Chroma reshaping of HDR video signals
Methods for the chroma reshaping of high-dynamic range (HDR) signals are presented. For each input pixel, a first scaler is generated based on the pixel luminance and a luma-based chroma reshaping (LCR) function. A second scaler is generated based on a saturation-based chroma reshaping (SCR) function and the saturation value of the pixel. A cascaded scaler is generated by multiplying the first scaler with the second scaler. Reshaped chroma values are generated by multiplying the input chroma values with the cascaded scaler. The characteristics of preferred LCR and SCR functions are discussed. Inverse decoding methods based on chroma-reshaping metadata that define the LCR and SCR functions are also described.
1. In a video encoder, a method to reshape chroma components of a high dynamic range image, the method comprising:
receiving an input high dynamic range image comprising pixels, wherein each pixel is coded in the IC T C P color format and comprises a luma component I, a first chroma component C T and a second chroma component C p ; and
for a pixel in the input high dynamic range image:
generating a first scaler as a function of the luma component of the pixel, wherein the function of the luma component comprises a bell-shaped function;
generating the colour saturation of the pixel as a function of the first and second chroma components according to the function Sat=√{square root over (C T 2 +C P 2 )};
generating a second scaler as a piecewise, monotonically decreasing function of the colour saturation of the pixel; and
generating, as first and second reshaped chroma components for the pixel, i) the multiplicative product of the first chroma component, the first scaler and the second scaler and ii) the multiplicative product of the second chroma component, the first scaler and the second scaler.
2. The method of claim 1 , wherein the function suitable for generating values of the first scaler is suitable for generating a highest value of the first scaler for luma values in the range 100 to 1000 nits.
3. The method of claim 1 , wherein the bell-shaped function comprises a generalized Gaussian function of the form
s
lcr
=
R
*
e
-
(
a
bs
(
lo
g
10
(
PQ
2
L
(
I
)
+
0.001
)
-
u
)
1.414
*
σ
)
β
+
(
1
-
R
)
,
where I denotes the input luma value of the pixel in the ICtCp color format, PQ2L( )denotes the SMPTE 2084 transfer function to convert normalized PQ values to luminance, R denotes a range, u denotes a mean, β denotes steepness, and σ adenotes a width.
4. The method of claim 1 , wherein the piecewise, monotonically decreasing function is such that the second scaler is greater than 1 in a lowest-saturation interval of the colour saturation of the pixel.
5. The method of claim 4 , wherein the piecewise, monotonically decreasing function is such that the second scaler is less than 1 in another interval of the colour saturation of the pixel.
6. The method of claim 1 , further comprising generating reshaping metadata.
7. The method of claim 6 , wherein the reshaping metadata comprises at least one parameter of the function of at least the luma component of the pixel.
8. The method of claim 6 , wherein the reshaping metadata comprises at least one parameter of the function of the colour saturation of the pixel.
9. In a decoder, a method to perform inverse reshaping of chroma components of a high dynamic range image, the method comprising:
receiving a reshaped high dynamic range image comprising pixels, wherein each pixel comprises a luminance component I, a first reshaped chroma component CT ccr and a second chroma reshaped component CP ccr ; and
for a pixel in the reshaped high dynamic range image:
generating a first inverse scaler as a function of the luma component of the pixel, by applying, to the luma component of the pixel, the inverse of a function that generates values of a first forward scaler;
generating the reshaped colour saturation of the pixel as a function of the first and second reshaped chroma components according to the function Sat ccr =√{square root over (CT ccr 2 +CP ccr 2 )};
generating a second inverse scaler as a function of the reshaped colour saturation of the pixel; and
generating, as first and second inverse reshaped chroma components in the IC T C P color format for the pixel, i) the multiplicative product of the first reshaped chroma component, the first inverse scaler and the second inverse scaler and ii) the multiplicative product of the second reshaped chroma component, the first inverse scaler and the second inverse scaler.
10. The method of claim 9 , wherein the function suitable for generating values of the first forward scaler is suitable for generating a highest value of the first forward scaler for luma values in the range 100 to 1000 nits.
11. The method of claim 9 , wherein the function for generating values of the first forward scaler comprises a bell-shaped function.
12. An apparatus comprising a processor and configured to perform the method of claim 1 .
13. A non-transitory computer-readable storage medium having stored thereon computer-executable instruction for executing a method with one or more processors in accordance with claim 1 .