Image-dependent contrast and brightness control for HDR displays
Methods and systems to adjust differently brightness and contrast for dark and bright pictures on a display are provided. Given a tone-mapping curve mapping an input dynamic range to a display comprising a minimum and maximum display luminance value, the maximum display luminance value is lowered to an adjusted luminance value according to user defined parameters. The input dynamic range is tone-mapped to the display dynamic range using the adjusted luminance value. For brightness control, the tone mapped image is stretched linearly back to the maximum display luminance value. For contrast control, a gamma or power EOTF of the display is adjusted according to the adjusted luminance. For displays with global backlight control, the global backlight is adjusted only when contrast is adjusted.
1. A computer-implemented method for providing an image-dependent brightness control based on a user-defined value of a peak luminance adjustment parameter to reduce brightness in bright image portions relatively stronger than in dark image portions, the method comprising:
receiving an input image in an image dynamic range;
receiving a minimum display luminance value and a maximum display luminance value defining a display dynamic range of a target display;
generating ( 420 ) a first adjusted luminance value for the target display based on the peak luminance adjustment parameter and the maximum display luminance value, wherein the first adjusted luminance value is lower than the maximum display luminance value;
generating ( 430 ) a tone-mapped image with a tone-mapping function and the input image, wherein the tone-mapping function maps the image dynamic range to the minimum display luminance value and the first adjusted luminance value; and
generating ( 415 ) an output image for the target display based on the tone-mapped image; wherein generating the first adjusted luminance value (Tmax Adj1 ) comprises computing
T
max
Adj
1
=
T
max
*
1
2
β
,
wherein Tmax denotes the maximum display luminance value and β denotes the peak luminance adjustment parameter.
2. The method of claim 1 , wherein the peak luminance adjustment parameter is between 0 and 1.
3. The method of claim 1 , further comprising adjusting global backlight dimming for the target display based on the first adjusted luminance value, and the tone-mapping function maps the image dynamic range to an adjusted minimum display luminance value and the first adjusted luminance value.
4. The method of claim 1 , wherein for a target display with a gamma or power electro-optical transfer function (EOTF), before applying the EOTF, an input to the EOTF is normalized by a delta factor comprising a difference of the minimum display luminance value from the maximum display luminance value when there is no backlight dimming adjustment or a difference of an adjusted minimum luminance value from the first adjusted luminance value when there is backlight dimming adjustment.
5. The method of claim 4 , wherein given a color component (RGB) of the input to the EOTF image, generating a color component (RGB′) of an EOTF output comprises computing:
RGB
′
=
R
G
B
T
max
-
T
min
1
/
γ
if there is no global backlight dimming adjustment on the target display, and computing
RGB
′
=
R
G
B
T
max
Adj
1
-
T
min
Adj
1
/
γ
.
if there is global backlight dimming adjustment on the target display, wherein γ denotes the gamma or power factor, Tmin denotes the minimum display luminance value, Tmax denotes the maximum display luminance value, Tmax Adj1 denotes the first adjusted luminance value, and Tmin Adj denotes the adjusted minimum luminance value due to the global backlight dimming adjustment.
6. The method of claim 1 , wherein the method is further based on a user-defined value of an average picture level boost adjustment parameter to increase brightness in dark image portions relatively stronger than in bright image portions, the method further comprising:
generating ( 440 ) a second adjusted luminance value for the target display based on the average picture level boost adjustment parameter and the first adjusted luminance value, wherein the second adjusted luminance value is lower than the maximum display luminance value;
generating ( 455 ) the tone-mapped image with the tone-mapping function and the input image, wherein the tone-mapping function maps the image dynamic range to the minimum display luminance value and the second adjusted luminance value; and
generating ( 460 ) a first output image for the target display by applying a linear mapping to the tone-mapped image, wherein parameters of the linear mapping are based on the minimum display luminance value, the first adjusted luminance value, and the second adjusted luminance value;
wherein generating the second adjusted luminance value (Tmax Adj2 ) comprises computing
T
max
Adj
2
=
T
max
Adj
1
*
1
2
a
,
wherein Tmax Adj1 denotes the first adjusted luminance value and α denotes the average picture level boost adjustment parameter.
7. The method of claim 6 , wherein for an RGB color component in the tone-mapped image applying the linear mapping to generate a color component (RGB o ) in the output image comprises computing
RGB o =RGB* m−b,
wherein,
m
=
T
max
Adj
1
-
T
min
T
max
Adj
2
-
T
min
and
b
=
T
min
*
T
max
Adj
1
-
T
min
2
T
max
Adj
2
-
T
min
-
T
min
denote the parameters of the linear mapping, Tmin denotes the minimum display luminance value, Tmax Adj1 denotes the first adjusted luminance value, and Tmax Adj2 denotes the second adjusted luminance value.
8. A non-transitory computer-readable storage medium having stored thereon computer-executable instructions for executing with one or more processors the method of claim 1 .
9. An apparatus comprising a processor and configured to perform the method of claim 1 .
10. A computer-implemented method for providing an image-dependent brightness control based on a user-defined value of an average picture level boost adjustment parameter to increase brightness in dark image portions relatively stronger than in bright image portions, the method comprising:
receiving an input image in an image dynamic range;
receiving a minimum display luminance value and a maximum display luminance value defining a display dynamic range of a target display;
generating ( 405 ) an adjusted maximum luminance value for the target display based on the average picture level boost adjustment parameter, wherein the adjusted maximum luminance value is lower than the maximum display luminance value;
generating ( 410 ) a tone-mapped image with a tone-mapping function and the input image, wherein the tone-mapping function maps the image dynamic range to the minimum display luminance value and the adjusted maximum luminance value; and
generating ( 415 ) an output image for the target display by applying a linear mapping to the tone-mapped image, wherein parameters of the linear mapping are based on the minimum display luminance value, the maximum display luminance value, and the adjusted maximum luminance value;
wherein generating the adjusted maximum luminance value (Tmax Adj ) comprises computing
T
max
Adj
=
T
max
*
1
2
α
,
wherein Tmax denotes the maximum display luminance value and α denotes the average picture level boost adjustment parameter;
wherein for an RGB color component in the tone-mapped image applying the linear mapping to generate a color component (RGB 0 ) in the output image comprises computing
RGB o =RGB* m−b,
wherein
m
=
T
max
-
T
min
T
max
Adj
-
T
min
and
b
=
T
min
*
T
max
-
T
min
2
T
max
Adj
-
T
min
-
T
min
denote the parameters of the linear mapping, Tmin denotes the minimum display luminance value, Tmax denotes the maximum display luminance value, and Tmax Adj denotes the adjusted maximum luminance value.
11. The method of claim 10 , wherein the average picture level boost adjustment parameter is between 0 and 1.