HDR image encoding and decoding methods and devices

1. An image encoder comprising:

an input for a high dynamic range input image;

an image grading unit arranged to allow a human color grader to specify a color mapping from a representation of the high dynamic range input image, to a first low dynamic range image by a human-determined color mapping algorithm, and configured to output data specifying the color mapping; and

an automatic grading unit configured to derive a second low dynamic range image by applying an automatic color mapping algorithm to the high dynamic range input image, with a color mapping algorithm fulfilling a condition that a HDR reconstructed image falling within a second predefined accuracy from the high dynamic range input image can be calculated by applying a second color mapping algorithm which is the inverse of the automatic color mapping algorithm to the second low dynamic range image.

2. The image encoder as claimed in claim 1 , wherein the representation of the high dynamic range input image is one of the high dynamic range input image and the second low dynamic range image.

3. The image encoder as claimed in claim 1 , wherein the image grading unit and the automatic grading unit are configured to apply a monotonous mapping function on a luminance-correlate of pixels in their respective input image, in at least a geometrical region of the respective input image corresponding to a same geometrical region of the high dynamic range input image.

4. The image encoder as claimed in claim 3 , wherein the automatic grading unit is configured to determine the monotonous mapping function from luminance-correlates of pixels of the high dynamic range input image to luminance-correlates of pixels of the second low dynamic range image according to a criterion which determines respective ranges of luminance-correlates of pixels of the high dynamic range input image allocated to respective single values of a luminance-correlate of pixels of the second low dynamic range image, the respective ranges forming a set of luminance-correlate ranges covering the total range of possible luminance-correlate values for the high dynamic range input image.

5. The image encoder as claimed in claim 1 , comprising a data formatter configured to output into an image signal the second low dynamic range image and at least one of, or both of, data describing the color mapping between the high dynamic range input image and the second low dynamic range image, and data describing the color mapping between the first low dynamic range image and the second low dynamic range image.

6. An image decoder configured to receive via an image signal input an image signal comprising a low dynamic range image, and data describing a first color mapping enabling reconstruction of a reconstruction of a high dynamic range image on the basis of the low dynamic range image, and data describing a second color mapping allowing calculation of a further low dynamic range image on the basis of the low dynamic range image, the image decoder comprising an image derivation unit configured to derive at least the low dynamic range image) on the basis of the data describing the second color mapping and the pixel colors encoded in the low dynamic range image.

7. The image decoder as claimed in claim 6 , comprising a system configuration unit, configured to determine whether the decoder is connected to at least one of a high dynamic range display and a low dynamic range display, and the system configuration unit being arranged to configure the image derivation unit to determine at least the reconstruction in case of a connection to the high dynamic range display, and arranged to configure the image derivation unit to determine at least the further low dynamic range image in case of a connection to the low dynamic range display.

8. The image decoder as claimed in claim 6 , further comprising:

an output port including at least one of a wired connection and a wireless connection to any connectable display; and

a signal formatter configured to transmit at least one of the reconstruction of the high dynamic range image and the low dynamic range image to any connected display.

9. The image decoder as claimed in claim 6 , wherein the image derivation unit is configured to determine a further image based on the reconstruction of the high dynamic range image and the low dynamic range image, or the second low dynamic range image and data describing the first color mapping and data describing the second color mapping.

10. The image decoder as claimed in claim 6 , wherein the image signal input is connected to a reading unit configured to read the image signal from a memory.

11. The image decoder as claimed in claim 6 , wherein the image signal input is connectable via a network connection to a source of the image signal.