader, and apply the function to the uniform output variable (u), yielding a transformed uniform value (TU); a linearization unit (204) arranged to transform the transformed uniform value (TU) to a linear domain value (LU); a multiplication factor determination unit (205) arranged to determine a multiplication factor (a) being equal to the linear domain value (LU) divided by the maximum (M); and a multiplier (104) arranged to multiply at least three linear color components (R,G,B) by the multiplication factor (a), yielding the output color.

An intra-frame encoding method, an intra-frame decoding method, an encoder, and a decoder, where the intra-frame encoding method includes obtaining reference pixel values of a current picture block obtaining a predicted value of a first-type pixel in the current picture block according to the reference pixel values of the current picture block using an intra-frame prediction algorithm, obtaining residual values of the first-type pixel, obtaining transform coefficients according to the residual values of the first-type pixel, quantizing the transform coefficients, reconstructing the first-type pixel according to the quantized transform coefficients and the predicted value of the first-type pixel, and obtaining a reconstructed value of the second-type pixel according to the reconstructed value of the first-type pixel using an interpolation method. Hence, flexible encoding can be implemented, and encoding efficiency can be improved.

An intra-frame encoding method, an intra-frame decoding method, an encoder, and a decoder, where the intra-frame encoding method includes obtaining reference pixel values of a current picture block obtaining a predicted value of a first-type pixel in the current picture block according to the reference pixel values of the current picture block using an intra-frame prediction algorithm, obtaining residual values of the first-type pixel, obtaining transform coefficients according to the residual values of the first-type pixel, quantizing the transform coefficients, reconstructing the first-type pixel according to the quantized transform coefficients and the predicted value of the first-type pixel, and obtaining a reconstructed value of the second-type pixel according to the reconstructed value of the first-type pixel using an interpolation method. Hence, flexible encoding can be implemented, and encoding efficiency can be improved.

In a system for coding high dynamic range (HDR) images using lower-dynamic range (LDR) images, a reshaping function allows for a more efficient distribution of the codewords in the lower dynamic range images for improved compression. A trim pass of the LDR images by a colorist may satisfy a director's intent for a given “look,” but may also result in unpleasant clipping artifacts in the reconstructed HDR images. Given an original forward reshaping function which maps HDR luminance values to LDR pixel values, a processor identifies areas of potential clipping and generates modified forward and backward reshaping functions to reduce the visibility of potential artifacts from the trim pass process while preserving the director's intent.

In a system for coding high dynamic range (HDR) images using lower-dynamic range (LDR) images, a reshaping function allows for a more efficient distribution of the codewords in the lower dynamic range images for improved compression. A trim pass of the LDR images by a colorist may satisfy a director's intent for a given “look,” but may also result in unpleasant clipping artifacts in the reconstructed HDR images. Given an original forward reshaping function which maps HDR luminance values to LDR pixel values, a processor identifies areas of potential clipping and generates modified forward and backward reshaping functions to reduce the visibility of potential artifacts from the trim pass process while preserving the director's intent.

Techniques are provided to encode and decode image data comprising a tone mapped (TM) image with HDR reconstruction data in the form of luminance ratios and color residual values. In an example embodiment, luminance ratio values and residual values in color channels of a color space are generated on an individual pixel basis based on a high dynamic range (HDR) image and a derivative tone-mapped (TM) image that comprises one or more color alterations that would not be recoverable from the TM image with a luminance ratio image. The TM image with HDR reconstruction data derived from the luminance ratio values and the color-channel residual values may be outputted in an image file to a downstream device, for example, for decoding, rendering, and/or storing. The image file may be decoded to generate a restored HDR image free of the color alterations.