Methods and apparatuses for creating code mapping functions for encoding an HDR image, and methods and apparatuses for use of such encoded images
To enable better encoding of the currently starting to appear high dynamic range images for use in full high dynamic range technical systems (containing an HDR display, and e.g. in an HDR grading application of a HDR movie), we invented a method of encoding a high dynamic range image, comprising the steps of: —inputting pixel colors of an input high dynamic range image, wherein the pixel colors have information of a luminance and a chromaticity; —applying an inverse of a mapping function to derive a luma code (v) of the luminance of a pixel color, which mapping function is predetermined as comprising a first partial function which is defined as (I), in which rho is a tuning constant, and v is the luma code corresponding to a luminance to be encoded, and a second partial mapping defined as L=L m P γ in which Lm is a peak luminance of a predefined reference display, and gamma is a constant which is preferably equal to 2.4, —outputting a matrix of pixels having a color encoding comprising the luma codes.
1. An encoding method, comprising the acts of:
inputting pixel colors of an input high dynamic range video image, wherein the pixel colors have information of a luminance and a chromaticity;
applying an inverse of a mapping function, to derive a luma code (v) of the luminance of a pixel color, which mapping function is predetermined as comprising a first partial function which is defined as
P
=
(
ρ
v
-
1
ρ
-
1
)
,
in which rho is a tuning constant, and v is the luma code corresponding to a luminance to be encoded, and a second partial mapping defined as L=L m P γ in which Lm is a peak luminance of a predefined reference display, and gamma is a constant which is preferably equal to 2.4,
outputting a matrix of pixels having a color encoding comprising the luma codes.
2. The method of claim 1 in which Lm equals 5000 nit.
3. The method of claim 1 in which rho equals 25.
4. The method of claim 1 in which the gamma function is composed of an equivalent gamma of a Rec. 709 encoding gamma and a gamma 2.4 function.
5. The method of claim 1 in which the parameters rho and gamma, are further optimized to yield an encoded image which looks good according to a human color grader on a 100 nit display, whereby the at least one of the parameters rho and gamma is preferably optimized by a human grader.
6. The method of claim 1 , in which chromaticity coordinates (u,v) of the color encoding are defined referenced from a CIE XYZ representation of the colors of pixels in the high dynamic range image (HDR_ORIG) by fractional equations of the type:
u
=
a
X
+
b
Y
+
c
Z
d
X
+
e
Y
+
f
Z
and
v
=
g
X
+
h
Y
+
i
Z
j
X
+
k
Y
+
l
Z
,
with a . . . l constants, and preferably with values: a=4, b=c=0, d=1, e=15, f=3, h=9, g=i=0, j=1, k=15, l=3.
7. The method of claim 1 , in which the chromaticity coordinates (u,v) are defined relative to a predetermined white point such as preferably D 65 .
8. The method of claim 1 , in which an image signal (S_im) is formed which comprises a pixel matrix image with pixel colors encoded with one color component being the luma code, and associated therewith metadata comprising at least one of the rho and gamma parameter.
9. A video encoding apparatus, comprising:
an input for obtaining pixel colors of an input high dynamic range video image, wherein the pixel colors have information of a luminance and a chromaticity;
a grading managing unit arranged for applying an inverse of a mapping function to derive a luma code (v) of the luminance of a pixel color, which mapping function is predetermined as comprising a first partial function which is defined as
P
=
(
ρ
v
-
1
ρ
-
1
)
,
in which rho is a tuning constant, and v is the luma code corresponding to a luminance to be encoded, and a second partial mapping defined as L=L m P γ which is a gamma transform, and wherein Lm is a peak luminance of a predefined reference display, and gamma is a constant which is preferably equal to 2.4,
an encoder connected to a video transmission connection, connectable to a video memory or network, arranged to encode and transmit an image signal S_im comprising a pixel matrix image with pixel colors encoded with one color component being the luma code, and associated therewith metadata comprising at least one of the rho and gamma parameter.
10. The video encoding apparatus of claim 9 comprising a user interface unit allowing a human grader to select a particular value of rho and/or gamma.
11. The A video encoding apparatus as of claim 9 comprising an automatic image analysis unit arranged to determine a particular value of rho and/or gamma based upon a statistical analysis of the luminances of objects present in at least one of the high dynamic range images (HDR_ORIG).
12. The video encoding apparatus of claim 9 in which the grading managing unit is arranged to determine chromatic components of the pixels of the high dynamic range image (HDR_ORIG) as:
u
=
a
X
+
b
Y
+
c
Z
d
X
+
e
Y
+
f
Z
and
v
=
g
X
+
h
Y
+
i
Z
j
X
+
k
Y
+
l
Z
,
with a . . . l constants, and preferably with values: a=4, b=c=0, d=1, e=15, f=3, h=9, g=i=0, j=1, k=15, l=3.
13. A video decoding apparatus comprising:
a receiving and formatting unit arranged to receive high dynamic range image encoding (S_im) and derive therefrom an image encoding (Im_1) comprising luma codes, resulting from an encoding method of claim 1 , to be processed;
a color mapping unit arranged to apply a color mapping strategy to derive from the image encoding Im_1 a high dynamic range image (REC_HDR), wherein the color mapping unit is arranged to apply on pixel lumas v in the image encoding (Im_1) a predetermined mapping function defined as comprising a first partial function which is
P
=
(
ρ
v
-
1
ρ
-
1
)
,
in which rho is a tuning constant, and v is the luma code corresponding to a luminance to be encoded, and a second partial mapping defined as L=L m P γ in which Lm is a peak luminance of a predefined reference display, and gamma is a constant which is preferably equal to 2.4 to obtain luminances L of pixels of the high dynamic range image (REC_HDR).
14. The video decoding apparatus of claim 13 wherein:
the receiving and formatting unit is arranged to derive from the high dynamic range image encoding (S_im) at least one of the parameters rho, gamma, or Lm.
15. The video decoding apparatus of claim 9 , in which the color mapping unit is further arranged to receive chromaticity coordinates (u,v) for the pixel colors, and to apply a transformation to map together with the information of the luminances the u and v components of the pixel colors of the image Im_1 to a universal color representation like e.g. a CIE XYZ color representation, or to a device dependent color representation like RGB.
16. The video decoding apparatus of claim 9 in which the color mapping unit is further arranged to apply a second color mapping strategy using further color mapping parameters co-encoded as metadata specifying a color mapping to an image with a dynamic range different from the dynamic range defined by the high dynamic range image (REC_HDR).
17. A display comprising the video decoding apparatus of claim 9 .
18. A method of video decoding, comprising:
receiving an encoded high dynamic range video of images (S_im) and deriving therefrom an image encoding (Im_1) to be processed, and
color mapping by applying a color mapping strategy to derive from the image encoding (Im_1) a high dynamic range image (REC_HDR), wherein the color mapping unit is arranged to apply on pixel lumas v in the image encoding (Im_1) a predetermined mapping function defined as comprising a first partial function which is
P
=
(
ρ
v
-
1
ρ
-
1
)
,
in which rho is a tuning constant, and v is the luma code corresponding to a luminance, and a second partial mapping defined as L=L m P γ which is a gamma transform, and wherein Lm is a peak luminance of a predefined reference display, and gamma is a constant which is preferably equal to 2.4 to obtain luminances L of pixels of the high dynamic range image (REC_HDR).