Tone mapping method
Generating by a digital processing device, of a first digital image from a second digital image, by: for each pixel of the second image, determining a pixel luminance; dividing the interval ranging from the lowest to the highest luminance into a plurality of sub-intervals; and determining the value of at least one pixel of the first image by multiplying the value of a pixel of the second image by a gain determined by interpolation by taking into account the distance of the pixel luminance of the second image to the limits of the sub-interval containing this luminance.
1. A method, comprising:
generating, using a configured digital processing device, a first digital image from a second digital image, by:
determining a luminance value of each pixel of the second image;
dividing a luminance interval ranging from a lowest determined luminance value to a highest determined luminance of the pixels of the second image into a plurality of sub-intervals; and
determining a luminance value of at least one pixel of the first image by multiplying the determined luminance value of a pixel of the second image by a gain determined by interpolation based on a distance of the luminance value of the pixel of the second image to limits of a sub-interval containing the determined luminance value of the pixel of the second image.
2. The method of claim 1 , comprising generating a first histogram H representative of a distribution of luminance values in the second image.
3. The method of claim 2 wherein the first histogram comprises a number of discrete values smaller than a total number of available discrete values for luminance values of the second image.
4. The method of claim 2 , comprising determining a second cumulative histogram based on the first histogram.
5. The method of claim 2 wherein dividing the luminance interval into sub-intervals comprises dichotomous division in which, at each step of division of an initial interval or sub-interval ranging from a value L min to a value L max :
an intermediate value β of the interval or sub-interval is determined, such that:
∑
k
=
L
min
β
H
(
k
)
=
∑
k
=
β
L
max
H
(
k
)
,
where k designates luminance values for which a histogram value H(k) has been calculated; and
a cut point C of the initial interval or sub-interval is determined, such that:
C
=
L
max
+
L
min
2
+
α
(
β
-
L
max
+
L
min
2
)
where α designates a control parameter greater than or equal to 0 and smaller than or equal to 1.
6. The method of claim 5 wherein control parameter α ranges between 0.4 and 0.6.
7. The method of claim 1 wherein a number of sub-intervals into which said interval is divided is smaller than an available number of luminance values in the luminance interval.
8. The method of claim 1 wherein the first and second images are mosaic color images, the luminance value of each pixel represents a luminance level of a single color of a color base.
9. The method of claim 8 wherein pixels of the first and second images are grouped in elementary macroblocks of pixels having a same base color distribution pattern.
10. The method of claim 9 wherein the luminance value determined for a pixel of the second image is proportional to a weighted average of luminance values of pixels of a macroblock containing the pixel.
11. The method of claim 9 wherein a same gain is applied to all pixels of a macroblock of the second image.
12. The method of claim 1 wherein luminance values of pixels of the first image are coded over a smaller number of bits than luminance values of pixels of the second image.
13. A device, comprising:
a memory; and
image processing circuitry, which, inoperation,
determines a luminance value of each pixel of a digital image;
divides a luminance interval between a lowest determined luminance value and a highest determined luminance value of the pixels of the digital image into a plurality of sub-intervals; and
determines a luminance value of at least one pixel of an output image based on a determined luminance value of a pixel of the digital image and a gain, wherein the gain is determined by interpolation based on a distance of the luminance value of the pixel of the digital image to limits of a sub-interval containing the determined luminance value of the pixel of the digital image.
14. The device of claim 13 wherein the image processing circuitry, in operation, generates a first histogram H representative of a distribution of luminance values in the digital image.
15. The device of claim 14 wherein the first histogram comprises a number of discrete values smaller than a total number of available discrete values for luminance values of the digital image.
16. The device of claim 14 wherein the image processing circuitry, in operation, divides the luminance interval into sub-intervals based on dichotomous division in which, at each step of division of an initial interval or sub-interval ranging from a value L min to a value L max :
an intermediate value β of the interval or sub-interval is determined, such that:
∑
k
=
L
min
β
H
(
k
)
=
∑
k
=
β
L
max
H
(
k
)
,
where k designates luminance values for which a histogram value H(k) has been calculated; and
a cut point C of the initial interval or sub-interval is determined, such that:
C
=
L
max
+
L
min
2
+
α
(
β
-
L
max
+
L
min
2
)
where α designates a control parameter greater than or equal to 0 and smaller than or equal to 1.
17. The device of claim 13 wherein a number of sub-intervals into which said interval is divided is smaller than an available number of luminance values in the luminance interval.
18. The device of claim 13 wherein pixels of the digital image are grouped in elementary macroblocks of pixels having a same base color distribution pattern.
19. The device of claim 18 wherein the image processing circuitry, in operation, determines a luminance value of a pixel of the digital image based on a weighted average of luminance values of pixels of a macroblock containing the pixel.
20. The device of claim 18 wherein a same gain is applied to all pixels of a macroblock of the digital image.
21. The device of claim 13 wherein luminance values of pixels of the output image are coded over a smaller number of bits than luminance values of pixels of the digital image.
22. A system, comprising:
image acquisition circuitry, which, in operation, acquires a first digital image; and
image processing circuitry, couple to the image acquisition circuitry, and which, in operation, generates a second digital image from the first digital image, by:
determining a luminance value of each pixel of the first image;
dividing a luminance interval, the luminance interval ranging from a lowest determined luminance value to a highest determined luminance of the pixels of the first image, into a plurality of sub-intervals; and
determining a luminance value of at least one pixel of the second image by multiplying the determined luminance value of a pixel of the first image by a gain determined by interpolation based on a distance of the luminance value of the pixel of the first image to limits of a sub-interval containing the determined luminance value of the pixel of the first image.
23. The system of claim 22 , comprising a memory coupled between the image acquisition circuitry and the image processing circuitry and configured to store the first image.
24. The system of claim 22 wherein the image processing circuitry is configured to:
group pixels of the first image into elementary macroblocks of pixels having a same base color distribution pattern;
determine a luminance value of a pixel of the first image based on a weighted average of luminance values of pixels of a macroblock containing the pixel; and
apply a same gain to all pixels of the macroblock.